Transcriber’s Note:

   This work features some large and wide tables. These are best viewed
   with a wide screen.




                                  THE

                       CAMBRIDGE NATURAL HISTORY


                               EDITED BY

S. F. HARMER, M.A., Fellow of King’s College, Cambridge; Superintendent
                  of the University Museum of Zoology

                                  AND

      A. E. SHIPLEY, M.A., Fellow of Christ’s College, Cambridge;
        University Lecturer on the Morphology of Invertebrates

                              VOLUME III

  [Illustration]

  [Illustration: Map to illustrate

    THE GEOGRAPHICAL DISTRIBUTION
    of the
    LAND OPERCULATE MOLLUSCA]

  _The figures indicate the number of known species._


MOLLUSCS

   By the Rev. A. H. COOKE, M.A., Fellow and Tutor of
   King’s College, Cambridge


BRACHIOPODS (RECENT)

   By A. E. SHIPLEY, M.A., Fellow of Christ’s College,
   Cambridge


BRACHIOPODS (FOSSIL)

   By F. R. C. REED, M.A., Trinity College, Cambridge

                               New York
                           MACMILLAN AND CO.
                              AND LONDON

                                 1895

                         _All rights reserved_


   “Why, you might take to some light study: conchology, now; I
   always think that must be a light study.”
                                   GEORGE ELIOT, _Middlemarch_.


                           COPYRIGHT, 1895,
                         BY MACMILLAN AND CO.


                            Norwood Press:
                 J. S. Cushing & Co.--Berwick & Smith.
                        Norwood, Mass., U.S.A.




                        PREFACE TO THE MOLLUSCA


The general plan of classification adopted in this work is not that of
any single authority. It has been thought better to adopt the views of
recognised leading specialists in the various groups, and thus place
before the reader the combined results of recent investigation. This
method may, perhaps, occasion a certain number of small discrepancies,
but it is believed that the ultimate effect will be to the advantage of
the student.

The classification adopted for the recent Cephalopoda is that of
Hoyle (‘Challenger’ Reports, _Zoology_, vol. xvi.), for the fossil
Cephalopoda (Nautiloidea) that of Foord (_Catalogue of the Fossil
Cephalopoda in the British Museum_, 1888–91), and (Ammonoidea) P.
Fischer (_Manuel de Conchyliologie_, 1887). In the Gasteropoda the
outlines are those adopted by Pelseneer (_Mém. Soc. Malacol. Belg._
xxvii. 1894), while the details are derived, in the main, from P.
Fischer. The Amphineura, however, have not been regarded as a separate
class. The grouping of the Nudibranchiata is that of Bergh (Semper,
_Reisen im Archipel der Philippinen_, ii. 3). The Pelecypoda are
classified according to Pelseneer’s most recent grouping.

Acknowledgment of the principal sources of information has been made in
footnotes, and a short list of leading authorities has been appended
to the chapters on anatomy, for the use of students desirous to pursue
the subject further. In the case of geographical distribution the
authorities are too numerous and scattered to admit of a list being
given.

A special word of thanks is due to Mr. Edwin Wilson for his patient
care in preparing the illustrations, the majority of which are taken
from specimens in the University Museum of Zoology. Mr. Edgar Smith,
besides affording the kind help which visitors to the British Museum
always experience at his hands, has permitted me to use many specimens
for the purposes of illustration.

                                                         A. H. COOKE.

    KING’S COLLEGE, CAMBRIDGE,
       _20th December 1894_.




                               CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK.


                               MOLLUSCA


                               CHAPTER I

    INTRODUCTION--POSITION OF MOLLUSCA IN THE ANIMAL
        KINGDOM--CLASSIFICATION--ORIGIN OF LAND AND FRESH-WATER
        MOLLUSCA                                                   1


                              CHAPTER II

    LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND GENERAL
        ECONOMY                                                   23


                              CHAPTER III

    ENEMIES OF THE MOLLUSCA--MEANS OF DEFENCE--MIMICRY AND
        PROTECTIVE COLORATION--PARASITIC
        MOLLUSCA--COMMENSALISM--VARIATION                         56


                              CHAPTER IV

    USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION
        OF THE OYSTER, MUSSEL, AND SNAIL--SNAILS AS
        MEDICINE--PRICES GIVEN FOR SHELLS                         96


                               CHAPTER V

    REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE
        FERTILISED OVUM--DIFFERENCES OF SEX--DIOECIOUS AND
        HERMAPHRODITE MOLLUSCA--DEVELOPMENT OF FRESH-WATER
        BIVALVES                                                 123


                              CHAPTER VI

    RESPIRATION AND CIRCULATION--THE MANTLE                      150


                              CHAPTER VII

    ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE
        FOOT--THE NERVOUS SYSTEM                                 177


                             CHAPTER VIII

    THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS      209


                              CHAPTER IX

    THE SHELL, ITS FORM, COMPOSITION, AND GROWTH--DESIGNATION
        OF ITS VARIOUS PARTS                                     244


                               CHAPTER X

    GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER
        MOLLUSCA--THE PALAEARCTIC, ORIENTAL, AND
        AUSTRALASIAN REGIONS                                     277


                              CHAPTER XI

    GEOGRAPHICAL DISTRIBUTION OF LAND MOLLUSCA
        (_continued_)--THE ETHIOPIAN, NEARCTIC, AND
        NEOTROPICAL REGIONS                                      328


                              CHAPTER XII

    DISTRIBUTION OF MARINE MOLLUSCA--DEEP-SEA MOLLUSCA AND
        THEIR CHARACTERISTICS                                    360


                             CHAPTER XIII

    CLASS CEPHALOPODA                                            378


                              CHAPTER XIV

    CLASS GASTEROPODA--AMPHINEURA AND PROSOBRANCHIATA            400


                              CHAPTER XV

    CLASS GASTEROPODA (_continued_): OPISTHOBRANCHIATA AND
        PULMONATA                                                427


                              CHAPTER XVI

    CLASSES SCAPHOPODA AND PELECYPODA                            444


                         BRACHIOPODA (RECENT)


                             CHAPTER XVII

    INTRODUCTION--SHELL--BODY--DIGESTIVE SYSTEM--BODY
        CAVITY--CIRCULATORY SYSTEM--EXCRETORY
        ORGANS--MUSCLES--NERVOUS SYSTEM--REPRODUCTIVE
        SYSTEM--EMBRYOLOGY--HABITS--DISTRIBUTION--CLASSIFICATION
                                                                 463


                         BRACHIOPODA (FOSSIL)


                             CHAPTER XVIII

    INTRODUCTION--DIVISION I. ECARDINES--EXTERNAL
        CHARACTERS--INTERNAL CHARACTERS--DIVISION II.
        TESTICARDINES--EXTERNAL CHARACTERS--INTERNAL
        CHARACTERS--SYNOPSIS OF FAMILIES--STRATIGRAPHICAL
        DISTRIBUTION--PHYLOGENY AND ONTOGENY                     491




           SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK


                               MOLLUSCA

      Class               Order                Sub-order                  Section
                                        { OCTOPODA (p. 382).
                                        {                           { Phragmophora (p. 386).
                 { =Dibranchiata=       {                           { Sepiophora (p. 388).
                 {                      {                           {
                 {                      { DECAPODA                  { Chondrophora   { _Myopsidae_ (p. 389).
                 {                      {                           {                { _Oigopsidae_ (p. 390).
 =CEPHALOPODA=   {
                 {                      { NAUTILOIDEA               { Retrosiphonata (p. 393).
                 {                      {                           { Prosiphonata (p. 395).
                 { =Tetrabranchiata=    {
                 {                      { AMMONOIDEA                { Prosiphonata (p. 397).
                 {                      {                           { Prosiphonata (p. 397).

                 { =Amphineura=         { POLYPLACOPHORA (p. 400).
                 {                      { APLACOPHORA (p. 404).
                 {
                 {                      {                           { Docoglossa (p. 405).
                 {                      { DIOTOCARDIA               {                  { _Zygobranchiata_
                 {                      {                           { Rhipidoglossa    {   (p. 406).
                 {                      {                           {                  { _Azygobranchiata_
                 { =Prosobranchiata=    {                           {                  {    (p. 407).
                 {                      {
                 {                      {                           { Ptenoglossa (p. 411).
                 {                      {                           {                  { _Platypoda_ (p. 411).
                 {                      {                           { Taenioglossa     { _Heteropoda_ (p. 420).
                 {                      { MONOTOCARDIA              {
 =GASTEROPODA=   {                      {                           { Gymnoglossa (p. 422).
                 {                      {                           {
                 {                      {                           { Toxoglossa (p. 426).
                 {
                 {                      {                           { Bulloidea (p. 429).
                 {                      { TECTIBRANCHIATA           { Aplysioidea (p. 430).
                 {                      {                           { Pleurobranchoidea (p. 431).
                 {                      {                           { Siphonarioidea (p. 431).
                 {                      {
                 { =Opisthobranchiata=  { ASCOGLOSSA (p. 431).
                 {                      {
                 {                      { NUDIBRANCHIATA            { Cladohepatica (p. 432).
                 {                      {                           { Holohepatica (p. 433).
                 {                      {
                 {                      { PTEROPODA                 { Thecosomata (p. 435).
                 {                      {                           { Gymnosomata (p. 437).
                 {                      {
                 {
                 { =Pulmonata=          { BASOMMATOPHORA (p. 438).
                 {                      { STYLOMMATOPHORA (p. 439).


 =SCAPHOPODA=
 (p. 444).

    Class                 Order                             Suborder
                 { =Protobranchiata= (p. 447).
                 {                                    { ANOMIACEA (p. 448).
                 { =Filibranchiata=                   { ARCACEA (p. 448).
                 {                                    { MYTILACEA (p. 448).
                 {
                 { =Pseudolamellibranchiata= (p. 449).
                 {
 =PELECYPODA=    {                                    { SUBMYTILACEA (p. 451).
                 {                                    { TELLINACEA (p. 453).
                 {                                    { VENERACEA (p. 454).
                 { =Eulamellibranchiata=              { CARDIACEA (p. 454).
                 {                                    { MYACEA (p. 456).
                 {                                    { PHOLADACEA (p. 457).
                 {                                    { ANATINACEA (p. 458).
                 { =Septibranchiata= (p. 459).


                              BRACHIOPODA

                               Order                         Family
                 {                  { Lingulidae (pp. 487 and 503).
                 {                  { Obolidae (p. 504).
                 { ECARDINES        { Discinidae (pp. 487 and 504).
 =Brachiopoda=   {                  { Craniidae (pp. 487 and 504).
                 {                  { Trimerellidae (p. 504).
                 {
                 {                  { Productidae (p. 504).
                 {                  { Strophomenidae (p. 505).
                 {                  { Koninckinidae (p. 505).
                 {                  { Spiriferidae (p. 505).
                 {                  { Atrypidae (p. 505).
                 { TESTICARDINES    { Rhynchonellidae (pp. 487 and 505).
                 {                  { Terebratulidae (pp. 487 and 506).
                 {                  { Argiopidae (p. 506).
                 {                  { Stringocephalidae (p. 506).
                 {                  { Thecidiidae (pp. 487 and 506).




                             LIST OF MAPS


    THE GEOGRAPHICAL DISTRIBUTION OF THE LAND
      OPERCULATE MOLLUSCA                                _Frontispiece_

    THE GEOGRAPHICAL DISTRIBUTION OF THE LAND
      MOLLUSCA OF THE EAST INDIAN ARCHIPELAGO _Between pp. 308 and 309_

    THE RELATIONS OF THE LAND MOLLUSCA OF NEW GUINEA
      WITH THOSE OF NORTH AUSTRALIA                    _To face p. 322_

    THE GEOGRAPHICAL DISTRIBUTION OF THE LAND
      MOLLUSCA OF THE WEST INDIES             _Between pp. 344 and 345_




                               MOLLUSCS


                                  BY

                        REV. A. H. COOKE, M.A.
             Fellow and Tutor of King’s College, Cambridge




                               CHAPTER I

   INTRODUCTION--POSITION OF MOLLUSCA IN THE ANIMAL
     KINGDOM--CLASSIFICATION--ORIGIN OF LAND AND
     FRESH-WATER MOLLUSCA


It is the generally accepted opinion among men of science that all
life originated in the sea. Not that all parts of the sea are equally
favourable to the development of forms of life. The ocean surface,
with its entire absence of shelter or resting-place, and the deep sea,
whose abysses are always dark and cold and changeless, offer little
encouragement to plant or animal life, as an original starting-point.
True, both the surface and the depths of the sea have become colonised
by myriads of forms, Mollusca amongst them, but these quarters are in
the truest sense colonised, for the ancestors of those who inhabit them
in all probability migrated from elsewhere.

It was no doubt the littoral region and the shallow waters immediately
below it, a region of changeable currents, of light and shade, of
variation, within definite limits, of temperature and tide effects,
which became the scene of the original development of plant life,
in other words, of the food-supply which rendered possible its
colonisation by higher animals. But the littoral region, besides the
advantages of tenancy which it offers to animal life, has also its
drawbacks. The violence of the surf may beat its inhabitants in pieces,
the retreat of the tide exposes them, not merely to innumerable enemies
in the shape of predatory birds and beasts, but also to a change in
the atmospheric medium by which they are surrounded. Hence, in all
probability, have arisen the various forms of adaptation which are
calculated to bring about the ‘survival of the fittest’; hence, to
narrow our point of view to the MOLLUSCA, the development of
hard shells, or exoskeletons, hence the sand-burrowing, rock-boring,
rock-clinging instincts of various genera and species.[1]

What was the primitive form of molluscan life is little likely to be
ever positively known, although, on grounds of comparative anatomy,
something approaching to the archi-mollusc is often constructed, with
more or less probability, by careful observers. From one of the oldest
known geological strata, the Cambrian, nearly four hundred species
of Mollusca are known, which include representatives of nearly all
the great Orders as they exist at the present day, and without the
slightest sign of approximation to one another. With regard to the
origin of the land and fresh-water Mollusca some definite conclusions
can be arrived at, which will be given in their proper place.

Scarcely any portion of the coast-line of the world is destitute of
molluscan life, except in regions where extreme cold forbids its
existence. Thus along the shores of Northern Asia there is no proper
littoral fauna, the constant influence of travelling ice sweeping it
all away; animal life begins at about three fathoms. But in every coast
region not positively hostile to existence Mollusca make their home.
Each description of habitat has its own peculiar species, which there
flourish best, and exist precariously, if at all, elsewhere. Thus the
sandy waste of estuaries, the loose and shingly beaches, the slimy
mud-flats beset with mangroves, the low stretches of jagged rock, and
even the precipitous cliffs, from whose base the sea never recedes,
have all their own special inhabitants. The same is true of the deep
sea, and of the ocean surface. And when we come to examine the land and
fresh-water Mollusca, it is found not merely that some Mollusca are
terrestrial and others fluviatile, but that certain species haunt the
hills and others the valleys, some the recesses of woods and others the
open meadow sides, some prefer the limestone rocks, others the sandy
or clayey districts, some live only in still or gently moving waters,
while others are never found except where the current is rapid and
powerful.

It is within the tropics that the Mollusca become most numerous, and
assume their finest and quaintest forms. A tropical beach, especially
where there is a good tide-fall and considerable variety of station,
abounds in molluscan life to an extent which must literally be seen to
be believed. The beach at Panama, to select an instance familiar to
the present writer, is astonishingly rich in species, which probably
amount in all to several hundreds. This is due to the immense variety
of habitat. On the rocks at high-water mark, and even above them,
occur _Truncatella_, _Melampus_, _Littorina_, and _Siphonaria_;
where a mangrove-swamp replaces the rock, on the branches overhead
are huge _Littorina_, while three species of _Cerithidea_ crawl on
the mud, and _Cyrena_ and _Arca_ burrow into it. Lower down, in the
rock pools at half-tide mark are _Cerithium_, _Purpura_, _Omphalius_,
_Anachis_ (2 sp.), _Nassa_, and several _Crepidula_. At low-water
mark of ordinary tides, under stones half buried in clean sand, are
_Coecum_ and _Vitrinella_; under the blocks which rest on solid rock
are _Cypraea_ (4 or 5 sp.), _Cantharus_, more _Anachis_, _Columbella_
(3 sp. including the graceful _C. harpiformis_), and _Nitidella_.
Where the blocks of rock are rather muddy, _Conus_ lurks, and with it
_Turritella_ and _Latirus_. Where the rocks form a flat-topped platform
2 or 3 feet high, with here and there a deep crack, huge Chitons 3
inches long conceal themselves, with two species of _Turbo_, _Purpura_,
and _Clavella_. At extreme low-water mark of spring tides, _on_ the
isolated rocks are _Monoceros_, _Leucozonia_, and _Vermetus_, _in_ them
are _Pholas_ and a burrowing _Mytilus_, _under_ them are more _Conus_,
_Dolium_, and huge frilled _Murices_. Patches of clean gravelly sand
here produce _Strombus_; on the operculum of the great _Str. galea_
is sure to be a _Crepidula_, exactly fitting its breadth. On the
liquid mud-flats to the north glide about _Marginella_, _Nassa_, and
_Truncaria_, in the clean sandstretch to the west _Olivella_ ploughs
about by hundreds with several species of _Natica_, and _Tellina_
and _Donax_ bury themselves deep, while farther down are _Artemis_,
_Chione_, and, where mud begins to mix with the sand, _Mytilus_
and more _Arca_. Each of these species has its own habitat, often
circumscribed to a few square feet at the most, and it would be utterly
useless to seek for it anywhere except in its own special domain.

Equally abundant are the land Mollusca of the tropics. Prof. C. B.
Adams relates that within the limits of a single parish in Jamaica,
named Manchester, which measures no more than four miles long and one
mile broad, he obtained no fewer than one hundred species. Mr. J. S.
Gibbons, in a description of the Mollusca he obtained near St. Ann’s,
Curaçao, gives a lively picture of their abundance in an exceptionally
favoured locality:--[2]

“Near the outskirts of the town a waste piece of ground supplied
me with occupation for all the time I had to spare. Neither grass
nor water was to be seen, the only vegetation consisting of a few
stunted cacti and still fewer acacia bushes. This, however, was so
rich in shells that of several species enough specimens could have
been collected in a few yards to supply, I should suppose, all the
shell cabinets in the world.... The stones, plants, and ground were
covered with _Strophia uva_ L., _Tudora megacheila_, P. and M., was
in equal abundance, suspended by its silk-like thread from acacia
boughs, or strewed thickly on the ground underneath. A _Bulimulus_
(_B. multilineatus_ var. _sisalensis_) abounded on the smaller boughs,
while under masses of coral _Macroceramus inermis_ Gundl., _Pupa
parraiana_ d’Orb, and _P. pellucida_ Pfr., were abundant. In the loose
soil _Cylindrella Raveni_ Bland, _Cistula Raveni_ Bland, and a curious
_Cionella_ were so numerous that a spade would have been the best
instrument with which to collect them. I wasted a good deal of valuable
time in separating them from the soil, when by simply taking away a few
handfuls of mould, I might have obtained a larger number of specimens.
A species of _Stenogyra_ and a _Succinea_ complete a list, all of which
might have been gathered from almost any square yard of ground on the
hillside.”


=Position of Mollusca in the Animal Kingdom.=--Up to very recent times
it was usual to regard the Mollusca as one of the four subdivisions
of a great family known as _Malacozoa_, the subdivisions being (1)
Mollusca, (2) Tunicata, (3) Brachiopoda, (4) Polyzoa or Bryozoa. This
classification is still retained in the leading modern manual on the
subject.[3] The progress, however, of investigation leads to the belief
that the Mollusca are not so closely related to these other groups as
such a classification would seem to imply. The Tunicata, for instance,
appear, from the whole course of their development, to occupy a
position near to the Vertebrata. The relations of the Brachiopoda
and Polyzoa will be more particularly referred to in that part of
this History which deals especially with those groups. The position
of the Mollusca is, in many respects, one of considerable isolation.
Any attempt, therefore, definitely to relate them to one group or
another, is, in all probability, to go further than the present state
of our knowledge warrants. Especially to be deprecated are systems of
classification which confidently _derive_ the Mollusca in general from
this or that group. The first undisputed traces of animal life, which
appear in the Cambrian epoch, exhibit the same phyletic distinctions as
now exist. Sponges, Echinoderms, Mollusca, and Worms, formed already,
in those immeasurably remote ages, groups apparently as generally
distinct from one another as they are at the present time. It would
seem that any theory of development, which confidently teaches the
derivation of any one of these groups from any other, is, in the
present state of the evidence before us, hazardous in the extreme.

Some indications of relationship, which must not be pushed too far, may
be drawn from a consideration of embryonic resemblance. An especial
characteristic of the Mollusca is the possession of a particular form
of larva, which occurs in one of the stages of development, known as
the _trochosphere_ (see p. 130). This form of larva is shared with two
orders of Annelida, the Chaetopoda and the Gephyrea armata, and, in
all probability, with the Polyzoa as well. It may also be significant
that the adult form in Rotifera bears a close resemblance to the
trochosphere larva in those groups.


=Basis of Classification.=--The Mollusca are divided into four
great Orders--=Cephalopoda=, =Gasteropoda=, =Scaphopoda=, and
=Pelecypoda=.[4] Each name, it will be noticed, bears reference to the
‘foot,’ _i.e._ to the organ of motion which corresponds in function to
the foot in the Vertebrata.

In the _Cephalopoda_ the feet, or, as they are more frequently termed,
the ‘arms,’ are arranged symmetrically round the head or mouth. The
common forms of ‘cuttle-fish’ (_Octopus_, _Loligo_) are familiar
examples of Cephalopods.

The _Gasteropoda_ crawl on the flat under-surface or ‘sole’ of the
foot. Snails, slugs, sea-hares, whelks, periwinkles, and coats-of-mail
or chitons are examples of this Order.

The _Scaphopoda_ possess a long tubular shell open at both ends; with
their small and elongated foot they are supposed to dig into the mud in
which they live. The common _Dentalium_ or tusk-shell of our coasts is
a representative of this Order.

  [Illustration: FIG. 1.--Examples of the four Orders: =A=,
  Cephalopoda; =B=, Gasteropoda; =C=, Scaphopoda, and =D=,
  Pelecypoda.

  =A=, _Ommastrephes sagittatus_ Lam., Naples: _a_, _a_,
  arms surrounding the mouth; _f_, funnel; _t_, _t_, the two
  ‘tentacular’ arms, × ⅖. =B=, _Buccinum undatum_ L., Britain: _f_,
  foot; _pr_, proboscis. × ½. =C=, _Dentalium entalis_ L., Norway:
  _f_, foot. =D=, _Cardium oblongum_ Chem., Naples: _f_, foot; _s_,
  efferent or anal siphon; _s’_, efferent or branchial siphon, × ½.]

The _Pelecypoda_[5] are enclosed in a bivalve shell fastened by a
muscular hinge, the adjacent part of the valves being generally more or
less toothed; the foot is as a rule roughly comparable to the shape of
an axe-head.

To these four Orders is frequently added a fifth, the _Pteropoda_,
whose exact position is at present not absolutely settled. The
Pteropoda[6] are ‘pelagic,’ _i.e._ they live in the open waters of the
ocean, rising to the surface at night, and sinking into cooler water by
day. They are provided with a pair of wing-like appendages or ‘feet,’
on each side of the head, by means of which they are enabled to swim.
Some authorities regard the Pteropoda as a subdivision of Gasteropoda,
others as forming a separate Order, of equivalent value to the other
four. The question will be further discussed below (see chap. xv.),
but for the present it will be sufficient to state that the weight of
evidence appears to show that the Pteropoda are modified Gasteropoda,
with special adaptations to pelagic life, and are therefore not
entitled to rank as a separate Order.

Some writers conveniently group together the first three of these
Orders, the Cephalopoda, Gasteropoda, and Scaphopoda, under the
title =Glossophora=,[7] or Mollusca furnished with a _radula_ or
ribbon-shaped ‘tongue,’ set with rows of teeth and situated in
something of the nature of a head, as distinguished from the =Aglossa=
(or _Lipocephala_),[8] _i.e._ those Mollusca which have no radula and
no head. To the latter belong only the fourth Order, the Pelecypoda.
This view postulates, for the primitive ancestral Mollusc, a body
with a more or less developed head, and possibly the rudiments of an
apparatus for grinding or triturating food. This form, it is held,
either developed or degenerated. In the former case, in consequence
of the more active mode of life upon which it may be supposed to have
entered, it gave rise to all the more highly organised forms which
are grouped under the three great Orders. When, on the other hand,
the ancestral form associated itself with an inactive or sedentary
life, it was, we may believe, modified accordingly, and either lost
by atrophy or failed to acquire those special points of organisation
which characterise the highly-developed form. Hence the Pelecypoda, or
bivalves, whose characteristic is the absence of any definite cephalic
region or masticatory apparatus. It is a remarkable fact in support
of this theory of the origin of the Aglossa that certain of their
larvae are known to possess traces of higher organisation, _e.g._ an
external mouth and eyes, the former of which becomes covered by the
mantle lobes, while the latter disappear long before the adult stage is
reached.

Thus we have

                                MOLLUSCA
                                    |
                   +----------------+----------------------------------+
                   |                                                   |
           =Glossophora=                                    =Aglossa=
                   |                                                   |
          +--------+----------+-------------------+                    |
          |                   |                   |                    |
 _Cephalopoda_   _Gasteropoda_   _Scaphopoda_    _Pelecypoda_


=Classification of Gasteropoda.=--The _Gasteropoda_ are numerically
very largely in excess of the two other Orders of the Glossophora,
far more complicated as regards classification, and contain a large
proportion of those examples of the Mollusca which are most familiar
to the ordinary observer. It will therefore be convenient to postpone
for the present a fuller discussion of the subdivisions of the
Cephalopoda and Scaphopoda, as well as of the Aglossa, returning to
them again in special chapters (chaps. xiii. and xvi.), and to devote
a few introductory words to the classification and relations of the
Gasteropoda.

The Gasteropoda are divided into four Classes, _Amphineura_,
_Prosobranchiata_, _Opisthobranchiata_, and _Pulmonata_.

  [Illustration: FIG. 2.--An example of the Polyplacophora: _Chiton
  spinosus_ Brug.]

  [Illustration: FIG. 3.--An example of the Aplacophora, _Neomenia
  carinata_ Tullb.: _a_, anus; _gr_, ventral groove; _m_, mouth.]

(1) The _Amphineura_[9] are bilaterally symmetrical Mollusca, _i.e._
with organs either single and central, or paired and disposed on
either side of the longer axis of the animal. The shell, when
present, is never spiral, but consists of eight overlapping plates,
kept together by an elliptical girdle. The Amphineura are divided
into (_a_) _Polyplacophora_,[10] or Chitons, and (_b_) _Aplacophora_
(_Chaetoderma_ and _Neomenia_).

(2) The _Prosobranchiata_[11] are so named from the fact that the
breathing organ (branchia or ctenidium[12]) is as a rule situated _in
front of_ the heart, the auricle at the same time being in front of
the ventricle. They are asymmetrical, almost always furnished with a
shell, which is at some time spiral, and with an operculum. The sexes
are separate. They are either marine animals, or can be shown to be
more or less directly derived from genera which are marine. They
are divided into (_a_) _Diotocardia_[13] (_Haliotis_, _Fissurella_,
_Trochus_, _Nerita_, _Patella_), which have, or whose immediate
ancestors are believed to have had, two auricles to the heart, two sets
of breathing organs, two kidneys, but no proboscis, penis, or siphon,
and (_b_) _Monotocardia_,[14] in which the heart has only one auricle,
the true breathing organ is single, and there is a single kidney. To
this division belong the great majority of marine univalve Mollusca,
_e.g._ _Cypraea_, _Buccinum_, _Murex_, _Littorina_, _Ianthina_,
all the land and fresh-water operculates (_Cyclostoma_, _Melania_,
_Paludina_, etc.), as well as the _Heteropoda_, which are a group of
Prosobranchiata which have betaken themselves to a pelagic life.

  [Illustration: FIG. 4.--Example of a Heteropod, _Carinaria
  mediterranea_ Lam., Naples: _a_, anus; _br_, branchia; _f_, foot;
  _i_, intestine; _m_, mouth; _p_, penis; _s_, sucker; _sh_, shell;
  t, tentacles. × ½. The animal swims foot uppermost.]

(3) In the _Opisthobranchiata_[5] the breathing organs (when present)
are _behind_ the heart, and the auricle of the heart is consequently
behind the ventricle. They are asymmetrical marine animals; usually,
but by no means always, without a shell, scarcely ever with an
operculum in the adult state. The sexes are united in the same
individual. The _Opisthobranchiata_ fall into two divisions: (_a_)
_Tectibranchiata_, in which the breathing organ is more or less covered
by the mantle, and a shell is usually present, which is sometimes
rudimentary, _e.g._ _Bulla_, _Aplysia_, _Umbrella_, and the whole
group of _Pteropoda_; (_b_) _Nudibranchiata_, or sea slugs, which have
no shell and no true ctenidia, but breathe either by the skin, or by
‘cerata’ or papilliform organs prominently developed on the back:
_e.g._ _Doris_, _Aeolis_, _Dendronotus_.

  [Illustration: FIG. 5.--=A=, A Tectibranchiate Opisthobranch,
  _Umbrella mediterranea_ Lam., Naples: _a_, anus; _br_, branchia;
  _f_, foot; _m_, mouth; _rh_, rhinophores; _sh_, shell.

  =B=, A Pteropod, _Hyalaea tridentata_ Forsk., Naples: _sh_,
  shell; _l_, _l_, swimming lobes of foot.

  =C=, A Nudibranchiate Opisthobranch, _Aeolis peregrina_, Naples:
  _f_, foot; _c_, cerata.]

  [Illustration: FIG. 6.--Examples of--=A=, Pulmonata
  Basommatophora, the common _Limnaea peregra_ Müll.: _e_, _e_,
  eyes; _t_, _t_, tentacles. =B=, Pulmonata Stylommatophora, _Helix
  hortensis_ Müll.: _e_, _e_, eyes; _t_, _t_, tentacles; _p. o_,
  pulmonary orifice (the position of the pulmonary orifice in
  _Limnaea_ will be seen by reference to Fig. 101).]

(4) The _Pulmonata_[15] are asymmetrical air-breathing non-marine
Mollusca, generally, but not always, furnished with a shell. The sexes
are always united in the same individual, and the operculum is always
wanting, except in _Amphibola_. They are conveniently divided into
_Stylommatophora_,[16] in which the eyes are at the tip of the upper
tentacles, which are retractile (_Helix_, _Limax_, _Bulimus_, and all
true land slugs and snails), and _Basommatophora_, in which the eyes
are at the base of the tentacles, which are not retractile (_Limnaea_,
_Planorbis_, _Physa_, and all the _Auriculidae_).

Thus we have

                { _Amphineura_         { Polyplacophora
                {                      { Aplacophora
                {
                { _Prosobranchiata_    { Diotocardia
 =Gasteropoda=  {                      { Monotocardia (incl. Heteropoda)
                {
                { _Opisthobranchiata_  { Tectibranchiata (incl. Pteropoda)
                {                      { Nudibranchiata[17]
                {
                { _Pulmonata_          { Stylommatophora
                {                      { Basommatophora

The relation of the four great Orders to one another will be better
discussed when we come to deal with each Order separately. The problem
of the origin and mutual relationship of the various forms of molluscan
life is of extreme subtlety, and its solution can only be approached
after a comprehensive survey of many complicated anatomical details.
But there is one branch of the Mollusca--the land and fresh-water
genera--whose origin is, comparatively speaking, of recent date, and
whose relationships are therefore less likely to have suffered complete
obliteration.

=Origin of the Land and Fresh-water Mollusca.=--The ultimate
derivation of the whole of the land and fresh-water molluscan fauna
must, as has already been remarked, be looked for in the sea. In
certain cases the process of conversion, if it may be so termed, from
a marine to a non-marine genus, is still in progress, and can be
definitely observed; in others the conversion is complete, but the
modification of form has been so slight, or the date of its occurrence
so recent, that the connexion is unmistakable, or at least highly
probable; in others again, the modification has been so great, or the
date of its occurrence so remote, that the actual line of derivation is
obscured or at best only conjectural.

  [Illustration: FIG. 7.--=A=, the common cockle (_Cardium edule_
  L.). =B=, _Adacna plicata_ Eichw., Caspian Sea. =C=, _Didacna
  trigonoides_ Pall., Caspian Sea.]

This passage from a marine to a non-marine life--in other words, this
direct derivation of non-marine from marine genera--is illustrated
by the faunal phenomena of an inland brackish-water sea like the
Caspian, which is known to have been originally in connexion with the
Mediterranean, and therefore originally supported a marine fauna.
The Mollusca of the Caspian, although without exception brackish- or
fresh-water species, are in their general facies distinctly marine.
Of the 26 univalve species which inhabit it 19 belong to 4 peculiar
genera (_Micromelania_, _Caspia_, _Clessinia_, _Nematurella_), all of
which are modified forms of the marine _Rissoidae_. The characteristic
bivalves belong to the genera _Adacna_, _Didacna_, and _Monodacna_, all
of which can be shown to be derived from the common _Cardium edule_.
We have here a case where complete isolation from the sea, combined
no doubt with a gradual freshening of the water, has resulted in the
development of a number of new genera. The singularly marine facies
of several of the fresh-water genera now inhabiting Lake Tanganyika,
has given rise to the belief, among some authorities, that that lake
was at one time an inlet of the Indian Ocean. In the upper waters of
the Baltic, marine and fresh-water Mollusca flourish side by side.
So complete is the intermixture, that an observer who had lived on
no other shores would probably be unable to separate the one set of
species from the other.[18] Thus between Dagö and Papen-Wiek[19]
_Mytilus edulis_, _Cardium edule_, _Tellina balthica_, _Mya arenaria_,
_Littorina rudis_, and _Hydrobia balthica_ are the only true marine
species; with these live _Unio_, _Cyclas_, _Neritina_, _Limnaea_, and
_Bithynia_. The marine species and _Neritina_ live down to 15–20 fath.,
the rest only down to 3 fath. Under stones close to the shore of the
Skärgård at Stockholm[20] are found young _Cardium_ and _Tellina_,
and at 3 to 6 fath. _Limnaea peregra_, and _Physa fontinalis_. Near
Gothland _Limnaea_ is found in the open sea at 8–12 fath., and with it
occur _Cardium_ and _Tellina_. At the Frisches Haff[21] _Mya arenaria_
is the only marine species, and lives in company with 6 sp. _Limnaea_,
1 _Physa_, 9 _Planorbis_, 1 _Ancylus_, 4 _Valvata_, 2 _Sphaerium_. Were
the Sound to become closed, and the waters of the Baltic perfectly
fresh, it would be inevitable that _Mya arenaria_, and such other
marine species as continued to live under their changed conditions,
should in course of time submit to modifications similar in kind to
those experienced by the quondam marine species of the Caspian.

It seems probable, however, that the origin, at least in a great part,
of the land and fresh-water Mollusca need not be accounted for by such
involuntary changes of environment as the enclosure of arms of the sea,
or the possible drying up of inland lakes. These cases may be taken as
illustrations of the much more gradual processes of nature by which the
land and fresh-water fauna must have been developed. The ancestry of
that fauna must be looked for, as far as the Gasteropoda are concerned,
in _the littoral and estuarine species_; for the Pelecypoda, in _the
estuarine alone_. The effect of the recess of the tide, in the one
case, and the effect of the reduced percentage of salt, in the other,
has tended to produce a gradual adaptation to new surroundings, an
adaptation which becomes more and more perfect. It may be safely
asserted that no marine species could pass into a land or fresh-water
species except after a period, more or less prolonged, of littoral
or estuarine existence. Thus we find no land or fresh-water species
exhibiting relationships with such deep-sea genera as the _Volutidae_,
_Cancellariidae_, _Terebridae_, or even with genera trenching on the
lowest part of the littoral zone, such as the _Haliotidae_, _Conidae_,
_Olividae_, _Capulidae_. The signs of connexion are rather with the
_Neritidae_, _Cerithiidae_, and above all the _Littorinidae_, which
are accustomed to live for hours, and in the case of _Littorina_ for
days or even weeks, without being moistened by the tide. Similarly
the fresh-water Pelecypoda exhibit relationships, not with genera
exclusively marine, but with genera known to inhabit estuaries, such as
the _Mytilidae_, _Corbulidae_, _Cardiidae_.

It would be natural to expect that we should find this process of
conversion still going on, and that we should be able to detect
particular species or groups of species in process of emigration
from sea to land, or from sea to fresh water. Such species will be
intermediate between a marine and a land or fresh-water species, and
difficult to classify distinctly as one or the other. Cases of Mollusca
occupying this intermediate position occur all over the world. They
inhabit brackish swamps, damp places at high-water mark, and rocks only
at intervals visited by the tide. Such are _Potamides_, _Assiminea_,
_Siphonaria_, _Melampus_, _Hydrobia_, _Truncatella_, among the
univalves, and many species of _Cyrena_ and _Arca_ among the bivalves.


                    =Origin of the Fresh-water Fauna=

(_a_) =Pelecypoda.=--Estuarine species, which have become accustomed to
a certain admixture of fresh water, have gradually ascended the streams
or been cut off from the sea, and have at last become habituated to
water which is perfectly fresh.

  [Illustration: FIG. 8.--=A=, The common _Mytilus edulis_ L.,
  a marine genus and species. =B=, _Dreissensia_, a fresh-water
  genus, closely allied to _Mytilus_.]

  [Illustration: FIG. 9.--=A=, _Arca navicella_ Reeve, Philippines,
  a marine species. =B=, _Arca (Scaphula) pinna_ Bens., R.
  Tenasserim, a fresh-water species which lives many miles above
  the tide-way.]

Thus _Dreissensia_ (rivers and canals throughout N. Europe and N.
America) and _Mytilopsis_ (rivers of America) are scarcely modified
_Mytili_ (Fig. 8); _Scaphula_ is a modified _Arca_, and lives in
the Ganges, the Jumna, and the Tenasserim at a distance of 1600
miles from the sea (Fig. 9). _Pholas rivicola_ is found imbedded in
floating wood on the R. Pantai many miles from its mouth. _Cyrena_,
_Corbicula_, and probably _Sphaerium_ and _Pisidium_ are derived, in
different degrees of removal, from the exclusively marine _Veneridae_;
_Potamomya_ (rivers of S. America), and _Himella_ (R. Amazon) are forms
of _Corbula_. The Caspian genera derived from _Cardium_ (_Adacna_,
_Didacna_, _Monodacna_), have already been referred to. _Nausitora_ is
a form of _Teredo_, which lives in fresh water in Bengal. _Rangia_,
_Fischeria_, and _Galatea_ probably share the derivation of the
Cyrenidae, while in _Iphigenia_ we have one of the Donacidae which
has not yet mounted rivers, but is confined to a strictly estuarine
life. The familiar _Scrobicularia piperata_ of our own estuaries is a
_Tellina_, which lives by preference in brackish water.

  [Illustration: FIG. 10.--_Trigonia pectinata_ Lam., Sydney,
  N.S.W.]

The great family of the Unionidae is regarded by Neumayr[22] as derived
from _Trigonia_, the points of similarity being the development of a
nacreous shell, the presence of a strong epidermis, and the arrangement
of the muscular scars. It is remarkable, too, that on many Uniones of
Pliocene times there is found shell ornamentation of such a type as
occurs elsewhere among the Pelecypoda only on _Trigonia_.

The genera of fresh-water Pelecypoda are comparatively few in number,
and their origin is far more clearly discernible than that of any other
group. This is perhaps due to the fact that the essential changes of
structure required to convert a marine into a fresh-water bivalve
are but slight. Both animals “breathe water,” and both obtain their
nutriment from matter contained in water. Similar remarks apply to
fresh-water operculate Gasteropoda. But the passage from a marine to an
aerial life involves much profounder changes of environment, which have
to be met by correspondingly important changes in the organism. This
may be in part the reason why the ancestry of all Pulmonata, whether
land or fresh-water, is so difficult to trace.

  [Illustration: FIG. 11.--=A=, _Cominella_, a marine genus, which
  lives between tide marks, and from which is probably derived =B=,
  _Clea_, a genus occurring only in fresh water.]

  [Illustration: FIG. 12.--=A=, _Cerithium columna_ Sowb. (marine).
  =B=, _Potamides microptera_ Kien. (brackish water). =C=, _Io
  spinosa_ Lea, one of the _Pleuroceridae_ (fresh water).]

(_b_) =Gasteropoda.=--(1) _Operculate._ _Canidia_ and _Clea_
are closely allied, with but little modification, to the marine
_Cominella_[23] (Fig. 11), as is also _Nassodonta_ to _Nassa_.
They occur (in fresh water) in the rivers of India, Indo-China,
Java, and Borneo, associated with essentially fresh-water species.
_Potamides_, with its various sub-genera (_Telescopium_, _Pyrazus_,
_Pirenella_, _Cerithidea_, etc.), all of which inhabit swamps and
mud-flats just above high-water mark in all warm countries, are
derived from _Cerithium_ (Fig. 12); _Assiminea_, _Hydrobia_, and
perhaps _Truncatella_, from _Rissoa_. It is a remarkable fact that in
_Geomelania_ (with its sub-genera _Chittya_ and _Blandiella_) we have
a form of _Truncatella_ which has entirely deserted the neighbourhood
of the sea, and lives in woody mountainous localities in certain of
the West Indies. _Cremnoconchus_, a remarkable shell occurring only on
wet cliffs in the ghâts of southern India, is a modified _Littorina_.
_Neritina_ and _Nerita_ form a very interesting case in illustration
of the whole process. _Nerita_ is a purely marine genus, occurring
on rocks in the littoral zone; one species, however, (_N. lineata_,
Chem.) ascends rivers as far as 25 miles from their mouth, and others
haunt marshes of brackish water. _Neritina_ is the fresh-water form,
some species of which are found in brackish swamps or even creeping on
wet mud between tide marks, while the great majority are fluviatile,
one group (_Neritodryas_) actually occurring in the Philippines on
trees of some height, at a distance of a quarter of a mile from any
water. _Navicella_ is a still further modified form of _Neritina_,
occurring only on wet rocks, branches, etc., in non-tidal streams (Fig.
13).

  [Illustration: FIG. 13.--Illustrating the development of the
  fresh-water genus _Navicella_, through the brackish-water
  _Neritina_, from the marine _Nerita_, with corresponding changes
  in the operculum. 1. _Nerita_; 2, 3. _Neritina_; 4. _Neritina_,
  intermediate form; 5, 6. _Navicella_.]

The great family of the Melaniidae, which occurs in the rivers of warm
countries all over the world, and that of the Pleuroceridae, which
is confined to North America, are, in all probability, derived from
some form or forms of _Cerithium_. The origin of the Paludinidae,
Valvatidae, and Ampullariidae is more doubtful. Their migration from
the sea was probably of an early date, since the first traces of all
three appear in the lower Cretaceous, while Melaniidae are not known
until Tertiary times. _Ampullaria_, however, shows distinct signs
of relationship to _Natica_, while the affinities of _Paludina_ and
_Valvata_ cannot as yet be approximately affirmed.

(2) _Pulmonata._--Intermediate between the essentially fresh-water
and the essentially marine species come the group sometimes known as
Gehydrophila, consisting of the two families Auriculidae and Otinidae.
These may be regarded as Mollusca which, though definitely removed
from all marine species by the development of a true lung or lung
cavity in the place of a gill, have yet never become, in respect of
habitat, genuine fresh-water species. Like _Potamides_, they haunt salt
marshes, mangrove swamps, and the region about high-water mark. In
some cases (_Otina_, _Melampus_, _Pedipes_) they live on rocks which
are moistened, or even bathed by the spray, in others (_Cassidula_,
_Auricula_) they are immersed in some depth of brackish water at
high tide, in others again (_Scarabus_) they are more definitely
terrestrial, and live under dead leaves in woods at some little
distance from water. Indeed one genus of diminutive size (_Carychium_)
has completely abandoned the neighbourhood of the sea, and inhabits
swampy ground almost all over the world.

  [Illustration: FIG. 14.--Examples of the _Auriculidae_: =A=,
  _Auricula Judae_ Lam., Borneo; =B=, _Scarabus Lessoni_ Blainv.,
  E. Indies; =C=, _Cassidula mustelina_ Desh., N. Zealand;
  =D=, _Melampus castaneus_ Mühlf., S. Pacific; =E=, _Pedipes
  quadridens_ Pfr., Jamaica.]

  [Illustration: FIG. 15.--An example of _Amphibola_ (_avellana_
  Chem.), the only true Pulmonate which possesses an operculum.]

To this same section Gehydrophila have been assigned two remarkable
forms of air-breathing “limpet,” _Siphonaria_ and _Gadinia_ (see
page 151), and the aberrant _Amphibola_, a unique instance of a true
operculated pulmonate. _Siphonaria_ possesses a pulmonary cavity
as well as a gill, while _Gadinia_ and _Amphibola_ are exclusively
air-breathing. _Siphonaria_ lives on rocks at or above high-water
mark, _Gadinia_ between tide marks, _Amphibola_ (Fig. 15) in
brackish water at the estuaries of rivers, half buried in the sand.
There can be little doubt that all these are marine forms which
are gradually becoming accustomed to a terrestrial existence. In
_Gadinia_ and _Amphibola_ the process is so far complete that they
have exchanged gills for a pulmonary cavity, while in _Siphonaria_
we have an intermediate stage in which both organs exist together.
A curious parallel to this is found in the case of _Ampullaria_,
which is furnished with two gills and a pulmonary chamber, and
breathes indifferently air and water. It is a little remarkable that
_Siphonaria_, which lives at a higher tide level than _Gadinia_, should
retain the gill, while _Gadinia_ has lost it.

The ultimate affinities of the essentially fresh-water groups,
_Limnaea_, _Physa_, _Chilina_, cannot be precisely affirmed. The
form of shell in _Latia_, _Gundlachia_, and perhaps _Ancylus_, may
suggest to some a connexion with the Otinidae, and in _Chilina_,
a similar connexion with the Auriculidae. But, in a question of
derivation, similarities of shell alone are of little value. It is
not a little remarkable, for instance, that we should find a simple
patelliform shell in genera so completely distinct from one another
in all anatomical essentials as _Ancylus_, _Patella_, _Siphonaria_,
_Propilidium_, _Hipponyx_, _Cocculina_, and _Umbrella_.

Some recent authors, on grounds of general organisation, regard the
Limnaeidae and their allies as Opisthobranchs adapted to an aerial
life. It is held[24] that the Nudibranchiate Opisthobranchs have
given birth to the Pulmonata Stylommatophora or land _snails_, and
the Tectibranchiate Opisthobranchs to the Pulmonata Basommatophora or
fresh-water _snails_. Such a view seems at first sight open to some
objection from other views than those which deal simply with anatomy.
The _Opisthobranchiata_ are not, to any marked extent, littoral genera,
nor do they specially haunt the mouths of rivers. On the contrary,
they inhabit, as a rule, only the very lowest part of the littoral
zone, and are seldom found, except where the water is purely salt.
In other cases, when the derivation of land or fresh-water genera is
fairly well established, intermediate forms persist, which indicate,
with more or less clearness, the lines along which modification has
proceeded. It has, however, recently been shown that _Siphonaria_[25]
and _Gadinia_,[26] which have, as has been already mentioned, hitherto
been classified as _Pulmonata_, are in reality modified forms of
Opisthobranchiata, which are in process of adaptation to a life partly
marine, partly on land. They may therefore be regarded as supplying the
link, hitherto missing, between the land _Pulmonata_ and the marine
groups from one or other of which the latter must have been derived.
The general consensus of recent opinion inclines towards accepting
these views, some writers[27] being content to regard the _Pulmonata_,
as a whole, as derived from the Tectibranchiate Opisthobranchs, while
others[28] go further and regard the Stylommatophora as derived
directly from the Basommatophora.


                       =Origin of the Land Fauna=

=Gasteropoda.=--(1) _Operculate._ On _a priori_ grounds, one might
predict a double origin for land operculates. Marine species might
be imagined to accustom themselves to a terrestrial existence, after
a period, more or less prolonged, of littoral probation. Or again,
fresh-water species, themselves ultimately derived from the sea, might
submit to a similar transformation, after a preliminary or intermediate
stage of life on mudbanks, wet swamps, branches overhanging the water,
etc. Two great families in this group, and two only, seem to have
undergone these transformations, the Littorinidae and the Neritidae.
The derivation of almost all existing land operculates may be referred
to one or other of these groups.

  [Illustration: FIG. 16.--Two rows of the radula of _Littorina
  littorea L._, × 72.]

The power of the Littorinidae to live for days or even weeks without
being moistened by the sea may be verified by the most casual observer.
In the tropics this power seems even greater than on our own shores. I
have seen, in various parts of Jamaica, _Littorina muricata_ living at
the top of low cliffs among grass and herbage. At Panama I have taken
three large species of _Littorina_ (_varia_, _fasciata_, _pulchra_), on
trees at and above high-water mark. Cases have been recorded in which
a number of _L. muricata_, collected and put aside, have lived for
three months, and _L. irrorata_ for four months.[29] These facts are
significant, when we know that the land operculates almost certainly
originated in a tropical climate.

The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, as
contrasted with the other land operculates, form one group, have very
close relations, particularly in the length and formation of the
radula, or lingual ribbon, with the Littorinidae.

  [Illustration: FIG. 17.--Two rows of the radula of _Cyclophorus_
  sp., India, × 40.]

On the other hand, the Helicinidae, Hydrocenidae, and Proserpinidae are
equally closely related to _Neritina_. The Proserpinidae (restricted
to the Greater Antilles, Central America and Venezuela) may perhaps
be regarded as the ultimate term of the series. They have lost the
characteristic operculum, which in their case is replaced by a number
of folds or lamellae in the interior of the shell. It has already been
noticed how one group of _Neritina_ (_Neritodryas_) occurs normally
out of the water. This group furnishes a link between the fresh-water
and land forms. It is interesting to notice that here we have the most
perfect sequence of derivatives; _Nerita_ in the main a purely marine
form, with certain species occurring also in brackish water; _Neritina_
in the main fresh-water, but some species occurring on the muddy shore,
others on dry land; _Helicina_ the developed land form; and finally
_Proserpina_, an aberrant derivative which has lost the operculum.[30]

   [Illustration: FIG. 18.--=A=, _Neritina reticularis_ Sowb.,
  Calcutta (brackish water); =B=, _Helicina neritella_ Lam.,
  Jamaica (land); =C=, _Proserpina_ (_Ceres_) _eolina_ Ducl.,
  Central America (land).]

=Gasteropoda.=--(2) _Pulmonata._ The origin of these, the bulk of the
land fauna, must at present be regarded as a problem not yet finally
solved. Some authorities, as we have seen, regard them as derived
from the Nudibranchiate, others, probably more correctly, from the
Tectibranchiate Opisthobranchs.

The first known members of the land Pulmonata (_Pupa_ [?], _Hyalinia_)
are from the Carboniferous of North America. Similar but new forms
appear in the Cretaceous, from which time to the present we have
an unbroken series. The characteristically modern forms, according
to Simroth,[31] are Helices with thick shells. According to the
same author, _Vitrina_ and _Hyalinia_ are ancestral types, which
give origin not only to many modern genera with shells, but to many
shell-less genera also, _e.g._ _Testacella_ is probably derived through
_Daudebardia_ from _Hyalinia_, while from _Vitrina_ came _Limax_ and
_Amalia_. A consideration of the radulae of the genera concerned
certainly tends in favour of these views.

Godwin-Austen, speaking generally, considers[32] genera of land
Pulmonata with strongly developed mantle-lobes and rudimentary shell as
more advanced in development than genera in which the shell is large
and covers all or nearly all the animal.




                              CHAPTER II

            LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND
                            GENERAL ECONOMY


The majority of the Land Mollusca are probably more sensitive than is
usually believed. The humidity of the air must affect the surface of
their skin to a considerable extent. Every one has noticed how the
snails ‘come out’ on a damp evening, especially after rain. As a rule,
they wait till rain is over, probably objecting to the patter of the
drops upon their delicate tentacles. Snails kept in captivity under
a bell-glass are acutely sensitive of a damp atmosphere, and will
bestir themselves after rain just as if they were in the open air.
Certain Helices which are accustomed to live in moist places, will
find their way to water, if removed from their usual haunts. A case is
recorded[33] of a specimen of _H. arbustorum_, kept in a kitchen, which
used to find its way directly under the cold water tap, and appeared to
enjoy the luxury of a douche. How delicately the conditions of life are
balanced in some of these creatures is seen in the case of _Omalonyx_,
a genus akin to _Succinea_, which is found in Brazil and the northern
parts of South America. It lives creeping on plants which overhang the
margin of water, but perishes equally, if placed in the water itself,
or removed to a distance from it for any length of time.[34]


=Endurance of Heat and Cold.=--The Mollusca are capable, at least
as far as some species are concerned, of enduring severe extremes
both of cold and heat. The most northern pulmonate yet observed is
a fresh-water species, _Physa_ (_Aplecta_) _hypnorum_ L. This hardy
mollusc, whose shell is so fragile as to need most careful handling,
has been noticed on the peninsula of Taimyr, North Siberia, in 73° 30’
N. lat, a region whose mean annual temperature is below 10° F. with a
range of from 40° F. in July to -30° F. in January.

It is well known that the Limnaeidae, and probably most fresh-water
Mollusca of sub-temperate regions, can continue to live not merely
under, but enveloped in ice, and themselves frozen hard. Garnier
relates[35] that, during the winter of 1829–30, some large _Limnaea
auricularia_, which had been placed in a small basin, were frozen into
a solid mass, experiencing a cold of -2° F. He supposed they were dead,
but, to his surprise, when the basin thawed, the Limnaea gradually
revived. _Paludina vivipara_ and _Anodonta anatina_ have been known
to resist a temperature of 23° F., and the former has produced young
shortly after being thawed out of the ice.[36] As far north as Bodø in
Norway (67° 37’ N. lat., well within the Arctic circle) there are found
no less than fourteen species of terrestrial Mollusca, among them being
_Balea perversa_ and _Clausilia rugosa_.[37]

_Vitrina_ is one of our most hardy molluscs, and may be observed
crawling on bright mornings over the frost-covered leaves of a wood or
copse. _V. glacialis_ is said by Charpentier to live in the Alps at a
height where the stones are covered with snow from nine to ten months
of the year. Many of the _Hyaliniae_ are very hardy. _Arion_, in spite
of having no external shell to protect it, is apparently less affected
by the cold than _Helix_, and does not commence hibernation till a
later period in the autumn. The operculate land Mollusca, in spite of
the protection which their operculum may be supposed to afford, are
exceedingly sensitive to cold, and the whole group is without doubt a
product of tropical or semi-tropical regions (see map at frontispiece).
A species of _Helicina_ which inhabits the southern States of North
America has been known to be almost exterminated from certain districts
by the occurrence of an unusually severe winter.

One of the highest altitudes at which a land shell is known to live
appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous
altitude, two species of _Buliminus_ (_arcuatus_ Hutt. and _nivicola_
Bens.) live on juniper bushes among patches of snow. An _Anadenus_ is
said to have been found in a similar locality at 15,000 ft., while
_Limnaea Hookeri_ has been taken from over 16,400 ft. in Landour. In
the Andes of Peru and Bolivia, five species of _Bulimulus_, one of
_Pupa_, and one of _Limax_ occur at an elevation of 10,500 to 15,000
ft. Several fresh-water Mollusca inhabit Lake Titicaca, which stands at
a height of 12,550 ft. in the Bolivian table-land.

In certain parts of the desert of Algeria, where there is not a trace
of vegetation to be seen, and the temperature at mid-day is 110° F.,
the ground is sometimes so covered with _Helix lactea_ as to appear
perfectly white. Dr. F. H. H. Guillemard has told me that he noticed,
in somewhat similar surroundings between Fez and Tangier, _H. pisana_
in such extraordinary abundance that they hung from the low scrub in
bunches the size of a man’s two fists. It is singular that Mollusca
should live, and not only live, but flourish, in localities apparently
so unpromising. Shells which occur in the Algerian Sahara are actually
larger and altogether finer than the ordinary European form of the
same species. In order to protect themselves to some extent against
the scorching heat and consequent evaporation, desert species are
frequently modified in one of two ways; the shell becomes either white
or a light dusky brown, as in the familiar _Helix desertorum_, or else
it gains immensely in thickness. Specimens of _H. pomatia_, recently
procured from Fez, are of extraordinary thickness as compared with
forms from our own chalk downs of Kent and Surrey.

Fresh-water Mollusca are frequently found inhabiting hot springs. Thus
_Neritina fluviatilis_ lives at Bagnères de Bigorre in water at about
68° F. In another hot spring in the eastern Pyrenees a _Bithynia_ lives
at a temperature of over 73° F.; while Blainville mentions another case
of a _Bithynia_ living in water at 122° F.

=Hibernation and Aestivation.=--As autumn begins to draw on, and the
first frosts to nip vegetation, terrestrial species retire beneath
stones, into cracks in old walls, holes in tree trunks, deep fissures
in rocks, and nooks and crannies of every kind, or else bury themselves
deeply in the earth or in moss and heaps of leaves. They thus commence
their period of _hibernation_, which varies in length according to
the duration of winter. Frequently masses of Helices may be found
attached to one another, probably not so much for the sake of warmth,
for their temperature is but low, as to share the comforts of a cosy
retreat in common. Slugs generally hibernate alone, excavating a sort
of nest in the earth, in which they encyst themselves, contracting
their bodies until they are almost round, and secreting a covering of
their own slime. The Helices usually close up the mouth of their shell
by the formation of a membranous or chalky _epiphragm_, which will be
further described below. Both snails and slugs take care to be in good
condition at the time their winter sleep begins, and for this reason
the former are said to be most esteemed by foreign epicures if captured
just at this period.[38]

During hibernation, the action of the heart in land Pulmonata ceases
almost entirely. This appears to be directly due to the effect of cold.
Mr. C. Ashford has related[39] some interesting experiments made upon
_H. hortensis_ and _Hyal. cellaria_, with the view of ascertaining the
effect of cold upon their pulsations. His observations may be tabulated
as follows:--

       Number of pulsations per minute
    _Helix hortensis_   _Hyal. cellaria_   At degrees Fahr.
            22                 21                52°
            14                 12                44°
            10                 11                38°
             4                  9                30°

At low temperatures the character, as well as the number of the
pulsations changed; they became imperfect and intermittent, although
exceptionally at 31° F. a _H. rufescens_ gave five or six pulsations a
minute, very full and deliberate. The result of taking the _Hyalinia_
suddenly into the heat of a greenhouse was to bring on palpitations.
Further experiments resulted in evidence of a similar kind. _Hyal.
radiatula_, placed upon a deal table in a room, showed 52 pulsations
per minute at 62° F. Placed upon the palm of the hand, the action
soon rose to 108. _Hyal. alliaria_, similarly treated, rose from 72
pulsations to 110. Floated upon water, the action of the heart of the
latter suddenly fell to 29.

Fresh-water Pulmonata do not appear to hibernate. _Unio_ and
_Anodonta_, however, bury themselves more deeply in the mud, and
_Dreissensia_ casts off its byssus and retires under the mud in deeper
water.[40] _Limnaea_ and _Planorbis_ have often been noticed to crawl
about under the lower surface of a thick coating of ice. In periods of
prolonged drought, when the water in the ponds dries up, the majority
of genera bury themselves in the mud. I have known _Limnaea peregra_
bury itself three inches deep, when surprised by a sudden fall of the
water in the ditch on Coe Fen, behind Peterhouse, Cambridge. _Physa
hypnorum_ frequents by preference ditches which dry up in summer, as
does also _Planorbis spirorbis_, the latter often forming a sort of
epiphragm against evaporation. _Ancylus_ has been observed to spend the
whole winter out of water, and _P. spirorbis_ has been noticed alive
after four months’ desiccation.[41]

True aestivation, however, occurs mainly in the tropics, where there is
no winter, but only a period when it is not quite so hot as the rest
of the year, or on a coast like the Mediterranean, which is subject to
sudden and severe heat. This period is usually rainless, and the heat
is therefore a dry heat. At this season, which may last for three or
four months, most of the land Mollusca enter upon a period of inaction,
either burying themselves deeply in the ground, or else permanently
attaching themselves to the stalks of grass and other herbage, or the
under sides of rocks. For instance, the large and beautifully painted
_Orthalicus_, _Corona_, and _Porphyrobaphe_, which inhabit Brazil,
Ecuador, and eastern Peru, bury themselves deeply in the ground during
the dry season, while in the rains they climb to the topmost branches
of the great forest trees.[42] Thus it may well happen that a visitor
to a tropical island, Ceylon for instance, or one of the Greater
Antilles, if he times his visit to coincide with the rainless season,
may be grievously disappointed at what seems its unaccountable poverty
in land Mollusca. But as soon as the weather breaks, and the moisture
penetrates their retreats, every bush and every stone, in favoured
localities, will be alive with interesting species.

=The Epiphragm.=--A considerable number of the land Pulmonata (and
a very few of the fresh-water) possess the power of closing the
aperture of their shell by means of what is known as an _epiphragm_
or covering of hardened mucus. This epiphragm is habitually formed by
certain species during hibernation or aestivation, or even during
shorter periods of inactivity and retirement, the object being, either
to check evaporation of the moisture of the body, or to secure the
animal against the cold by retaining a thin layer of slightly warm air
immediately within the aperture of the shell.

The epiphragm differs widely in character in different species,
sometimes (_Clausilia_, _Pupa_, _Planorbis_) consisting of the merest
pellicle of transparent membrane, while at others (_Helix aperta_,
_H. pomatia_) it is a thick chalky substance, with a considerable
admixture of carbonate of lime, with the consistency of a hardened
layer of plaster of Paris. Within these extremes every variety of
thickness, solidity, and transparency occurs. During long hibernation
several epiphragms are not unfrequently formed by the same individual
snail, one within the other, at gradually lessening distances. The
epiphragm thus performs, to a certain extent, the part of an operculum,
but it must be remembered that it differs radically from an operculum
physiologically, in being only a temporary secretion, while the
operculum is actually a living part of the animal.

The actual mode of formation of the epiphragm would seem to differ in
different species. According to Fischer,[43] the mollusc withdraws into
its shell, completely blocking all passage of air into the interior,
and closing the pulmonary orifice. Then, from the middle part of the
foot, which is held exactly at the same plane as the aperture, is
slowly secreted a transparent pellicle, which gradually thickens,
and in certain species becomes calcareous. Dr. Binney, who kept a
large number of _Helix hortensis_ in confinement, had frequently
an opportunity of noticing the manner in which the epiphragm was
formed.[44] The aperture of the shell being upward, and the collar
of the animal having been brought to a level with it, a quantity of
gelatinous matter is thrown out [? where from]. The pulmonary orifice
is then opened, and a portion of the air within suddenly ejected, with
such force as to separate the viscid matter from the collar, and to
project it, like a bubble of air, from the aperture. The animal then
quickly withdraws farther into the shell, and the pressure of the
external air forces back the vesicle to a level with the aperture,
when it hardens and forms the epiphragm. In some of the European
species in which the gelatinous secretion contains more carbonate of
lime, solidification seems to take place at the moment when the air is
expelled, and the epiphragm in these is in consequence strongly convex.

=Thread-spinning.=--A considerable number of fresh-water Mollusca
possess the power of stretching a thread, which is no more than an
exceedingly elongated piece of mucus, to the surface of the water, and
of using it as a means of locomotion. This thread bears no analogy
whatever to the fibrous byssus of certain bivalves, being formed in an
entirely different manner, without the need of a special gland.

The threads are ‘spun’ by several species of _Limnaea_, _Physa_,
and _Planorbis_, by _Bithynia tentaculata_, and several of the
_Cycladidae_. They are anchored to the surface by a minute concavity at
the upper end, which appears to act like a small boat in keeping the
thread steady. The longest threads are those of the _Physae_, which
have been noticed to attain a length, in confinement, of 14 inches.
They are always spun in the ascent, and as a rule, when the animal
descends, it rolls the thread up and carries it down as it goes. A
single thread is never spun on the descent, but occasionally, when a
thread has become more or less of a permanence, it becomes stronger by
the addition of more mucus each time it is used, whether for ascending
or descending purposes. _Cyclas cornea_ appears to be an exception to
the rule that threads are only spun on the ascent. This species, which
is particularly fond of crawling along the under surface of the water,
has been noticed to spin a thread half an inch in length while on the
surface, and to hang suspended from it for a considerable time.

What the exact use of the thread may be, must to a certain extent
be matter of conjecture. The Limnaeidae are, in the great majority
of cases, compelled to make periodic visits to the surface in order
to inspire oxygen. It is also a favourite habit with them to float
just under the surface, or crawl about on its under side, perhaps in
pursuit of tiny vegetable organisms. Whatever may be the object of an
excursion to the surface, a taut thread will obviously be a nearer way
up than any other which is likely to present itself; indeed, without
this thread-spinning power, which insures a tolerably rapid arrival at
the surface, the animal might find itself asphyxiated, or at least
seriously inconvenienced, before it could succeed in taking in the
desired supply of oxygen. With the Cycladidae, which do not breathe
air, such an explanation is out of place; in their case the thread
seems to be a convenient means of resting in one position in the
intervals of the periods of active exercise to which several of the
species are so much addicted.

The power of suspension by a thread is also possessed by certain of
the _Cyclostomatidae_, by some _Cerithidea_, several _Rissoa_ and
other marine genera, prominent among which is _Litiopa bombyx_, whose
name expresses its power of anchoring itself to the Sargasso weed by a
silken thread of mucus. Several species of slugs are known to be able
to let themselves down by threads from the branches of trees. _Limax
arborum_ is especially noted for this property, and has been observed
suspended in pairs during the breeding time. According to Binney, all
the American species of _Limax_, besides those of _Tebennophorus_,
possess this singular property. _Limax arborum_ appears to be the only
slug which has been noticed to ascend, as well as descend, its thread.
It has also been observed[45] that when this species is gorged with
food, its slime is thin and watery, and unable to sustain its weight,
but that after the process of digestion has been performed, the mucus
again becomes thick and tenacious. It appears therefore that when
the animal is hungry and most in need of the power of making distant
excursions in search of food, its condition enables it to do so, but
that when no such necessity is pressing, the thread-forming mucus is
not secreted, or is perhaps held in suspense while the glands assist in
lubricating the food before digestion.[46]

=Food of Land and Fresh-water Mollusca.=--_Arion ater_, the great black
slug, although normally frugivorous, is unquestionably carnivorous
as well, feeding on all sorts of animal matter, whether decaying,
freshly killed, or even in a living state. It is frequently noticed
feeding on earthworms; kept in captivity, it will eat raw beef; it does
not disdain the carcases of its own dead brethren. An old man near
Berwick-on-Tweed, going out one morning to mow grass, found a black
slug devouring, as he supposed, a dead mouse. Being of an inquisitive
turn, and wishing to ascertain if it were really thus engaged, he drew
the mouse a little back. When he returned in the evening, the mouse was
reduced almost to a skeleton, and the slug was still there.[47] Indeed
it would seem almost difficult to name anything which _Arion ater_ will
not eat. Dr. Gray mentions[48] a case of a specimen which devoured
sand recently taken from the beach, which contained just enough animal
matter to render it luminous when trodden on in the dark; after a
little time the faeces of the slug were composed of pure sand, united
together by a little mucus. A specimen kept two days in captivity was
turned out on a newspaper, and commenced at once to devour it. The same
specimen ate dead bodies of five other species of slugs, a dead _Unio_,
pupae of _Adimonia tanaceti_, part of the abdomen of a dragon-fly, and
Pears’ soap, the latter reluctantly.[49]

According to Simroth[50] and Scharff[51] the food of several of our
British slugs, _e.g._ _Limax maximus_, _L. flavus_, _Arion subfuscus_,
_A. intermedius_, consists of non-chlorophyllaceous substances only,
while anything containing chlorophyll is as a rule refused. On the
other hand _L. agrestis_ and _Amalia carinata_ feed almost entirely on
green food, and are most destructive in gardens. The latter species
lives several inches under ground during the day, and comes to the
surface only at night. It is largely responsible for the disappearance
of bulbs, to which it is extremely partial. _L. marginatus_ (=
_arborum_ Bouch.) feeds exclusively on lichens, and in captivity
absolutely refuses green leaves and a flesh diet. It follows therefore,
if these observations are correct, that the popular notions about slugs
must be revised, and that while we continue to exterminate from our
gardens those species which have a taste for chlorophyll, we ought
to spare, if not encourage those whose tastes lie in the opposite
direction.

_Limax agrestis_ has been seen devouring the crushed remains of _Arion
ater_. Five specimens of the same species were once noticed busily
devouring a May-fly each, and this in the middle of a large meadow,
where it may be presumed there was no lack of green food. The capture
and eating of insects by Mollusca seems very remarkable, but this
story does not stand alone. Mr. T. Vernon Wollaston once enclosed in
a bottle at least three dozen specimens of Coleoptera together with
4 _Helix cantiana_, 5 _H. hispida_, and 1 _H. virgata_, together
with an abundant supply of fresh leaves and grass. About a fortnight
afterwards, on the bottle being opened, it was found that every single
specimen of the Coleoptera had been devoured by the snails.[52] _Amalia
marginata_ in captivity has been fed upon the larvae of _Euchelia
jacobaeae_, eating three in two hours.[53]

_Limax maximus_ (Fig. 19) has been seen frequently to make its way into
a dairy and feed on raw beef.[54] Individuals kept in confinement are
guilty of cannibalism. Mr. W. A. Gain kept three specimens in a box
together, and found one of them two-thirds eaten, “the tail left clean
cut off, reminding one of that portion of a fish on a fishmonger’s
stall.” That starvation did not prompt the crime was proved by the
fact that during the preceding night the slug had been supplied with,
and had eaten, a considerable quantity of its favourite food. On two
other occasions the same observer found one of his slugs deprived of
its slime and a portion of its skin, and in a dying condition.[55] An
adult _L. maximus_, kept for thirty-three days in captivity with a
young _Arion ater_, attacked it frequently, denuded it of its slime,
and gnawed numerous small pieces of skin off the body and mantle.[56]
The present writer has found no better bait for this species on a warm
summer night than the bodies of its brethren which were slain on the
night preceding; it will also devour dead _Helix aspersa_. Mr. Gain
considers it a very dainty feeder, preferring fungi to all other foods,
and apparently doing no harm in the garden.

  [Illustration: FIG. 19.--_Limax maximus_ L. =PO=, pulmonary
  orifice: × ⅔.]

_Limax flavus_, which is fond of inhabiting the vicinity of cellars,
makes its presence most disagreeable by attacking articles of food,
and especially by insinuating itself into vessels containing meal and
flour.[57] It is particularly partial to cream.

Slugs will sometimes bite their captor’s hands. Mr. Kew relates
that a _Limax agrestis_, on being stopped with the finger, while
endeavouring to escape from the attack of a large _Arion_, attempted
to bite fiercely, the rasping action of its radula being plainly felt.
According to the same authority, probably all the slugs will rasp the
skin of the finger, if it is held out to them, and continue to do so
for a considerable time, without however actually drawing blood.[58]
While Mr. Gain was handling a large _Arion ater_, it at once seized one
of the folds of skin between the fingers of the hand on which it was
placed; after the action of the radula had been allowed to continue for
about a minute, the skin was seen to be abraded.[59] Another specimen
of _Arion ater_, carried in the hand for a long time enclosed in a
dock leaf, began to rasp the skin. The operation was permitted until
it became too painful to bear. Examination with a lens showed the skin
almost rasped away, and the place remained tender and sore, like a
slight burn, for several days.[60]

_Helix pisana_, if freshly caught, and placed in a box with other
species, will set to work and devour them within twenty-four hours.
The present writer has noticed it, in this position, attack and kill
large specimens of _H. ericetorum_, cleaning them completely out, and
inserting its elongated body into the top whorls of its unfortunate
victims in a most remarkable manner. Amongst a large number of species
bred in captivity by Miss F. M. Hele,[61] was _Hyalinia Draparnaldi_.
In the first summer the young offspring were fed on cabbage, coltsfoot,
and broadleafed docks. They would not hibernate even in the severest
frosts, and, no outdoor food being available, were fed on chopped beef.
This, Miss Hele thinks, must have degenerated their appetites, for in
the following spring and summer they constantly devoured each other.

_Zonites algirus_ feeds on decayed fruit and vegetables, and on
stinking flesh.[62] _Achatina panthera_ has been known to eat meat,
other snails (when dead), vegetables, and paper.[63] The common
_Stenogyra decollata_ of the South of Europe has a very bad character
for flesh-eating habits, when kept in captivity. Mr. Binney[64] kept
a number for a long time as scavengers, to clean the shells of other
snails. As soon as a living _Helix_ was placed in a box with them, one
would attack it, introduce itself into the upper whorls, and completely
remove the animal. One day a number of _Succinea ovalis_ were left
with them for a short time, and disappeared entirely! The _Stenogyra_
had eaten shell as well as animal. This view of _Stenogyra_ is quite
confirmed by Miss Hele, who has bred them in thousands. “I can keep,”
she writes,[65] “no small _Helix_ or _Bulimus_ with them, for they at
once kill and eat them. They will also eat raw meat.”

Even the common _Limnaea stagnalis_, which is usually regarded as
strictly herbivorous, will sometimes betake itself, apparently by
preference, to a diet of flesh. Karl Semper frequently observed the
Limnaeae in his aquarium suddenly attack healthy living specimens of
the common large water newt (_Triton taeniatus_), overcome them, and
devour them, although there was plenty of their favourite vegetable
food growing within easy reach.[66] The same species has also been
noticed to devour its own ova, and the larvae of _Dytiscus_. _Limnaea
peregra_ has been detected capturing and partially devouring minnows
in an aquarium, when deprived of other food, and Dr. Jeffreys has seen
the same species attack its own relatives under similar circumstances,
piercing the spire at its thinnest point near to the apex.[67] _L.
stagnalis_, kept in an aquarium, has succeeded in overpowering and
partially devouring healthy specimens of the common stickleback.[68]


=Powers of Intelligence, Homing, and finding Food.=--It is not easy to
discover whether land Mollusca possess any faculties which correspond
to what we call intelligence, as distinct from their capacities for
smell, sight, taste, and hearing. Darwin mentions[69] a remarkable
case, communicated to him by Mr. Lonsdale. A couple of _Helix pomatia_,
one of which was sickly, were placed in a small and ill-provided
garden. The stronger of the two soon disappeared over the wall into
the next garden, which was well furnished with food. It was concluded
that the snail had deserted its weakly mate, but after twenty-four
hours it returned, and apparently communicated the results of its
expedition, for after a short time both started off along the same
track, and disappeared over the wall. According to Dr. W. H. Dall,[70]
a young girl who possessed a remarkable power over animals succeeded
in training a snail (_H. albolabris_) to come out of its lurking-place
at her call. If placed in a room, it would shrink into its shell at
the sound of any other voice, but it would always start off in the
direction of hers.

Snails and slugs possess to a considerable extent the faculty of
‘homing,’ or returning to the same hiding-place day after day, after
their night excursions in search of food. Mr. C. Ashford once marked
with a dab of white paint seven _Helix aspersa_ found lurking under a
broken flagstone; at 10 P.M. the same evening three had disappeared
on the forage; the next morning all were ‘at home.’ The following
night at 10 P.M. five were gone out, two being discovered with some
difficulty ‘in a small jungle’ six feet away; the next morning six
out of the seven were safely beneath the flagstone. According to the
same authority, _Helix aspersa_ will find its way across a cinder-path
(which it specially detests) to get to its favourite food, and will
return by the same way to its old quarters, although it could easily
have found new lodgings nearer the food-supply. A snail has been
observed to occupy a hole in the brick wall of a kitchen-garden about
four feet from the ground. Leaning against the wall, and immediately
under the hole, was a piece of wood, the lower end of which rested in
a bed of herbs. For months the snail employed this ladder between its
food and its home, coming down as soon as it was dark, and retiring to
rest during the day.

In greenhouses a slug will forage night after night--as gardeners know
to their cost--over the same beat, and will always return to the same
hiding-place. _Limax flavus_ has been noticed crawling with great
regularity to a sink from a hole near the water-pipe, and keeping to
a well-marked circular track. In all probability the scent, either of
the desired object of food, or of the creature’s own trail, plays
a considerable part in keeping it to the same outward and homeward
track, or at least in guiding it back to its hiding-place. Yet even
scent is occasionally at fault, for on one occasion a _Limax flavus_
was accustomed to make nightly excursions to some basins of cream,
which were kept in a cool cellar. When the basins were removed to a
distant shelf, the creature was found the next morning ‘wandering
disconsolately’ about in the place where the basins had formerly
stood.[71]

A remarkable case of the power of smell, combined with great
perseverance on the part of a _Helix_, is recorded by Furtado.[72] He
noticed a _Helix aspersa_ lodged between a column on a verandah and a
flower-pot containing a young banana plant, and threw it away into a
little court below, and six or seven yards distant. Next morning the
snail was in precisely the same place on the flower-pot. Again he threw
it away, to the same distance, and determined to notice what happened.
Next morning at nine o’clock, the snail was resting on the rail of a
staircase leading up to the verandah from the court; in the evening
it started again, quickening its space as it advanced, eventually
attacking the banana in precisely the same place where it had been
gnawed before.

For further instances of the power of smell in snails, see chap. vii.

Slugs have been known to make their way into bee-hives, presumably
for the sake of the honey.[73] ‘Sugaring’ the trees at night for
moths will often attract a surprising concourse of slugs. Sometimes a
particular plant in a greenhouse will become the object of the slugs’
persistent attacks, and they will neglect every other food in order
to obtain it. _Farfugium grande_ is one of these favourite foods,
“the young leaves and shoots being always eaten in preference to all
other plants growing in the houses; where no _Farfugiums_ were kept
the slugs nibbled indiscriminately at many kinds.”[74] The flowers
of orchidaceous plants exercise a special attraction over slugs,
which appear to have some means of discovering when the plants are in
bloom. “I have often observed,” says Mr. T. Baines, “that a slug will
travel over the surface of a pot in which is growing a _Dendrobium
nobile_, a _Cattleya_, _Vanda_, or similar upright plant for a score
of times without ever attempting to ascend into the head of the plant
unless it is in bloom, in which case they are certain to find their
way straight to the flowers; after which they will descend, and
return to some favourite hiding-place, often at the opposite end of
the house.”[75] Mr. R. Warner has “actually seen many little slugs
suspending themselves by slime-threads from the rafters and descending
on the spikes of the beautiful _Odontoglossum alexandrae_; and thus
many spikes, thickly wadded round with cotton wool (which the slugs
could not travel over), and growing in pots surrounded by water, had
been lost.”[76] Perhaps the most singular instance of a liking for
a particular food is that related by Mr. E. Step.[77] In a London
publishing house, slugs were observed, during a period of nearly twelve
months, to have fed almost nightly on the colouring matter in certain
bookcovers, and though the trails were often seen over the shelves,
and cabbage and lettuce leaves laid down to tempt the creatures, they
continued their depredations with impunity for the time above mentioned.

_Limnaea peregra_ has been observed feeding on old fish-heads thrown
into a dirty stream, and a large gathering of _Limnaea stagnalis_ has
been noticed feeding upon an old newspaper in a pond on Chislehurst
Common, ‘so that for the space of about a square foot nothing else
could be seen.’[78]

=Tenacity of Life.=--Land Mollusca have been known to exhibit, under
unusual conditions, remarkable tenacity of life. Some of the most
noteworthy and best authenticated instances of this faculty may be here
mentioned.

The well-known story of the British Museum snail is thus related by Mr.
Baird.[79] On the 25th March 1846 two specimens of _Helix desertorum_,
collected by Charles Lamb, Esq., in Egypt some time previously, were
fixed upon tablets and placed in the collection among the other
Mollusca of the Museum. There they remained fast gummed to the tablet.
About the 15th March 1850, having occasion to examine some shells in
the same case, Mr. Baird noticed a recently formed epiphragm over the
mouth of one of these snails. On removing the snails from the tablet
and placing them in tepid water, one of them came out of its shell, and
the next day ate some cabbage leaf. A month or two afterwards it began
repairing the lip of its shell, which was broken when it was first
affixed to the tablet.

While resident in Porto Santo, from 27th April to 4th May 1848, Mr.
S. P. Woodward[80] collected a number of Helices and sorted them out
into separate pill-boxes. On returning home, these boxes were placed in
empty drawers in an insect cabinet, and on 19th October 1850, nearly
two and a half years afterwards, many of them were found to be still
alive. A whole bagful of _H. turricula_, collected on the Ilheo de Cima
on 24th April 1849, were all alive at the above-mentioned date.

In September 1858 Mr. Bryce Wright sent[81] to the British Museum two
specimens of _H. desertorum_ which had been dormant for four years.
They were originally collected in Egypt by a Mr. Vernèdi, who, in May
1854, while stopping at one of the stations in the desert, found a
heap of thorn-bushes lying in a corner of the building, rather thickly
studded with the snails. He picked off fifteen or twenty specimens,
which he carried home and locked up in a drawer, where they remained
undisturbed until he gave two to Mr. Wright in September 1858.

In June 1855 Dr. Woodward placed specimens of _H. candidissima_ and _H.
aperta_ in a glass box, to test their tenacity of life; he writes of
their being still alive in April 1859.

Mr. R. E. C. Stearns records[82] a case of _Buliminus pallidior_ and
_H. Veatchii_ from Cerros I. living without food from 1859 to March
1865.

H. Aucapitaine mentions[83] a case of _H. lactea_ found in calcinated
ground in a part of the Sahara heated to 122° F., where no rain was
said to have fallen for five years. The specimen revived after being
enclosed in a bottle for three and a half years.

In August 1863, Mr. W. J. Sterland[84] put specimens of _H. nemoralis_
in a box and afterwards placed the box in his cabinet; in November 1866
one specimen was discovered to be alive.

Gaskoin relates[85] a case in which specimens of _H. lactea_ were
purchased from a dealer in whose drawer they had been for two years.
This dealer had them from a merchant at Mogador, who had kept them for
more than that time under similar conditions. One of these shells on
being immersed in water revived, and in April 1849 was placed quite
alone under a bell jar with earth and food. In the end of the following
October about thirty young _H. lactea_ were found crawling on the glass.

Mr. R. D. Darbishire bought[86] some _H. aperta_ in the market at Nice
on 18th February 1885. Two specimens of these, placed with wool in a
paper box, were alive in December 1888. This is a very remarkable case,
_H. aperta_ not being, like _H. desertorum_, _H. lactea_, _H. Veatchii_
and _Bul. pallidior_, a desert snail, and therefore not accustomed to
fasting at all.

=Age of Snails.=--It would appear, from the existing evidence, which
is not too plentiful, that five years is about the average age of
the common garden snail. Mr. Gain has published[87] some interesting
observations on the life of a specimen from the cradle to the grave,
which may be exhibited in a tabular form.

    Aug. 1882.        Eggs hatched; one attained diameter of ⅜ in.
                        before winter; fed on coltsfoot and cabbage.

    5th Oct. 1883.    Shell 1 in. in diameter, no lip formed.

    July 1884.        Shell finished; diameter 1⅛ in., including
                        perfect lip.

    3rd May 1885.     Left winter quarters; companion introduced,
                        with which it was seen in company on 5th
                        August.

    9th Aug.  „       Laid eggs in soil, which were hatched on
                        10th September, and feeding on 17th September;
                        in May 1886 the largest of these
                        was 11/16 in. diameter.

    13th Oct. 1887.   Old snail died, aged 5 years 2 months.

According to Clessin, the duration of life in _Vitrina_ is one year,
_Cyclas_ 2 years; _Hyalinia_, _Succinea_, _Limnaea_, _Planorbis_, and
_Ancylus_ are full grown in 2 to 3 years, _Helix_ and _Paludina_ in 2
to 4, and _Anodonta_ in 12 to 14. Hazay finds[88] that the duration of
life in _Hyalinia_ is 2 years, in _Helix pomatia_ 6 to 8, in _Helix
candicans_ 2 to 3, in _Paludina_ 8 to 10, in _Limnaea_ and _Planorbis_
3 to 4.

=Growth of the Shell.=--Mr. E. J. Lowe, many years ago, conducted[89]
some interesting experiments on the growth of snails. The facts arrived
at were--

(1) The shells of Helicidae increase but little for a considerable
period, never arriving at maturity before the animal has _once_
become dormant.

(2) Shells do not grow whilst the animal itself remains dormant.

(3) The growth of shells is very rapid when it does take place.

(4) Most species bury themselves in the ground to increase the
dimensions of their shells.

Six recently hatched _H. pomatia_ were placed in a box and regularly
fed on lettuce and cabbage leaves from August until December, when
they buried themselves in the soil for winter; at this period they had
gradually increased in dimensions to the size of _H. hispida_. On the
1st April following, the box was placed in the garden, and on the 3rd
the Helices reappeared on the surface, being _no larger_ in size than
they were in December. Although regularly fed up to 20th June, they
were not perceptibly larger, but on that day five of them disappeared,
having buried themselves, with the mouth of the shell _downwards_, in
the soil. After ten days they reappeared, having in that short time
grown so rapidly as to be equal in size to _H. pisana_. On the 15th
July they again buried themselves, and reappeared on 1st August, having
again increased in size. For three months from this date they did not
become perceptibly larger; on 2nd November food was withheld for the
winter and they became dormant.

A similar experiment, with similar results, was carried on with a
number of _H. aspersa_, hatched on 20th June. During the summer they
grew but little, buried themselves on 10th October with the _head
upwards_, and rose to the surface again on 5th April, not having
grown during the winter. In May they buried themselves with the _head
downwards_, and appeared again in a week double the size; this went on
at about fortnightly intervals until 18th July, when they were almost
fully grown.

_Helix nemoralis_, _H. virgata_, _H. caperata_, and _H. hispida_ bury
themselves to grow; _H. rotundata_ burrows into decayed wood; _Hyalinia
radiatula_ appears to remain on decaying blades of grass; _Pupa
umbilicata_, _Clausilia rugosa_, and _Buliminus obscurus_ bury their
heads only.

The observations of Mr. W. E. Collinge[90] do not at all agree with
those of Mr. Lowe, with regard to the mode in which land Mollusca
enlarge their shells. He bred and reared most of the commoner forms
of _Helix_ and also _Clausilia rugosa_, but never saw them bury any
part of their shell when enlarging it. While admitting that they may
increase their shells when in holes or burrows of earthworms, he
thinks that the process of burying would seriously interfere with the
action of the mantle during deposition, and in many cases damage the
membranaceous film before the calcareous portion was deposited. Mr.
Collinge has found the following species under the surface in winter:
_Arion ater_ (3–4 in.), _Agriolimax agrestis_, (6–8 in.), _Hyalinia
cellaria_ and _H. alliaria_ (6–8 in.), _Hyalinia glabra_ (5 in.),
_Helix aspersa_ (5–6 in.), _H. rufescens_ (4–6 in.), _H. rotundata_
(4–5 in.), _H. hispida_ (7 in.), _Buliminus obscurus_ (4–6 in.),
_B. montanus_[91] (24 in.), and the following in summer, _Hyalinia
cellaria_ and _alliaria_ (6–8 in.), _Helix rotundata_ (4–5 in.), _Balea
perversa_ (6–8 in.), _Cyclostoma elegans_ (3–4 in.). The same author
has found the following species of fresh-water Mollusca living in hard
dry mud: _Sphaerium corneum_ (3–14 in.), _S. rivicola_ (5–6 in.), _S.
lacustre_ (10–14 in.), all the British species of _Pisidium_ (4–12
in.), _Limnaea truncatula_ (18 in., a single specimen). All our species
of _Unio_, _Anodonta_, _Bithynia_, and _Paludina_ bury themselves
habitually in fine or thick wet mud, to a depth of from 4 to 14 inches.

This burying propensity on the part of Mollusca has been known to
play its part in detecting fraud. When my friend Mr. E. L. Layard was
administering justice in Ceylon, a native landowner on a small scale
complained to him of the conduct of his neighbour, who had, during his
absence from home, diverted a small watercourse, which ran between
their holdings, in such a way as to filch a certain portion of the
land. The offender had filled up and obliterated the ancient course
of the stream, and protested that it had never run but in its present
bed. Mr. Layard promptly had a trench sunk across what was said to
be the old course, and the discovery of numerous _living Ampullaria_,
buried in the mud, confirmed the story of one of the litigants and
confounded the other.[92]


=Depositing and Hatching of Eggs: Self-fertilisation.=--There appears
to be no doubt that Helices, when once impregnated, can lay successive
batches of eggs, and possibly can continue laying for several years,
without a further act of union. A specimen of _Helix aspersa_ was
noticed in company with another on 5th August; on 9th August it laid
eggs in the soil, and early in the following summer it laid a second
batch of eggs, although its companion had been removed directly after
its first introduction. An _Arion_ received from a distance laid 30
eggs on 5th September, and 70 more on the 23rd of the same month,
although quite isolated during the whole time.[93] By far the most
remarkable case of the kind is related by Gaskoin.[94] A specimen of
_Helix lactea_ was kept in a drawer for about two years, and then in
another drawer for about two years more. It was then taken out, and
placed in water, when it revived, and was placed alone under a bell jar
with earth and food. Six months after, about 30 young _H. lactea_ were
found crawling on the glass, the act of oviposition not having been
observed.

The observations of Mr. F. W. Wotton,[95] with regard to the
fertilisation and egg-laying of _Arion ater_, are of extreme interest
and value. A pair of this species, kept in captivity, united on 10th
September 1889, the act lasting about 25 minutes. From that date until
the eggs were laid, the animals looked sickly, dull of colour, with a
somewhat dry skin. Eggs were deposited in batches, one, which we will
call _A_, beginning three days before _B_. On 10th October _A_ laid 80
eggs; on the 16th, 110; on the 25th, 77; on 8th November, 82; and on
17th November, 47; making a total of 396. Specimen _B_, which began on
13th October, three days after _A_, made up for the delay by laying 246
eggs in 40 hours; on 26th October it laid 9, on 10th November, 121; and
on 30th November, 101; a total of 477. These eggs weighed 624 to the
ounce, and, in excluding the batch of 246, _B_ parted with ⅜ of its own
weight in 40 hours, while the whole number laid were rather over ¾ of
its own weight!

While depositing the eggs, the slug remained throughout in the
same position on the surface of the ground, with the head drawn up
underneath the mantle, which was lifted just above the reproductive
orifice. When taken into the hand, it went on laying eggs without
interruption or agitation of any kind. After it had finished laying
it ate half a raw potato and then took a bath, remaining submerged
for more than an hour. Bathing is a favourite pastime at all periods.
Specimens, says Mr. Wotton, have survived a compulsory bath, with total
submersion, of nearly three days’ duration.

Mr. Wotton’s account of the hatching of the eggs is equally
interesting. It is noticeable that the eggs of one batch do not hatch
by any means simultaneously; several days frequently intervene. The
average period is about 60 days, a damp and warm situation bringing out
the young in 40 days, while cold and dryness extended the time to 74
days, extremes of any kind proving fatal. Of the batch of eggs laid by
_B_ on 30th November, the first 2 were hatched on the following 16th
January, and 2 more on the 17th; others, from 10 to 20, followed suit
on the succeeding 5 days, until 82 in all were hatched, the remaining
19 being unproductive.[96]

By placing the egg on a looking-glass the act of exclusion can be
perfectly observed. For several days the inmate can be seen in motion,
until at last a small crack appears in the surface of the shell: this
gradually enlarges, until the baby slug is able to crawl out, although
it not unfrequently backs into the shell again, as if unwilling to risk
itself in the world. When it once begins to crawl freely, it buries
itself in the ground for 4 or 5 days without food, after which time it
emerges, nearly double its original size. At exclusion, the average
length is 9 mm., increasing to 56 mm. after the end of 5 months. Full
growth is attained about the middle of the second year, and nearly all
die at the end of this year or the beginning of the next. Death from
exhaustion frequently occurs after parturition. Death from suffocation
is sometimes the result of the formation of small blisters on the
margin of the respiratory aperture. The attacks of an internal parasite
cause death in a singular way. The upper tentacles swell at the base in
such a way as to prevent their extrusion; digestive troubles follow,
with rigidity and loss of moisture, and death ensues in 2 or 3 days.

Mr. Wotton isolated newly-hatched specimens, with the view of
experimenting on their power of self-fertilisation, if the opportunity
of fertilising and being fertilised by others was denied them. One
of these, after remaining in absolute solitude for 10½ months, began
to lay, scantily at first (11th January, 2; 25th January, 2; 11th
February, 2), but more abundantly afterwards (3rd April, 60; 15th and
16th, 70; 29th, 53, etc.), the eggs being hatched out in 42–48 days.
The precautions taken seem to have been absolutely satisfactory, and
the fact of the power of self-fertilisation appears established as far
as _Arion ater_ is concerned.

Braun took young individuals of _Limnaea auricularia_ on the day they
were hatched out, and placed them singly in separate vessels with
differing amounts of water. This was on 15th June 1887. In August 1888
specimen _A_ had only produced a little spawn, out of which three
young were hatched; specimen _B_ had produced four pieces of spawn
of different sizes, all of which were hatched; specimen _C_, which
happened to be living with three _Planorbis_, produced five pieces
of spawn distinctly Limnaeidan, but nothing is recorded of their
hatching. Self-impregnation, therefore, with a fruitful result, appears
established for this species of _Limnaea_.[97]


=Reproduction of Lost Parts.=--When deprived of their tentacles, eyes,
or portions of the foot, Mollusca do not seem to suffer severely, and
generally reproduce the lost parts in a short time. If, however, one of
the ganglia is injured, they perish. Certain of the Mollusca possess
the curious property of being able to amputate certain parts at will.
When _Prophysaon_, a species of Californian slug, is annoyed by being
handled, an indented line appears at a point about two-thirds of the
length from the head, the line deepens, and eventually the tail is
shaken completely off. Sometimes the _Prophysaon_ only threatens this
spontaneous dismemberment; this line appears (always exactly in the
same place), but it thinks better of it, and the indentation proceeds
no further.[98] According to Gundlach,[99] _Helix imperator_ and _H.
crenilabris_, two large species from Cuba, possess the same property,
which is said to be also characteristic of the sub-genus _Stenopus_
(W. Indies). Amongst marine species, _Harpa ventricosa_ and _Solen
siliqua_ have been observed to act in a similar way, _Harpa_ apparently
cutting off the end of the foot by pressure of the shell. Karl Semper,
in commenting on the same property in species of _Helicarion_ from the
Philippines (which whisk their tail up and down with almost convulsive
rapidity, until it drops off), considers[100] it greatly to the
advantage of the mollusc, since any predacious bird which attempted
to seize it, but only secured a fragment of tail, would probably be
discouraged from a second attack, especially as the _Helicarion_ would
meanwhile have had time to conceal itself among the foliage.


=Strength and Muscular Force.=--The muscular strength of snails is
surprisingly great. Sandford relates[101] an experiment on a _Helix
aspersa_, weighing ¼ oz. He found it could drag vertically a weight of
2¼ oz., or nine times its own weight. Another snail, weighing ⅓ oz.,
was able to drag in a horizontal direction along a smooth table twelve
reels of cotton, a pair of scissors, a screwdriver, a key, and a knife,
weighing in all no less than 17 oz., or more than fifty times its own
weight. This latter experiment was much the same as asking a man of 12
stone to pull a load of over 3¾ tons.

If a snail be placed on a piece of glass and made to crawl, it will
be seen that a series of waves appear to pursue one another along
the under surface of the foot, travelling from back to front in the
direction in which the animal is moving. Simroth has shown that the
sole of the foot is covered with a dense network of muscular fibres,
those which run longitudinally being chiefly instrumental in producing
the undulatory motion. By means of these muscles the sole is first
elongated in front, and then shortened behind to an equal extent. Thus
a snail _slides_, not on the ground, but on its own mucus, which it
deposits mechanically, and which serves the purpose of lubricating the
ground on which it travels. It has been calculated that an averaged
sized snail of moderate pace progresses at the rate of about a mile in
16 days 14 hours.[102]


=Sudden Appearance of Mollusca.=--It is very remarkable to notice how
suddenly Pulmonata seem to appear in certain districts where they have
not been noticed before. This sudden appearance is more common in the
case of fresh-water than of land Mollusca, and there can be little
doubt that, wherever a new pond happens to be formed, unless there is
something in its situation or nature which is absolutely hostile to
molluscan life, Mollusca are certain to be found in it sooner or later.
“Some 23 years ago,” writes Mr. W. Nelson,[103] “I was in the habit
of collecting shells in a small pond near to the Black Hills, Leeds.
At that time the only molluscan forms found there were a dwarf form
of _Sphaerium lacustre_, _Pisidium pusillum_, _Planorbis nautileus_,
and _Limnaea peregra_. About 10 years ago I resumed my visits to the
locality, and found, in addition to the species already enumerated,
_Planorbis corneus_. These were the only species found there until this
spring [1883], when, during one of my frequent visits, I was surprised
to find _Physa fontinalis_ and _Planorbis vortex_ were added to the
growing list of species. Later on _Pl. carinatus_, _Limnaea stagnalis_,
and _Ancylus lacustris_ turned up; and during June, _Pl. contortus_
was found in this small but prolific pond.” _Limnaea glutinosa_ is
prominent for these remarkable appearances and disappearances. In 1822
this species suddenly appeared in some small gravel pits at Bottisham,
Cambs., in such numbers that they might have been scooped out by
handfuls. After that year they did not appear numerous, and after three
or four seasons they gradually disappeared.[104] _Physa_ (_Aplecta_)
_hypnorum_ is noted in a similar way. In February 1852, for instance,
after a wet month, the water stood in small puddles about 3 feet by 2
in a particular part of Bottisham Park which was sometimes a little
swampy, though usually quite dry. One of these puddles was found to
contain immense numbers of the _Aplecta_, which up to that time had not
been noted as occurring in Cambridgeshire at all.[105] In a few days
the species entirely disappeared and was never again noticed in the
locality.[106]

Writing to the Zoological Society of London from New Caledonia, Mr. E.
L. Layard remarks:[107] “The West Indian species _Stenogyra octona_
has suddenly turned up here in thousands; how introduced, none can
tell. They are on a coffee estate at Kanala on the east coast. I have
made inquiries, and cannot find that the planter ever had seed coffee
from the West Indies. All he planted came from Bombay, and it would be
interesting to find out whether the species has appeared there also.”

Sometimes a very small event is sufficient to disturb the natural
equilibrium of a locality, and to become the cause either of the
introduction or of the destruction of a species. In 1883 a colony of
_Helix sericea_ occupied a portion of a hedge bottom twenty yards long
near Newark. It scarcely occurred outside this limit, but within it was
very plentiful, living in company with _H. nemoralis_, _H. hortensis_,
_H. hispida_, _H. rotundata_, _Hyalinia cellaria_ and _Hy. nitidula_,
and _Cochlicopa lubrica_. In 1888 the hedge was well trimmed, but the
bottom was not touched, and the next year a long and careful search was
required to find even six specimens of the _sericea_.[108]


=Showers of Shells.=--_Helix virgata_, _H. caperata_, and _Cochlicella
acuta_ sometimes occur on downs near our sea-coasts in such
extraordinary profusion, that their sudden appearance out of their
hiding-places at the roots of the herbage after a shower of rain has
led to the belief, amongst credulous people, that they have actually
descended with the rain. There seems, however, no reason to doubt that
Mollusca may be caught up by whirlwinds into the air and subsequently
deposited at some considerable distance from their original habitat,
in the same way as frogs and fishes. A very recent instance of such a
phenomenon occurred[109] at Paderborn, in Westphalia, where, on 9th
August 1892, a yellowish cloud suddenly attracted attention from its
colour and the rapidity of its motion. In a few moments it burst,
with thunder and a torrential rain, and immediately afterwards the
pavements were found to be covered with numbers of _Anodonta anatina_,
all of which had the shell broken by the violence of the fall. It was
clearly established that the shells could not have been washed into
the streets from any adjacent river or pond, and their true origin
was probably indicated when it was found that the funnel-shaped cloud
which burst over the town had passed across the one piece of water near
Paderborn, which was known to contain the _Anodonta_ in abundance.


=Cases of Singular Habitat.=--Mollusca sometimes accustom themselves
to living in very strange localities, besides the extremes of heat and
cold mentioned above (pp. 23–24). In the year 1852, when some large
waterpipes in the City Road, near St. Luke’s Hospital, were being taken
up for repairs, they were found to be inhabited in considerable numbers
by _Neritina fluviatilis_ and a species of _Limnaea_.[110] _Dreissensia
polymorpha_ has been found in a similar situation in Oxford Street,
and also in Hamburg, and has even been known to block the pipes and
cisterns of private houses. In an engine cistern at Burnley, 60 feet
above the canal from which the water was pumped into the cistern,
were found the following species: _Sphaerium corneum_, _S. lacustre_;
_Valvata piscinalis_, _Bithynia tentaculata_; _Limnaea peregra_, very
like _Succinea_ in form and texture; _Planorbis albus_, _P. corneus_,
_P. nitidus_, _P. glaber_, and thousands of _P. dilatatus_, much larger
than the forms in the canal below, a fact probably due to the equable
temperature of the water in the cistern all the year round.[111] In
certain parts of southern Algeria the fresh-water genera _Melania_ and
_Melanopsis_ inhabit abundantly waters so surcharged with salt that
the marine _Cardium edule_ has actually become extinct from excess
of brine. The common _Mytilus edulis_ is sometimes found within the
branchial chamber and attached to the abdomen of crabs (_Carcinus
maenas_), which are obliged to carry about a burden of which they
are powerless to rid themselves (see p. 78). A variety of the common
_Limnaea peregra_ lives in the hot water of some of the geysers of
Iceland, and has accordingly been named _geisericola_.


=Underground Snails.=--Not only do many of the land Mollusca aestivate,
or hibernate, as the case may be, beneath the surface of the soil, but
a certain number of species live permanently underground, like the
mole, and scarcely ever appear in the light of day. Our own little
_Caecilianella acicula_ lives habitually from 1 to 3 feet below
ground, appearing to prefer the vicinity of graveyards. _Testacella_,
the carnivorous slug, scarcely ever appears on the surface during
the day, except when driven by excessive rain, and even then it
lurks awhile under some protecting cover of leafage. There is a
curious little _Helix_ (_tristis_ Pfr.), peculiar to Corsica, which
is of distinctly subterranean habits. It lives in drifted sand above
high-water mark, always at the roots of _Genista Saltzmanni_, at a
depth which varies with the temperature and dryness of the air. In hot
and very dry weather it buries itself nearly 2 feet below the surface,
only coming up during rain, and burying itself again immediately the
rain is over. Like a _Solen_, it often has a hole above its burrow, by
which it communicates with the air above, so as to avoid being stifled
in the sand. The animal, in spite of its dry habitat, is singularly
soft and succulent, and exudes a very glutinous mucus. It probably
descends in its burrow until it arrives at the humid stratum, the
persistence of which is due to the capillarity of the sand.[112] I am
assured by Mr. E. L. Layard that precisely similar underground habits
are characteristic of _Coeliaxis Layardi_, which lives exclusively in
sand at the roots of scrub and coarse grass at East London.


=Rock-boring Snails.=--Cases have sometimes been recorded, from which
it would appear that certain species of snails possess the power of
excavating holes in rocks to serve as hiding-places. At Les Bois des
Roches, ten miles from Boulogne, occur a number of solid calcareous
rocks scattered about in the wood. The sides of the rocks which face
N.E. and E. are covered with multitudes of funnel-shaped holes, 1½
inch in diameter at the opening and contracting suddenly within to
½ inch. Sometimes the holes are 6 inches deep, and terminate, after
considerable windings, in a cup-shaped cavity. _Helix hortensis_
inhabits these holes, and has been observed to excavate them at the
rate of ½ inch each hibernation, choosing always the side of the rock
which is sheltered from the prevailing rains. It does not form an
epiphragm, but protrudes part of its body against the rock. That the
snails secrete an acid which acts as a solvent seems probable from the
fact that red litmus paper, on being applied to the place where the
foot has been, becomes stained with violet.[113] _Helix aspersa_ is
said to excavate holes 10 to 12 cm. deep at Constantine,[114] and _H.
Mazzullii_ is recorded as perforating limestone at Palermo.[115]


=Snails as Barometers.=--An American writer of more than thirty years
ago[116] gave his experience of Helices as weather-prophets. According
to him, _H. alternata_ is never seen abroad except shortly before rain;
it then climbs on the bark of trees, and stations itself on leaves.
_Helix clausa_, _H. ligera_, _H. pennsylvanica_, and _H. elevata_ climb
trees two days before rain, if it is to be abundant and continuous.
_Succinea_ does the same, and its body is yellow before rain and bluish
after it. Several of the Helices assume a sombre colour after rain,
when their bodies are exceedingly humid; after the humidity has passed
off they resume a clearer and lighter tint.


=Production of Musical and other Sounds.=--Certain molluscs are said
to be capable of producing musical sounds. Sir J. E. Tennent describes
his visit to a brackish-water lake at Batticaloa, in Ceylon, where the
fishermen give the name of the ‘crying shell’ to the animal supposed to
produce the sounds. “The sounds,” he says,[117] “came up from the water
like the gentle thrills of a musical chord, or the faint vibrations
of a wineglass when its rim is rubbed by a moistened finger. It was
not one sustained note, but a multitude of tiny sounds, each clear
and distinct in itself; the sweetest treble mingling with the lowest
bass. On applying the ear to the woodwork of the boat, the vibration
was greatly increased in volume. The sounds varied considerably at
different points as we moved across the lake, and occasionally we rowed
out of hearing of them altogether.” According to the fishermen, the
shells were _Pyrazus palustris_ and _Littorina laevis_. It appears
uncertain whether the sounds are really due to Mollusca. Fishermen in
other parts of India assert that the sounds are made by fish, and, like
those in Ceylon, produce the fish which they say ‘sings.’ The same,
or a similar sound, has also been noticed to issue from the water in
certain parts of Chili, and on the northern shores of the Gulf of
Mexico. _Dendronotus arborescens_, when confined in a glass jar of sea
water, has been noticed[118] to emit a sound like the clink of a steel
wire. According to Lieut.-Col. Portlock,[119] F.R.S., _Helix aperta_,
a very common species in South Europe, has the property of emitting
sounds when irritated. When at Corfu, he noticed that if the animal is
irritated by a touch with a piece of straw or other light material, it
emits a noise, as if grumbling at being disturbed. He kept a specimen
in his house for a considerable time, which would make this noise
whenever it was touched.

The Rev. H. G. Barnacle describes the musical properties of
_Achatinella_ in the following terms:[120] “When up the mountains
of Oahu I heard the grandest but wildest music, as from hundreds of
Aeolian harps, wafted to me on the breezes, and my companion (a native)
told me it came from, as he called them, the singing shells. It was
sublime. I could not believe it, but a tree close at hand proved it.
On it were many of the _Achatinella_, the animals drawing after them
their shells, which grated against the wood and so caused a sound; the
multitude of sounds produced the fanciful music. On this one tree I
took 70 shells of all varieties.”


=Habits of the Agnatha.=--Not much is known of the habits and mode of
life of the _Agnatha_, or carnivorous Land Mollusca. In this country
we have only two, or at most three, of this group, belonging to the
genus _Testacella_, and, in all probability, not indigenous to our
shores. There seems little doubt, when all the circumstances of their
discovery are taken into account, that both _Testacella haliotidea_
and _T. Maugei_ have been imported, perhaps from Spain or Portugal in
the first instance, along with roots imbedded in foreign earth, for
their earliest appearances can almost invariably be traced back to the
neighbourhood of large nursery grounds, or else to gardens supplied
directly from such establishments.

The underground life of _Testacella_ makes observation of its habits
difficult. It is believed to feed exclusively on earthworms, which
it pursues in their burrows. Continued wet weather drives it to the
surface, for though loving damp soil it is decidedly averse to
too much moisture, and under such circumstances it has even been
noticed[121] in considerable numbers crawling over a low wall. In the
spring and autumn months, according to Lacaze-Duthiers,[122] it comes
to the surface at night, hiding itself under stones and _débris_ during
the day. Earthworms are, at these periods, nearer the surface, and the
_Testacella_ has been seen creeping down into their burrows. The author
has taken _T. Maugei_ abundantly under clumps of the common white pink
in very wet weather, lying in a sort of open nest in the moist earth.
On the other hand, when the earth is baked dry by continued drought,
they either bury themselves deeper, sometimes at a depth of 3 feet, in
the ground, or else become encysted in a capsule of hardened mucus to
prevent evaporation from the skin. When first taken from the earth and
placed in a box, the _Testacella_ invariably resents its capture by
spitting up the contents of its stomach in the shape of long fragments
of half-digested worms.

  [Illustration: FIG. 20.--_Testacella haliotidea_ Drap.,
  protruding its pharynx (_ph_) and radula (_r_); _oe_, oesophagus;
  _p.o_, pulmonary orifice; _sh_, shell; _t_, tentacles (after
  Lacaze-Duthiers).]

It appears not to bite the worm up before swallowing it, but contrives,
in the most remarkable manner, to take down whole worms apparently
much too large for its stomach. Mr. Butterell relates[123] that, after
teasing a specimen of _T. Maugei_, and making it emit a quantity of
frothy mucus from the respiratory aperture, he procured a worm of about
three inches long, and rubbed the worm gently across the head of the
_Testacella_. The tongue was rapidly extended, and the victim seized.
The odontophore was then withdrawn, carrying with it the struggling
worm, which made every effort to escape, but in vain; in about five
minutes all had disappeared except the head, which was rejected. This
protrusion of the tongue (_radula_) and indeed of the whole pharynx,
is a very remarkable feature in the habits of the animal. It appears,
as it were, to harpoon its prey by a rapid thrust, and when the victim
is once pierced by a few of the powerful sickle-shaped teeth (compare
chap. viii.) it is slowly but surely drawn into the oesophagus (Fig.
20).

Most gardeners are entirely ignorant of the character of _Testacella_,
and confuse it, if they happen to notice it at all, with the common
enemies of their tender nurslings. Cases have been known, however,
when an intelligent gardener has kept specimens on purpose to kill
worms in ferneries or conservatories. In some districts these slugs
are very numerous; Lacaze-Duthiers once dug 182 specimens from a good
well-manured piece of ground whose surface measured only ten square
yards.

Towards the end of September or beginning of October the period of
hibernation begins. I infer this from the behaviour of specimens kept
in captivity, which, for about a fortnight before this time, gorged
themselves inordinately on as many worms as I chose to put into their
box, and then suddenly refused food, buried themselves deeply in the
earth, and appeared no more during the winter. The eggs are apparently
much less numerous than is the case with _Limax_ or _Helix_, and very
large, measuring about ⅙ inch in diameter. They are enveloped in a
remarkably tough and elastic membrane, and, if dropped upon any hard
surface, rebound several inches, just like an india-rubber ball.

The animal creeps rather rapidly, and has the power of elongating
its body to a remarkable extent. When placed on the surface of the
ground, in the full light of day, it soon betrays uneasiness, and
endeavours to creep into concealment. Its method of burying itself is
very interesting to watch. It first elongates its neck and inserts its
head into the soil; gradually the body begins to follow, while the tail
tilts upwards into the air. No surface motion of the skin, no writhing
or wriggling motion of any kind occurs; the creature simply works its
way down in a stealthy and mysterious way, until at last it is lost to
view.

The great _Glandina_, which attain their maximum development in Mexico
and the southern United States, are a very noticeable family in this
group. According to Mr. Binney,[124] _Glandina truncata_ Gmel., one
of the commonest species of the genus, is somewhat aquatic in its
habits. It is found in the sea islands of Georgia and around the keys
and everglades of Florida, where it attains a maximum length of 4
inches, while in less humid situations it scarcely measures more than
1 inch. It occurs most abundantly in the centre of clumps and tussocks
of coarse grass in marshes close to the sea-coast. By the action of
the sharp, sickle-shaped teeth of its radula the soft parts of its
prey (which consists chiefly of living Helices) are rapidly rasped
away; sometimes they are swallowed whole. It has been known to attack
_Limax_ when confined in the same box, rasping off large pieces of the
integument. In one case an individual was noticed to devour one of its
own species, thrusting its long neck into the interior of the shell,
and removing all the viscera.

  [Illustration: FIG. 21.--_Glandina sowerbyana_ Pfr. (Strebel).]

The _Glandinae_ of southern Europe, although scarcely rivalling those
of Central America in size or beauty, possess similar carnivorous
propensities. _Glandina Poireti_ has been observed,[125] on Veglia
Island, attacking a living _Cyclostoma elegans_. By its powerful teeth
it filed through two or three whorls of the shell of its victim, and
then proceeded to devour it, exactly in the same manner as a _Natica_
or _Buccinum_ perforates the shell of a _Tellina_ or _Mactra_ in order
to get at its contents.

Few observations appear to have been made on the habits or food of
_Streptaxis_, _Rhytida_, _Ennea_, _Daudebardia_, _Paryphanta_, and
other carnivorous Mollusca. A specimen of _Ennea sulcata_, enclosed in
the same box as a Madagascar _Helix_ (_sepulchralis_ Fér.) many times
its own size, completely emptied the shell of its inhabitant.[126]
Mr. E. L. Layard informs me that certain Cape _Rhytida_, _e.g._ _R.
capsula_ Bens., _R. dumeticola_ Bens., and _R. vernicosa_ Kr., eat
_Cyclostoma affine_, _Helix capensis_, _H. cotyledonis_, etc. To Mr.
Layard I am also indebted for the--perhaps apocryphal--tradition that
the best time to capture the great _Aerope caffra_ Fér. in numbers
was after an engagement between the Kaffirs and Zulus, when they might
be observed streaming from all points of the compass towards the field
of slaughter. The Cuban _Oleacina_ are known to secrete a very bitter
fluid which they emit; this perhaps produces a poisoning or benumbing
effect upon their victims when seized. They devour operculates, _e.g._
_Helicina regina_ and _sagraiana_.[127]




                              CHAPTER III

          ENEMIES OF THE MOLLUSCA--MEANS OF DEFENCE--MIMICRY
                 AND PROTECTIVE COLORATION--PARASITIC
                   MOLLUSCA--COMMENSALISM--VARIATION


                       =Enemies of the Mollusca=

The juicy flesh and defenceless condition of many of the Mollusca make
them the favourite food and often the easy prey of a host of enemies
besides man. Gulls are especially partial to bivalves, and may be
noticed, in our large sandy bays at the recess of the tide, busily
devouring _Tellina_, _Mactra_, _Mya_, _Syndosmya_, and _Solen_. On the
Irish coast near Drogheda a herring gull has been observed[128] to take
a large mussel, fly up with it in the air over some shingly ground and
let it fall. On alighting and finding that the shell was unbroken it
again took it up and repeated the process a number of times, flying
higher and higher with it until the shell was broken. Hooded crows,
after many unavailing attempts to break open mussels with their beak,
have been seen to behave in a similar way.[129] Crows, vultures, and
aquatic birds carry thousands of mussels, etc., up to the top of the
mountains above Cape Town, where their empty shells lie in enormous
heaps about the cliffs.[130]

The common limpet is the favourite food of the oyster-catcher, whose
strong bill, with its flattened end, is admirably calculated to
dislodge the limpet from its seat on the rock. When the limpet is
young, the bird swallows shell and all, and it has been calculated
that a single flock of oyster-catchers, frequenting a small Scotch
loch, must consume hundreds of thousands of limpets in the course
of a single year. Rats are exceedingly fond of limpets, whose shells
are frequently found in heaps at the mouth of rat holes, especially
where a cliff shelves gradually towards a rocky shore. A rat jerks the
limpet off with a sudden movement of his powerful jaw, and, judging
from the size of the empty shells about the holes, has no difficulty in
dislodging the largest specimens. ‘I once landed,’ relates a shepherd
to Mr. W. Anderson Smith,[131] ‘on the I. of Dunstaffnage to cut grass,
and it was so full of rats that I was afraid to go on; and the grass
was so full of limpets that I could scarcely use the scythe, and had
to keep sharpening it all the time.’ Sometimes, however, the limpet
gets the better both of bird and beast. The same writer mentions the
case of a rat being caught by the lip by a limpet shell, which it was
trying to dislodge. A workman once observed[132] a bird on Plymouth
breakwater fluttering in rather an extraordinary manner, and, on going
to the spot, found that a ring dotterel had somehow got its toe under a
limpet, which, in closing instantly to the rock, held it fast. Similar
cases of the capture of ducks by powerful bivalves are not uncommon,
and it is said that on some parts of the American coasts, where clams
abound, it is impossible to keep ducks at all,[133] for they are sure
to be caught by the molluscs and drowned by the rising tide.

The _Weekly Bulletin_ of San Francisco, 17th May 1893, contains an
account of the trapping of a coyote, or prairie wolf, at Punta Banda,
San Diego Co., by a _Haliotis Cracherodii_. The coyote had evidently
been hunting for a fish breakfast, and finding the _Haliotis_ partially
clinging to the rock, had inserted his muzzle underneath to detach it,
when the _Haliotis_ instantly closed down upon him and kept him fast
prisoner.

Rats devour the ponderous Uniones of North America. When _Unio_ moves,
the foot projects half an inch or more beyond the valves. If, when in
this condition, the valves are tightly pinched, the foot is caught, and
if the pinching is continued the animal becomes paralysed and unable to
make use of the adductor muscles, and consequently flies open even if
the pressure is relaxed. The musk-rat (_Fiber zibethicus_) seizes the
_Unio_ in his jaws, and by the time he reaches his hole, the _Unio_ is
ready to gape.[134] Rats also eat _Vivipara_, and even _Limnaea_, in
every part of the world.

Every kind of slug and snail is eaten greedily by blackbirds, thrushes,
chaffinches, and in fact by many species of birds. A thrush will very
often have a special sacrificial stone, on which he dashes the shells
of _Helix aspersa_ and _nemoralis_, holding them by the lip with his
beak, until the upper whorls are broken; heaps of empty shells will be
found lying about the place of slaughter. The bearded Titmouse (_Parus
biarmicus_) consumes quantities of _Succinea putris_ and small _Pupa_,
which are swallowed whole and become triturated in the bird’s stomach
by the aid of numerous angular fragments of quartz.[135]

Frogs and toads are very partial to land Mollusca. A garden attached to
the Laboratory of Agricultural Chemistry at Rouen had been abandoned
for three years to weeds and slugs. The director introduced 100 toads
and 90 frogs, and in less than a month all the slugs were destroyed,
and all kinds of vegetables and flowers, whose cultivation had until
then been impossible, were enabled to flourish.[136]

Certain Coleoptera are known to prey upon Helices and other land
Mollusca. Récluz noticed, near Agde, a beetle (_Staphylinus olens_)
attack _Helix ericetorum_ when crawling among herbage, sticking its
sharp mandibles into its head. Every time the snail retreated into
its shell the beetle waited patiently for its reappearance, until at
last the snail succumbed to the repeated assaults. M. Lucas noticed,
at Oran, the larva of a _Drilus_ attacking a _Cyclostoma_. The
_Drilus_ stood sentinel at the mouth of a shell, which was closed by
the operculum, until the animal began to issue forth. The _Drilus_
then with its mandibles cut the muscle which attaches the operculum
to the foot, disabling it sufficiently to prevent its being securely
closed, upon which it entered and took possession of the body of its
defenceless host, completing its metamorphosis inside the shell, after
a period of six weeks.[137] The female glow-worm (_Lampyris noctiluca_)
attacks and kills _Helix nemoralis_.

Among the Clavicornia, some species of _Silpha_ carry on a determined
warfare against small Helices. They seize the shell in their
mandibles, and then, throwing their head backwards, break the shell by
striking it against their prothorax.

The common water beetle, _Dytiscus marginalis_, from its strength and
savage disposition, is a dangerous enemy to fresh-water Mollusca. One
_Dytiscus_, kept in an aquarium, has been noticed to kill and devour
seven _Limnaea stagnalis_ in the course of one afternoon. The beetles
also eat _L. peregra_, but apparently prefer _stagnalis_, for when
equal quantities of both species were placed within their reach, they
fixed on the latter species first.[138]

In East Africa a species of Ichneumon (_Herpestes fasciatus_) devours
snails, lifting them up in its forepaws and dashing them down upon some
hard substance.[139] In certain islands off the south coasts of Burmah,
flat rocks covered with oysters are laid bare at low tide. A species
of Monkey (_Macacus cynomolgus_) has been noticed to furnish himself
with a stone, and knock the oysters open, always breaking the hinge-end
first, and then pulling out the mollusc with his fingers.[140]

The walrus is said to support himself almost entirely on two species
of _Mya_ (_truncata_ and _arenaria_), digging them out of the sand, in
which they live buried at a depth of about 1½ feet, with his powerful
tusks. Whales swallow enormous numbers of pelagic molluscs (_Clio_,
_Limacina_), which are at times so abundant in the Arctic seas, as to
colour the surface for miles. Many of the larger Cetacea subsist in
great part on Cephalopoda; as many as 18 lbs. of beaks of Teuthidae
have been taken from the stomach of a single _Hyperoodon_.

Fish are remarkably partial to Mollusca of various kinds. The
cat-fish (_Chimaera_) devours _Pectunculus_ and _Cyprina_, crushing
the stout shells with its powerful jaws, while flounders and soles
content themselves with the smaller _Tellina_ and _Syndosmya_ which
they swallow whole. As many as from 30 to 40 specimens of _Buccinum
undatum_ have been taken from the stomach of a single cod, and the same
‘habitat’ has been recorded for some of the rarer whelks, _e.g._ _Bucc.
humphreysianum_, _Fusus fenestratus_, the latter also occurring as the
food of the haddock and the red gurnard. No less than 35,000 _Turtonia
minuta_ have been found in the stomach of a single mullet. Nudibranchs
are no doubt dainty morsels for fish, and hence have developed, in many
cases, special faculties for concealment, or, if distasteful, special
means of remaining conspicuous (see pp. 71–74).

  [Illustration: FIG. 22.--Two valves of _Mytilus edulis_ L.,
  representing diagrammatically the approximate position of the
  holes bored by _Purpura_ in about 100 specimens of _Mytilus_,
  gathered at Newquay, Cornwall.]

Besides the dangers to which they are exposed from other enemies, many
of the weaker forms of Mollusca fall a prey to their own brethren.
_Nassa_ and _Murex_ on this side of the Atlantic, and _Urosalpinx_ on
the other, are the determined foes of the oyster. _Purpura lapillus_
prefers _Mytilus edulis_ to any other food, piercing the shell in about
two days’ time by its powerful radula, which it appears to employ
somewhat in gimlet fashion. If _Mytilus_ cannot be procured, it will
eat _Littorina_ or _Trochus_, but its attempts on the hard shell of
_Patella_ are generally failures. The statement which is sometimes
made, that the _Purpura_ makes its hole over the vital parts of the
_Mytilus_, appears, according to the evidence embodied in the annexed
figure, to be without foundation. The fact is that a hole in any part
of its shell is fatal to the _Mytilus_, since the long proboscis of
the _Purpura_, having once made an entrance, can reach from one end of
the shell to the other. The branchiae are first attacked, the adductor
muscles and edges of the mantle last. _Natica_ and _Nassa_ pierce in a
similar way the shells of _Mactra_, _Tellina_, _Donax_, and _Venus_.
_Murex fortispina_ is furnished with a powerful tooth at the lower part
of its outer lip. At Nouméa, in New Caledonia, its favourite food is
_Arca pilosa_, which lives half buried in coral refuse. The _Murex_ has
been seen to drag the _Arca_ from its place of concealment, and insert
the tooth between the valves, so as to prevent their closing, upon
which it was enabled to devour its prey at leisure.[141]

The carnivorous land Mollusca, with the exception of _Testacella_,
appear to feed by preference upon other snails (pp. 54, 55).


=Parasitic Worms, Mites, etc.=--A considerable number of the
Trematode worms pass one or more of the stages in the cycle of their
development within the bodies of Mollusca, attaining to the more
perfect or sexual form on reaching the interior of some vertebrate.
Thus _Distoma endolabum_ Duj. finds its first intermediate host in
_Limnaea stagnalis_ and _L. ovata_, its second in _L. stagnalis_, or
in one of the fresh-water shrimps (_Gammarus pulex_), or in the larvae
of one of the _Phryganeidae_ (_Limnophilus rhombicus_), attaining to
the sexual form in the common frog. _Distoma ascidia_ v. Ben. passes
firstly through _Limnaea stagnalis_ or _Planorbis corneus_, secondly
through certain flies and gnats (_Ephemera_, _Perla_, _Chironomus_),
and finally arrives within certain species of bats. _Distoma nodulosum_
Zed. inhabits firstly _Paludina impura_, secondly certain fishes
(_Cyprinus Acerina_), and lastly the common perch. The sporocyst of
_Distoma macrostomum_ inhabits _Succinea putris_, pushing itself up
into the tentacles, which become unnaturally distended (Fig. 23). While
in this situation it is swallowed by various birds, such as the thrush,
wagtail, and blackbird, which are partial to _Succinea_, and thus
obtains lodgment in their bodies. _Amphistoma subclavatum_ spends an
early stage in _Planorbis contortus_, after which it becomes encysted
on the skin of a frog. When the frog sheds its skin, it swallows it,
and with it the _Amphistoma_, which thus becomes established in the
frog’s stomach.[142]

  [Illustration: FIG. 23.--A Trematode worm (_Leucochloridium
  paradoxum_ Car.) parasitic in the tentacles of _Succinea putris_
  L. × 20 (after Baudon).]

The common liver-fluke, which in the winter of 1879–1880 cost Great
Britain the lives of no less than three million sheep, is perhaps the
best known of these remarkable parasitic forms of life. Its history
shows us, in one important particular, how essential it is for the
creature to meet, at certain stages of its existence, with the exact
host to which it is accustomed. Unless the newly-hatched embryo finds
a _Limnaea truncatula_ within about eight hours it becomes exhausted,
sinks, and dies. It has been tried with all the other common pond and
river Mollusca, with _Limnaea peregra_, _palustris_, _auricularia_,
_stagnalis_, with _Planorbis marginatus_, _carinatus_, _vortex_, and
_spirorbis_, with _Physa fontinalis_, _Bithynia tentaculata_, _Paludina
vivipara_, as well as with _Succinea putris_, _Limax agrestis_ and
_maximus_, _Arion ater_ and _hortensis_. Not one of them would it
touch, except occasionally very young specimens of _L. peregra_, and in
these its development was arrested at an early stage. But on touching
a _L. truncatula_ the embryo seems to know at once that it has got
what it wants, and sets to work immediately to bore its way into the
tissues of its involuntary host, making by preference for the branchial
chamber; those which enter the foot or other outlying parts of the
_Limnaea_ proceed no farther.[143]

Many similar cases occur, in which littoral Mollusca, such as
_Littorina_ and _Buccinum_, form the intermediate host to a worm which
eventually arrives within some sea-bird.

Certain Nematode worms (_Rhabditis_) are known to inhabit the intestine
of _Arion_, and the salivary glands of _Limax agrestis_. Diptera
habitually lay their eggs within the eggs of _Helix_ and _Limax_.
Many species of mite (_Acarina_) infest land Pulmonata. No adult
_Limax maximus_ is without at least one specimen of _Philodromus_ (?)
_limacum_, and the same, or an allied species, appears to occur on the
larger of our _Helices_, retiring upon occasion into the pulmonary
chamber.

Several of the Crustacea live associated with certain molluscs.
_Pinnotheres_ lives within the shell of _Pinna_, _Ostrea_, _Astarte_,
_Pectunculus_, and others. Apparently the females alone reside within
the shell of their host, while the males seize favourable opportunities
to visit them there. A specimen of the great pearl-oyster (_Meleagrina
margaritifera_) was recently observed which contained a male
Pinnotheres encysted in nacre. It was suggested that he had intruded at
an unfortunate time, when no female of his kind happened to be in, and
that, having penetrated too far beneath the mantle in the ardour of his
search, was made prisoner before he could escape.[144] _Ostracotheres
Tridacnae_ lives in the branchiae of the great _Tridacna_. A little
brachyurous crustacean inhabits the raft of _Ianthina_, and assumes the
brilliant blue colour of the mollusc.


                          =Means of Defence=

As a rule, among the Mollusca, the shell forms a passive mode of
resistance to the attacks of enemies. Bivalves are enabled, by closing
their valves, to baffle the assault of their smaller foes, and the
operculum of univalves, both marine and land, serves a similar purpose.
Many land Mollusca, especially _Helix_ and _Pupa_, as well as a number
of _Auriculidae_, have the inside of the aperture beset with teeth,
which are sometimes so numerous and so large that it is puzzling to
understand how the animal can ever come out of its shell, or, having
come out, can ever draw itself back again. Several striking cases of
these toothed apertures are given in Fig. 24. Whatever may be the
origin of these teeth, there can be little doubt that their extreme
development must have a protective result in opposing a barrier to
the entrance, predatory or simply inquisitive, of beetles and other
insects. Sometimes, it will be noticed (_G_), the aperture itself is
fairly simple, but a formidable array of obstacles is encountered a
little way in. It is possible that the froth emitted by many land
snails has a similar effect in involving an irritating intruder in
a mass of sticky slime. The mucus of slugs and snails, on the other
hand, is more probably, besides its use in facilitating locomotion, a
contrivance for checking evaporation, by surrounding the exposed parts
of their bodies with a viscid medium.

  [Illustration: FIG. 24.--Illustrating the elaborate
  arrangement of teeth in the aperture of some land Pulmonata.
  =A.= _Helix_ (_Labyrinthus_) _bifurcata_ Desh., Equador.
  =B.= H. (_Pleurodonta_) _picturata_ Ad., Jamaica. =C.= _H._
  (_Dentellaria_) _nux denticulata_ Chem., Demerara. =D.= _Anostoma
  carinatum_ Pfr., Brazil; a, tube communicating with interior of
  shell. =E.= _H._ (_Stenotrema_) _stenotrema_ Fér., Tennessee, ×
  3/2. =F.= _H._ (_Polygyra_) _auriculata_ Say, Florida, × 3/2.
  =G.= _H._ (_Plectopylis_) _refuga_ Gld., Tenasserim (a and b ×
  2).]

Some species of _Lima_ shelter themselves in a nest constructed of
all kinds of marine refuse, held together by byssiferous threads.
_Modiola adriatica_, _M. barbata_, and sometimes _M. modiolus_ conceal
themselves in a similar way. _Gastrochaena_ frequently encloses itself
in a sort of half cocoon of cement-like material. The singular genus
_Xenophora_ protects itself from observation by gluing stones, shells,
and various _débris_ to the upper side of its whorls (Fig. 25).
Sometimes the selection is made with remarkable care; the _Challenger_,
for instance, obtained a specimen which had decorated its body whorl
exclusively with long and pointed shells (Fig. 26).

  [Illustration: FIG. 25.--_Xenophora_ (_Phorus_) _conchyliophora_
  Born., concealed by the stones which it glues to the upper
  surface of its shell. (From a British Museum specimen.)]

  [Illustration: FIG. 26.--_Xenophora_ (_Phorus_) _pallidula_
  Reeve. A mollusc which escapes detection by covering itself with
  dead shells of other species. (From a _Challenger_ specimen in
  the British Museum, × ½.)]

The formidable spines with which the shells, _e.g._ of the _Murex_
family, are furnished must contribute greatly to their protection
against fishes, and other predatory animals. _Murex tenuispina_, for
instance (see chap. ix.), would prove as dangerous a morsel in the
mouth of a fish as a hedgehog in that of a dog. Whether the singular
tooth in the outer lip of _Leucozonia_ (see chap. xiv.), a feature
which is repeated, to a less marked extent, in _Monoceros_ and several
of the West Coast muricoids, is developed for defensive purposes,
cannot at present be decided.

The _Strombidae_ possess the power of executing long leaps, which they
doubtless employ to escape from their foes. In their case alone this
power is combined with singular quickness of vision. On one occasion
Mr. Cuming, the celebrated collector, lost a beautiful specimen of
_Terebellum_, by the animal suddenly leaping into the water, as he was
holding and admiring it in his hand. Miss Saul has informed me that the
first living specimen of _Trigonia_ that was ever obtained was lost
in a similar way. It was dredged by Mr. Stutchbury in Sydney Harbour,
and placed on the thwart of a small boat. He had just remarked to a
companion that it must be a _Trigonia_, and his companion had laughed
at the idea, reminding him that all known _Trigonia_ were fossil, when
the shell in question baffled their efforts to discover its generic
position by suddenly leaping into the sea, and it was three months
before Mr. Stutchbury succeeded in obtaining another.

Some genera possess more than merely passive means of defence. Many
Cephalopoda emit a cloud of inky fluid, which is of a somewhat viscous
nature, and perhaps, besides being a means of covering retreat, serves
to entangle or impede the pursuer. The formidable suckers and hooks
possessed by many genera in this Order are most dangerous weapons,
both for offence and defence. _Aplysia_, when irritated, ejects a
purple fluid which used to be considered dangerously venomous. Many of
the Aeolididae, including our own common _Aeolis papillosa_, possess
stinging cells at the end of their dorsal papillae, the effect of which
is probably to render them exceedingly distasteful to fish.

The common _Vitrina pellucida_ has a curious habit which in all
probability serves for a defence against birds in the winter. When
crawling on the edge of a stone or twig it has the power of suddenly
jerking its ‘tail,’ so as to throw itself on the ground, where it is
probably lost to sight among decaying leaves. At other times it rolls
away a few inches and repeats the jump. It also possesses the power of
attaching to itself bits of leaves or soil, which entirely cover and
conceal both shell and animal.[145] The property of parting with the
tail altogether, a remarkable form of self-defence, has already been
noticed on p. 44.

The poisonous nature of the bite of certain species of _Conus_ is well
authenticated. Surgeon Hinde, R.N., saw[146] a native on the I. of
Matupi, New Britain, who had been bitten by a _Conus geographus_, and
who had at once cut small incisions with a sharp stone all over his
arm and shoulder. The blood flowed freely, and the native explained
that had he not taken these precautions he would have died. Instances
have been recorded of poisonous wounds being inflicted by the bite
of _Conus aulicus_, _C. textile_, and _C. tulipa_. According to Mr.
J. Macgillivray[147] _C. textile_ at Aneitum (S. Pacific) is called
_intrag_, and the natives say it spits the poison upon them from
several inches off! Two cases of bites from _C. textile_ occurred to
this gentleman’s notice, one of which terminated fatally by gangrene.
Sir Edward Belcher, when in command of the _Samarang_, was bitten[148]
by a _Conus aulicus_ at a little island off Ternate in the Moluccas.
As he took the creature out of the water, it suddenly exserted its
proboscis and inflicted a wound, causing a sensation similar to that
produced by the burning of phosphorus under the skin. The wound was
a small, deep, triangular mark, succeeded by a watery vesicle. The
natives of New Guinea have a wholesome dread of the bite of Cones.
Mr. C. Hedley relates[149] that while collecting on a coral reef he
once rolled over a boulder and exposed a living _C. textile_. Before
he could pick it up, one of the natives hastily snatched it away, and
explained, with vivid gesticulations, its hurtful qualities. On no
account would he permit Mr. Hedley to touch it, but insisted on himself
placing it in the bottle of spirits.

  [Illustration: FIG. 27.--A tooth from the radula of _Conus
  imperialis_ L., × 50, showing barb and poison duct.]


                  =Mimicry and Protective Coloration.=

Cases of Mimicry, or protective resemblance, when a species otherwise
defenceless adopts the outward appearance of a better protected
species, are rare among the Mollusca. Karl Semper[150] mentions an
interesting case of the mimicry of _Helicarion tigrinus_ by _Xesta
Cumingii_, in the Philippines. It appears that all species of
_Helicarion_ possess the singular property of shaking off the ‘tail’ or
hinder part of the foot, when seized or irritated. Specimens captured
by collectors, _Hel. tigrinus_ amongst them, have succeeded in escaping
from the hand, and concealing themselves, by a sort of convulsive leap,
among the dry leaves on the ground. This power of self-amputation must
be of great value to _Helicarion_, not only as enabling it to escape
from the clutch of its enemies, but also as tending to discourage
them from attempting to capture it at all. Now the genus _Xesta_ is,
in anatomy, very far removed from _Helicarion_, and the majority
of the species are also, as far as the shell is concerned, equally
distinct. _Xesta Cumingii_, however, has, according to Semper, assumed
the appearance of a _Helicarion_, the thin shell, the long tail, and
the mantle lobes reflected over the shell; but it has not the power
of parting with its tail at short notice. It lives associated with
_Helicarion_, and so close is the resemblance between them that, until
Semper pointed out its true position, it had always been classified as
a member of that group.

In the same passage Semper draws attention to two other cases
of apparent mimicry. The first is another species of _Xesta_
(_mindanaensis_) which closely resembles a species of _Rhysota_
(_Antonii_), a genus not indeed so far removed from _Xesta_ as
_Helicarion_, but, as far as the shell is concerned, well distinguished
from it. In this case, however, there is no obvious advantage gained by
the resemblance, since _Rhysota_ as compared with _Xesta_ is not known
to possess any definite point of superiority which it would be worth
while to counterfeit. A second case of resemblance between certain
species of the genus _Chloraea_ and the characteristic Philippine group
_Cochlostyla_ will not hold good as affording evidence of mimicry, for
_Chloraea_ is now recognised as a sub-genus of _Cochlostyla_.

The Mollusca are not much mimicked by creatures of different
organisation. This appears at first sight strange, since it might have
been thought that the strong defensive house of a snail was worth
imitating. Still it is probably not easy for creatures bilaterally
symmetrical to curl themselves up into an elevated spiral for any
length of time. One or two instances, however, may be mentioned. The
larva of a moth belonging to the Psychidae, and occurring in France,
Germany, the Tyrol, and Syria, coils itself up into a sinistral spiral
of three whorls, and is aptly named _Psyche helix_, a kindred species
from Italy being known as _Ps. planorbis_.

An insect larva (_Cochlophora valvata_) from E. Africa is said to
resemble a _Valvata_ or young _Cyclostoma_. In this case the spiral
is indifferently dextral or sinistral, the ‘shell’ being formed of
masticated vegetable matter, united together by threads spun by the
larva. Certain larvae of the Phryganeidae (“Caddis-worms”) enclose
themselves in houses which more or less resemble a spiral shell, and
have in some cases actually been described as molluscan; such species,
some of which belong to _Helicopsyche_, have been noticed in S. Europe,
Ceylon, Further India, China, Tasmania, New Zealand, Tennessee, Mexico,
Central America, Venezuela, Brazil, and Argentina, and all[151] possess
a dextral ‘shell.’ In all these cases ‘mimicry’ is probably not so much
to be thought of as the practical advantages which accrue to the animal
in question from the spiral form, which gives it greater strength
to resist external blows, and enables it to occupy, during a very
defenceless portion of its existence, a very small amount of space.

The larva of some species of the Syrphidae (_Diptera_) fixes itself on
the under side of stones in the Tyrol, and closely resembles a small
slug. The naturalist Von Spix, in 1825, described to the Bavarian
Academy as a new genus of land Mollusca a somewhat similar larval form
found in decaying wood on the banks of a German lake.[152] Simroth
mentions[153] a curious case as occurring near Grimma. The caterpillars
of certain Microlepidoptera occur on slabs of porphyry, associated
with a species of _Clausilia_. Besides being of the same colour as the
Clausiliae, the caterpillars have actually developed cross lines on the
back, _i.e._ on the side turned away from the rock, in imitation of the
suture of the mollusc.

It has been suggested[154] that there is mimicry between _Aeolis
papillosa_ (a common British nudibranch) and _Sagartia troglodytes_
(an Actinian), and also between another species of _Sagartia_ and
_Aeolidiella Alderi_. The facts observed are not sufficient to warrant
a decided opinion, but it seems more probable that the Actinian
mimics the nudibranch than _vice versâ_, since _Aeolis_ is known to be
unpalatable to fishes.

  [Illustration: FIG. 28.--=A=, _Strombus mauritianus_ Lam., which
  mimics _Conus_ in shape. =B=, _Conus janus_ Hwass, Mauritius.]

Certain species of _Strombus_ (_mauritianus_ L., _luhuanus_ L.) show
a remarkable similarity in the shape of the shell to that of _Conus_,
so much so, that a tiro would be sure to mistake them, at first sight,
for Cones. In the case of _S. luhuanus_ at least, this similarity is
increased by the possession of a remarkably stout brown epidermis.
Now _Conus_ is a flesh-eating genus, armed with very powerful teeth
which are capable of inflicting even on man a poisonous and sometimes
fatal wound (see p. 66). _Strombus_, on the other hand, is probably
frugivorous, and is furnished with weak and inoffensive teeth. It is
possible that this resemblance is a case of ‘mimicry.’ It is quite
conceivable that powerful fishes which would swallow a _Strombus_ whole
and not suffer for it, might acquire a distaste for a Cone, which
was capable of lacerating their insides after being swallowed. And
therefore the more like a Cone the _Strombus_ became, the better chance
it would have of being passed over as an ineligible article of food.

_Protective coloration_ is not uncommon among the Mollusca. _Littorina
obtusata_ is habitually found, on our own coasts, on _Fucus
vesiculosus_, the air-bladders of which it closely resembles in colour
and shape. _Littorina pagodus_, a large and showy species, resembles so
closely the spongy crumbling rocks of Timor, on which it lives, that it
can hardly be discerned a pace off. _Helcion pellucidum_, the common
British ‘blue limpet,’ lives, when young, almost exclusively on the
iridescent leaves of the great Laminariae, with the hues of which its
own conspicuous blue lines harmonise exactly. In mature life, when the
_Helcion_ invariably transfers its place of abode to the lower parts
of the stalk and finally to the root of the Laminaria, which are quite
destitute of iridescence, these blue lines disappear or become much
less marked.

The specimens of _Purpura lapillus_ which occur at Newquay in Cornwall
are banded with rings of colour, especially with black and white, in a
more varied and striking way than any other specimens that have ever
occurred to my notice. I am inclined to refer this peculiarity to a
tendency towards protective coloration, since the rocks on which the
_Purpura_ occurs are often banded with veins of white and colour, and
variegated to a very marked extent.

_Ovula_ varies the colour of its shell from yellow to red, to match the
colour of the _Gorgonia_ on which it lives. The same is the case with
_Pedicularia_, which occurs on red and yellow coral.

_Helix desertorum_, by its gray-brown colour, harmonises well with
the prevailing tint of the desert sands, among which it finds a home.
Benson observes that the gaudy _H. haemastoma_, which lives on the
trunks of palm-trees in Ceylon, daubs its shell with its excrement.
Our own _Buliminus obscurus_, which lives principally on the trunks of
smooth-barked trees, daubs its shell with mud, and must often escape
the observation of its enemies by its striking resemblance to the
little knots on the bark, especially of beech trees, its favourite
haunt. Some species of _Microphysa_, from the West Indies, habitually
encrust their shells with dirt, and the same peculiarity in _Vitrina_
has already been mentioned. _Ariophanta Dohertyi_ Aldr., a recent
discovery from Sumatra, is of a green colour, with a singularly
delicate epidermis; it is arboreal in its habits, and is almost
invisible amongst the foliage.[155] Many of our own slugs, according
to Scharff, are coloured protectively according to their surroundings.
A claret-coloured variety of _Arion ater_ occurred to this observer
only in pine woods, where it harmonised with the general colouring of
the ground and the pine-needles, while young winter forms of the same
species choose for hiding-places the yellow fallen leaves, whose colour
they closely resemble. _Limax marginatus_ (= _arborum_ Bouch.) haunts
tree trunks, and may easily be mistaken for a piece of bark; _Amalia
carinata_ lives on and under the ground, and in colour resembles the
mould; _Arion intermedius_ feeds almost exclusively on fungi, to which
its colour, which is white, gray, or light yellow, tends to approximate
it closely; _Geomalacus maculosus_ conceals itself by its striking
resemblance to the lichens which grow on the surface of rocks, and
actually presumes on this resemblance so much as to expose itself,
contrary to the usual custom of its congeners, to the full light of the
afternoon sun.[156]

Several views have been advanced with regard to the dorsal papillae,
or _cerata_, in the Nudibranchs. Professor W. A. Herdman, who has
examined a considerable number of our own British species, in which
these processes occur, is of opinion[157] that they are of two quite
distinct kinds. In the first place, they may contain large offshoots,
or _diverticula_, of the liver, and thus be directly concerned in the
work of digestion. This is the case with _Aeolis_ and _Doto_. In the
second place, they may be simply lobes on the skin, with no connexion
with the liver, and no special function to perform. This is the case
with _Tritonia_, _Ancula_, and _Dendronotus_.

Professor Herdman is of opinion that although the cerata may in all
cases aid in respiration to a certain extent, yet that extent is
so small as to be left out of consideration altogether. He regards
the cerata in both the two classes mentioned above as “of primary
importance in giving to the animals, by their varied shapes and
colours, appearances which are in some cases protective, and in others
conspicuous and warning.”

Thus, for instance, _Tritonia plebeia_, which is fairly abundant at
Puffin and Hilbre Is., appears always to be found creeping on the
colonies of a particular polyp, _Alcyonium digitatum_, and nowhere
else. The specimens in each colony of the polyp differ noticeably
both in the matter of colour, and of size, and of varied degrees of
expansion. The _Tritonia_ differs also, being marked in varied tints
of yellow, brown, blue, gray, black, and opaque white, in such a way
as to harmonise with the varied colours of the _Alcyonium_ upon which
it lives. The cerata on the back of the _Tritonia_ contribute to this
general resemblance. They are placed just at the right distance apart,
and are just the right size and colour, to resemble the crown of
tentacles on the half-expanded polyp.

Similarly, _Doto coronata_, which, when examined by itself, is a very
conspicuous animal, with showy, bright-coloured cerata, is found by
Professor Herdman to haunt no other situations but the under side of
stones and overhanging ledges of rock which are colonised by a hydroid,
known as _Clava multicornis_. The _Doto_ is masked by the tentacles and
clusters of sporosacs on the zoophyte, with whose colouring and size
its own cerata singularly correspond. A similar and even more deceptive
correspondence with environment was noticed in the case of the very
conspicuous _Dendronotus arborescens_.

In these cases, the colouring and general shape of the cerata are
protective, _i.e._ they match their surroundings in such a way as to
enable the animal, in all probability, to escape the observation of
its enemies. According to Professor Herdman, however, the brilliant
and showy coloration of the cerata of _Aeolis_ is not protective
but ‘warning.’ _Aeolis_ does not hide itself away as if shunning
observation, like _Doto_, _Tritonia_, and _Dendronotus_; on the
contrary, it seems perfectly fearless and indifferent to being noticed.
Its cerata are provided with sting-cells, like those of Coelenterata,
at their tips, and its very conspicuousness is a warning to its
enemies that they had better not try to attack it, just as the showy
white tail of the skunk acts as a sort of danger-signal to its own
particular foes. It is important for the _Aeolis_, not merely to _be_
an unpalatable nettle in animal shape, but also to be conspicuous
enough to prevent its being experimented upon as an article of food, in
mistake for something less nasty.

Professor Herdman subsequently conducted some experiments[158] with
fishes, with the view of testing his theory that the shapes and colours
of Nudibranchs serve the purpose either of protection or warning, and
bear direct relation to the creature’s edibility. These experiments,
on the whole, distinctly tended to confirm the theory. _Aeolis_ was
evidently very nasty, and probably stung the mouths of the fishes who
tried it. For the complete success of the theory, they ought to have
let it severely alone, but the fish were evidently accustomed to make a
dash at anything that was dropped into their tank. Another conspicuous
mollusc, _Ancula cristata_, was introduced, Professor Herdman and his
collaborator each commencing operations by eating a live specimen
themselves. They found the taste pleasant, distinctly like that of an
oyster. The fish, however, when the experiments were conducted under
conditions which made the scene as much like ‘real life’ as possible,
did not agree with Professor Herdman. The _Ancula_ crawled over various
parts of the tank for several days untouched by the fish, who sometimes
went close to them and looked at them, but never attempted to taste
them. Experiments with species whose colours were protective, such as
_Dendronotus_, were also conducted, and the decided edibility of these
species was established, the fish competing eagerly for them, and
tearing them rapidly to pieces.

Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological
Association, confirms[159] Professor Herdman’s views as to the shape
and colour of Opisthobranchs. _Pleurobranchus membranaceus_ is known
to secrete, on the surface of the body, an acid which reddens blue
litmus paper. It is, therefore, no doubt distasteful to fish, which
all abominate the taste of acids, and is conspicuously marked with
red-brown and yellowish ‘warning’ colours. _Haminea_ and _Philine_, on
the other hand, are good to eat, and consequently possess ‘protective’
coloration. _Runcina Hancocki_, which is of a brown colour, crawls
over brown mud and weeds, but avoids green weeds, on whose surface it
would appear conspicuous. _Elysia viridis_ varies its colour according
to its habitat, being green when on green weeds, and dark olive,
brown, or reddish brown, on pools among tufts of littoral algae. Green
specimens of _Hermaea dendritica_ were kept in captivity, and placed in
a dish with green and red sea-weeds. They were never observed crawling
upon the red weed, upon which they would have been very conspicuous.
_Archidoris flammea_ occurred on bright red sponges, to which its
colour was so closely assimilated that Mr. Garstang at first quite
overlooked it. _Goniodoris castanea_ was found under stones, feeding on
compound Ascidians (_Botryllus_), which it sufficiently resembled to be
very inconspicuous in that position.

Again, _Jorunna Johnstoni_ lives[160] upon stones on our southern
coast, associated with a certain sponge (_Halichondria_ sp.), which it
resembles so closely in outline, in colour, in character of surface,
and in its projecting plumes, as to make it very difficult even for
the careful observer to distinguish the one from the other. And, since
fishes, are known to be distinctly averse to sponges of any kind as an
article of food, this resemblance must be decidedly to the advantage
of the _Jorunna_. Another Nudibranch (_Calma glaucoides_ A. and H.)
imitates the ova of certain fishes, on which it feeds. Its elongated
and depressed form of body, transparent integuments, and silvery gray
papillae combine to give it a strong resemblance to the spawn of the
fish, which is deposited on stones, the roots of _Laminaria_, etc.[161]

The common _Lamellaria perspicua_ appears to possess the power of
protectively assimilating its colour, markings, etc., to the Ascidians
on which it lives. A recent case, occurring off the Isle of Man, is
thus described by Professor Herdman.[162] “The mollusc was on a colony
of _Leptoclinum maculatum_, in which it had eaten a large hole. It lay
in this cavity so as to be flush with the general surface; and its
dorsal integument was not only whitish with small darker marks which
exactly reproduced the appearance of the _Leptoclinum_ surface with
the ascidiozooids scattered over it, but there were also two larger
elliptical clear marks which looked like the large common cloacal
apertures of the Ascidian colony.... Presumably the _Lamellaria_
escapes the observation of its enemies through being mistaken for part
of the _Leptoclinum_ colony; and the _Leptoclinum_, being crowded like
a sponge with minute sharp-pointed spicules, is, I suppose, avoided as
inedible (if not actually noxious through some peculiar smell or taste)
by carnivorous animals which might devour such things as the soft
unprotected mollusc.”


                          =Parasitic Mollusca=

Various grades of parasitism occur among the Mollusca, from the true
parasite, living and nourishing itself on the tissues and secretions of
its host, to simple cases of commensalism. Some authors have divided
these forms into endo- and ectoparasites, according as they live
inside or outside of their host. Such a division, however, cannot
be rigidly carried out, for certain forms are indifferently endo-
and ecto-parasitical, while others are ecto-parasitic in the young
form, and become endo-parasitic in the adult. It will be convenient,
therefore, simply to group the different forms according to the home on
which they find a lodgment.

  [Illustration: FIG. 29.--_Magilus antiquus_ L.: =A=, the adult,
  imbedded in coral, which has been broken away to show the tube;
  =B=, the young (free) form.]

On _Sponges_.--_Vulsella_ and _Crenatula_ almost invariably occur
in large masses of irregular shape, boring into sponges. They are
especially abundant on Porifera from the Red Sea. Corals form a
favourite home of many species, amongst which are several forms
of _Coralliophila_, _Rhizochilus_, _Leptoconchus_, and _Sistrum_.
_Rhizochilus_ is a very singular creature, inhabiting branching corals.
When adult, it forms irregular shelly extensions of both the inner and
outer lips, which adhere to the shafts of the coral, or to the surface
of neighbouring shells; at length the aperture becomes completely
closed with the exception of the siphonal tube, which becomes long, and
consists of the same shelly material. The common _Magilus_ (Fig. 29),
from the Red Sea and Indian Ocean, in the young form is shaped like
a small _Buccinum_. As the coral (_Meandrina_) to which it attaches
itself grows, the _Magilus_ develops at the mouth a long calcareous
tube, the aperture of which keeps pace with the growth of the coral,
and prevents the mollusc from being entombed. The animal lives at the
free, or outer, end of the tube, and is thus continually shifting its
position, while the space it abandons becomes completely closed by a
mass of solid calcareous matter. Certain species of _Ovula_ inhabit
Gorgonia, assuming the colour, yellow or red, of their host, and, in
certain cases, developing, probably for prehensile purposes, a pointed
extension of the two extremities of the shell. _Pedicularia_, a form
akin to _Cypraea_, but with a more patulous mouth, inhabits the common
_Corallium rubrum_ of the Mediterranean, and another species has been
noticed by Graeffe[163] on _Melithaea ochracea_ in Fiji.

On _Echinodermata_.--(_a_) _Crinoidea._ _Stylina comatulicola_ lives on
_Comatula mediterranea_, fixed to the outer skin, which it penetrates
by a very long proboscis; the shell is quite transparent.[164] A
curious case of a fossil parasite has been noticed by Roberts.[165]
A _Calyptrea_-shaped shell named _Platyceras_ always occurred on the
ventral side of a crinoid, encompassed by the arms. For some time this
was thought to afford conclusive proof of the rapacity and carnivorous
habits of the echinoderm, which had died in the act of seizing its
prey. Subsequent investigations, however, showed that in all the
cases noticed (about 150) the _Platyceras_ covered the anal opening
of the crinoid in such a way that the mouth of the mollusc must have
been directly over the orifice of the anus. (_b_) _Asteroidea._ The
comparatively soft texture of the skin of the starfishes renders
them a favourite home of various parasites. The brothers Sarasin
noticed[166] a species of _Stilifer_ encysted on the rays of _Linckia
multiformis_. Each shell was enveloped up to the apex, which just
projected from a hole at the top of the cyst. The proboscis was long,
and at its base was a kind of false mantle, which appeared to possess
a pumping action. On the under side of the rays of the same starfish
occurred a capuliform mollusc (_Thyca ectoconcha_), furnished with a
muscular plate, whose cuticular surface was indented in such a way
as to grip the skin of the _Linckia_. This plate was furnished with
a hole, through which the pharynx projected into the texture of the
starfish, acting as a proboscis and apparently furnished with a kind
of pumping or sucking action. Adams and Reeve[167] describe _Pileopsis
astericola_ as living ‘on the tubercle of a starfish,’ and _Stilifer
astericola_, from the coast of Borneo, as ‘living in the body of a
starfish.’ In the British Museum there is a specimen of _Pileopsis
crystallina_ ‘in situ’ on the ray of a starfish, (_c_) On the brittle
starfishes (_Ophiuroidea_) occur several species of _Stiliferina_.
(_d_) _Echinoidea._ Various species of _Stilifer_ occur on the ventral
spines of echinoids, where they probably subsist on the excreta, and
are sometimes found imbedded in the spines themselves. _St. Turtoni_
occurs on the British coasts on several species of _Echinus_, and
_Montacuta substriata_ frequents _Spatangus purpureus_ and certain
species of _Echinocardium_, _Cidaris_, and _Brissus_. _Lepton
parasiticum_ has been described from Kerguelen I. on a _Hemiaster_,
and a new genus, _Robillardia_, has recently been established[168]
for a _Hyalinia_-shaped shell, parasitic on an _Echinus_ from
Mauritius. (_e_) _Holothurioidea._ The ‘sea-cucumbers’ afford lodgment
to a variety of curious forms, some of which have experienced such
modifications that their generic position is by no means established.
_Entoconcha_ occurs fixed by its buccal end to the blood-vessels of
certain _Synapta_ in the Mediterranean and the Philippines. _Entocolax_
has been dredged from 180 fath. in Behring’s Straits, attached by its
head to certain anterior muscles of a _Myriotrochus_.[169] A curious
case of parasitism is described by Voeltzkow[170] as occurring on
a _Synapta_ found between tide-marks on the I. of Zanzibar. In the
oesophagus of the _Synapta_ was found a small bivalve (_Entovalva_),
the animal of which was very large for its shell, and almost entirely
enveloped the valves by its mantle. As many as five specimens occurred
on a single _Synapta_. In the gut of the same Holothurian lived a small
univalve, not creeping freely, but fixed to a portion of the stomach
wall by a very long proboscis which pierced through it into the body
cavity. This proboscis was nearly three times as long as the animal,
and the forward portion of it was set with sharp thorns, no doubt in
order to enable it to retain its hold and resist evacuation. Various
species of _Eulima_ have been noticed in every part of the world, from
Norway to the Philippines, both inside and outside Holothurians.[171]
_Stilifer_ also occurs on this section of Echinoderms.[172]

On _Annelida_.--_Cochliolepis parasiticus_ has been noticed under
the scales of _Acoetes lupina_ (a kind of ‘sea-mouse’) in Charleston
Harbour.[173]

On _Crustacea_.--A mussel, ⅜ in. long, has been found[174] living
under the carapace of the common shore-crab (_Carcinus maenas_), and
one case has been noticed[175] where two mussels, one of several
months’ growth, the other smaller, well secured by their byssi, were
found under the abdomen of the same species, in such a position as to
force the appendages apart and askew. These, however, are not so much
cases of parasitism as of involuntary habitat, the mussel no doubt
having become involved in the branchiae and the abdomen of the crab in
the larval form.

On _Mollusca_.--A species of _Odostomia_ (_pallida_ Mont.) is found on
our own coasts on the ‘ears’ of _Pecten maximus_, and also[176] on the
operculum of _Turritella communis_. Another species (_O. rissoides_)
frequently occurs in hiding under beds of mussels, but it is not clear
whether the habitat is due to parasitism, or simply to the fact that
the mass of mussels, knitted together and to the rock by the byssi,
affords the _Odostomia_ a safe lurking-place. At Panama the present
writer found _Crepidula_ (2 sp.) plentiful on the opercula of the
great _Strombus galea_ and of _Cerithium irroratum_. In each case the
parasite exactly fitted the size of the operculum, and had assumed its
colour, dark brown or chestnut. _Amalthea_ is very commonly found on
_Conus_, _Turbo_, and other large shells from the South Pacific, but
this is probably not a case of parasitism, but simply of convenience of
habitat, just as young oysters are frequently seen on the carapace and
even on the legs of large crabs.

  [Illustration: FIG. 30.--_Crepidula onyx_ Sowb., parasitic on the
  operculum of _Strombus galeatus_ Swains., Panama.]

On _Tunicata_.--_Lamellaria_ deposits its eggs and lives on an Ascidian
(_Leptoclinum_), and the common _Modiolaria marmorata_ lives in
colonies imbedded in the test of _Ascidia mentula_ and other simple
Ascidians.

  [Illustration: FIG. 31.--Two species of _Eulima_: =A= is sessile
  on the skin of a Holothurian, through which it plunges its
  sucking proboscis (_Pr_); =B= creeps freely in the stomach of a
  Holothurian. (After K. Semper.)]

Special points of interest with regard to parasitic Mollusca relate
to (1) _Colour_. This is in most cases absent, the shell being of
a uniform hyaline or milky white. This may be due, in the case of
the endo-parasitic forms, to absence of light, and possibly, in
those living outside their host, to some deficiency in the nutritive
material. A colourless shell is not necessarily protective, for
though a transparent shell might evade detection, a milk-white hue
would probably be conspicuous. (2) _Modifications of structure._
These are in many cases considerable. _Entoconcha_ and _Entocolax_
have no respiratory or circulatory organs, and no known nervous
system; _Thyca_ and certain _Stilifer_ possess a curious suctorial
apparatus; the foot in many cases has aborted, since the necessity for
locomotion is reduced to a minimum, and its place is supplied by an
enormous development of the proboscis, which enables the creature to
provide itself with nutriment without shifting its position. K. Semper
notices a case where a _Eulima_, whose habitat is the stomach of a
Holothurian, retains the foot unmodified, while a species occurring
on the outer skin, but provided with a long proboscis, has lost its
foot altogether.[177] Special provision for holding on is noticed in
certain cases, reminding us of similar provision in human parasites.
Eyes are frequently, but not always wanting, even in endo-parasitic
forms. A specially interesting modification of structure occurs in
(3) the _Radula_ or ribbon-shaped arrangement of the teeth. In most
cases of parasitism (_Eulima_, _Stilifer_, _Odostomia_, _Entoconcha_,
_Entocolax_, _Magilus_, _Coralliophila_, _Leptoconcha_) it is absent
altogether. In _Ovula_ and _Pedicularia_, genera which are in all
other respects closely allied to _Cypraea_, the radula exhibits marked
differences from the typical radula of the Cypraeidae. The formula
(3·1·3) remains the same, but the laterals are greatly produced
and become fimbriated, sometimes at the extremity only, sometimes
along the whole length. A very similar modification occurs in the
radula of _Sistrum spectrum_ Reeve, a species which is known to live
parasitically on one of the branching corals. Here the laterals differ
from those of the typical _Purpuridae_ in being very long and curved at
the extremity. The general effect of these modifications appears to be
the production of a radula rather of the type of the vegetable-feeding
_Trochidae_, which may perhaps be regarded as a link in the chain of
gradually-degraded forms which eventually terminate in the absence of
the organ altogether. The softer the food, the less necessity there
is for strong teeth to tear it; the teeth either become smaller and
more numerous, or else longer and more slender, and eventually pass
away altogether. It is curious, however, that the same modified form
of radula should appear in species of _Ovula_ (_e.g._ _ovum_) and that
the same absence of radula should occur in species of _Eulima_ (_e.g._
_polita_) known to be not parasitic. This fact perhaps points back to
a time when the ancestral forms of each group are parasitic and whose
radulae were modified or wanting, the modification or absence of that
organ being continued in some of their non-parasitical descendants.


                             =Commensalism=

Mollusca are concerned in several interesting cases of commensalism,
or the habitual association of two organisms, as distinguished from
parasitism, where one form preys more or less upon the other.

Mr. J. T. Marshall has given[178] an interesting account of the
association of _Montacuta ferruginosa_ with _Echinocardium cordatum_.
The Echinoderm lives in muddy sand in Torbay, at a depth of about 6
inches, and the _Montacuta_ lives in a burrow leading from its ventral
end and running irregularly in a sloping direction for 3 or 4 inches,
the burrow, which is made by a current from the Echinoderm, being
almost exactly the width of the _Montacuta_. The _Montacuta_ were
always arranged in the burrows in order of size, the largest being
close to the Echinoderm, and the smallest of a string of about six at
the other end of the burrow. In another part of S. Devon, where the
sand was soft and sloppy, the Echinocardia rise to the surface and
travel along the sand; in this case the _Montacuta_ were attached to
their host by means of a byssus, and were dragged along as it travelled.

The Rev. Dr. Norman has noted[179] a somewhat similar habitat for
_Lepton squamosum_. This rare little British species was found at
Salcombe, living in the burrows of _Gebia stellata_, in all probability
feeding upon the secretions from the body of the crustacean. Dr.
Norman suggests that the extreme flatness of the shell of the _Lepton_
is of great advantage in enabling it not to get in the way of the
_Gebia_ as he scuttles up and down his burrow. Another species of
_Lepton_ is found on the coast of Florida in a precisely similar
locality,[180] while a third species, occurring on the Oregon and
California coasts, actually attaches itself to the inner surface of the
abdomen of a _Gebia_.[181]

  [Illustration: FIG. 32.--_Ephippodonta Macdougalli_ Tate, S.
  Australia. =A=, Burrow of prawn, the X indicating the position of
  the mollusc; _sp_, sponge. =B=, Ventral view of _Ephippodonta_;
  _by_, byssus; _f_, foot; _m_, mantle; _mm_, fused mantle borders.
  =C=, View of interior of shells; _h_, hinge; m´m´, adductor
  muscles. (=A= × ½; =B= and =C= × 2.)]

A very singular case of commensalism has been recently discovered
with regard to a genus of Australian bivalve shells, _Ephippodonta_.
This genus is never found except in the burrow of a species of prawn
(_Axius plectorhynchus_ Str.). For some reason at present unexplained,
the burrow of this particular prawn appears to be exceedingly popular
as a habitat for certain bivalves, for, besides two species of
_Ephippodonta_, a _Kellia_ and three _Mylitta_ are found there, and
there alone. Sometimes the prawn, when the rock is hard, builds a
tunnel of mud upon it, at other times it excavates the soft calciferous
sandstone. “This burrow is lined with a tenacious brown mud, composed
of excrementitious matter; and, in addition to the mud lining, there
is always more or less present an orange-coloured sponge which I have
never found elsewhere. Upon the mud or sponge, and adhering very
closely, are found the _Ephippodonta_. They quickly form a pit-like
depression by means of their foot, and appear almost covered by the
mud.” During the winter months (March-July) the prawn appears to fill
his burrow, possibly as a provision against stormy weather, with large
quantities of minced seaweed, underneath which immense numbers of
very young _Ephippodonta_ are found living.[182] The extreme flatness
of the _Ephippodonta_ must be due to the same cause as the flatness
of the _Lepton_ noticed above, namely, the necessity of not impeding
or interfering with the lively motions of the prawn. In the case of
_Lepton_ the two valves close completely and the shell is still very
flat; in _Ephippodonta_, on the other hand, the same result is produced
by the valves being opened to their widest possible extent. As in
_Entovalva_, a continuation of the mantle covers the outer surface of
the shell.


                               =Variation=

It is a familiar experience to the student, not only of the Mollusca,
but of every branch of animal or vegetable life, to come across
examples which exhibit certain slight deviations from the type
form as usually understood. These deviations may be more or less
pronounced, but, as a rule, a series of forms can be discovered,
gradually leading up to or down from the type. The definition of what
constitutes a species,--and, still more, the rigid application of
such definition--will always remain a difficult task, so long as the
personal element persists in him who defines.[183] What seems to one
authority ample ground for distinction of species, another may regard
as of comparatively trivial importance. The practical outcome of these
divergent views is sufficiently illustrated by the attitude of Mr. F.
P. Marrat on the one hand, and of what may be called the modern French
school of conchologists on the other. Mr. Marrat holds, or held, that
the great genus _Nassa_, of which more than 150 species are generally
recognised, is one shell (species) in an endless variety of forms. The
modern French school go to the other extreme, and apparently proceed
upon the view that almost any difference in form, however slight, is
sufficient to constitute a separate species.

It will be generally admitted, however, that some _structural
difference_ in the organisation of the animal (as distinct from that
of the shell alone) is necessary for the permanent constitution of
specific rank.[184] What _amount_ of structural difference is required,
what particular organ or organs must exhibit this difference, will
depend largely upon the idiosyncrasy of the observer. But if this, or
something like this definition of a species be accepted, it will follow
that a so-called ‘variety’ will be a form which exhibits differences
from the type which do not amount to permanent structural differences
in the organisation of the animal. The final court of appeal as to what
affords sufficient evidence for ‘permanent structural differences’ will
have to be, as with Aristotle of old, the judgment of the educated man.

It is, however, more to our present purpose to discuss the _causes of
variation_ than to lay down definitions of what variation is. One of
the most obvious causes of variation lies in a change or changes in
the environment. If we may assume, for the moment, that the type form
of a species is the form which is the mean of all the extremes, and
that this form is the resultant of all the varied forces brought to
bear upon it, whether of food, climate, temperature, competition of
numbers, soil, light, amount of water, etc., it will follow that any
change in one or more of these forces, if continuous and considerable,
any change, in other words, of the environment, will produce its
effect upon the organism in question. And this effect will be for
the better or for the worse, according to the particular nature of
the change itself as tending towards, or away from, the _optimum_ of
environment for the species concerned. Hence may be produced varieties,
more or less marked according to the gravity of the change, although
it must be noted that at times a change apparently unimportant from
our point of view, will produce very marked results upon the species.
It is indeed scarcely possible to predict with any certainty, in the
present state of our knowledge (beyond certain broad results) what will
be the particular effect upon a species of any given change in its
surroundings.


=Effects of Change in the Environment as tending to produce Variation.=

(_a_) _Changes in Climate, Temperature, Elevation, etc._--In the
eastern basin of the Baltic the marine Mollusca are much more stunted
than in the western.[185] For instance, _Mytilus edulis_ near Kiel
is 8–9 cm. long, while near Gothland it only attains a length of
3–4 cm. Mollusca living at only a shallow depth (_e.g._ _Tellina
balthica_, _Mya arenaria_, _Cardium edule_) do not differ much in
size in different parts of the Baltic, but in the far eastern basin
the calcareous layers of the shells of _Mya arenaria_ and _Tellina
balthica_ are extraordinarily thin, and disappear very rapidly
after death, leaving only the cuticular membrane, still united by
the ligament, in a perfect state of preservation. These remarkable
variations are no doubt to a large extent due to the violent changes
of temperature which are experienced in the Baltic, and by which the
steady development of the animals in question is interrupted and
thrown out of gear. The same species occur on the coasts of Greenland
and Iceland, where they attain a considerably larger size than in the
Baltic, in spite of the lower mean temperature, probably because their
development is not interrupted by any sudden change from cold to heat
or _vice versâ_.

Karl Semper has shown that _Limnaea stagnalis_ is developed, lives
and feeds best in a mean temperature of about 20° C. (= 68° F.). This
mean, however, must not be the mean of two distant extremes, for the
_Limnaea_ cannot digest its food and grow in a temperature which is
less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C.
(= 86° to 90° F.). In certain localities, therefore, the interruption
to the growth of this species must be serious and prolonged, and may
tend towards the production of more or less dwarfed varieties. Thus
specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the
sea, are permanently dwarfed, and have a very thin and fragile shell.
_Limnaea peregra_ in the Pyrenees, Alps, and Himalayas is generally of
a very delicate form and dwarfed habit, while the small variety known
as _lacustris_ occurs, according to Jeffreys, only in mountain lakes in
Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr.
Bateson from lakes near the Sea of Aral, which are salt for some months
and comparatively fresh for others, exhibit clearly the effect of
changes in the environment (Figs. 33 and 34). Excess of heat produces
similar results to excess of cold. _L. peregra_ var. _thermalis_,
found in the warm springs of the Pyrenees and the Vosges, and the var.
_geisericola_, from the hot water of the Iceland geysers, are alike
thin and dwarfed forms.

Many instances may be given of ‘varieties due to locality.’ In some of
these, the cause which predisposes towards variation can be inferred
with some approach to certainty, in others we must be content to note
the fact, without at present being able to perceive its explanation.

  [Illustration: FIG. 33.--Four examples of _Limnaea peregra_
  Müll., from salt marshes near the Sea of Aral, showing different
  effects produced by abnormal conditions of life.]

  [Illustration: FIG. 34.--Four examples of _Limnaea stagnalis_
  L., from marshes in the Aral district which are salt for several
  months in the year, illustrating variation produced by changes in
  the environment. × ½.]

Desert specimens of widely distributed species, _e.g._ _Helix pomatia_,
_H. niciensis_, _H. pisana_, _Leucochroa candidissima_ are much thicker
than the type, and tend to lose all trace of coloured bands. These
modifications are clearly the means of preventing evaporation of
moisture, the dull white or grayish brown colour being calculated to
absorb the smallest possible amount of heat. Desert shells in all parts
of the world (_e.g._ N. Africa, Arabia, Central Asia, S. Africa, W.
America) have been noticed to exhibit these peculiarities.

A very singular case of the reverse process, _i.e._ the production
of darkened forms of shell through cold, has been noticed by Fischer
as characteristic of the marine shells of the west coast of South
America.[186] This melanism is especially noticeable in _Trochus_,
_Turbo_, _Chiton_, _Mitra_, and _Pleurotoma_, and is attested by the
specific names, not merely expressive of actual blackness (_e.g._
_nigerrimus_, _ater_, _atramentarius_, _maurus_), but also of a
generally lugubrious tone (_e.g._ _moestus_, _funebralis_, _tristis_,
_lugubris_, _luctuosus_). It is highly probable that this concurrence
of specific melanism (which stands quite alone in the world) is due
to the cold polar current which impinges on the Chilian coasts, for
the same genera occur on the opposite shores of the continent without
exhibiting any trace whatever of this mournful characteristic.

It is a well-known fact, attested by many observers, that our common
_Limax agrestis_ as well as the young of _Arion ater_ become decidedly
darker in summer than in winter. If these slugs were accustomed to
disport themselves in the sun, it might have been suggested that this
increased darkness of colour tended to absorb more of the heat rays.
But since this is not the case, the result is probably due to some
unexplained effect of higher temperature. According to Lessona and
Pollonera, the length of the keel in _Limax arborum_ varies greatly in
different parts of Italy, being shorter in specimens from low ground,
but much longer in those inhabiting more elevated regions. The longer
the keel, the more obscure the colouring becomes, so that in the
Upper Alps of Piedmont individuals are practically black. Roebuck has
observed that Scottish specimens of this same slug are much darker
and less translucent than English forms. According to Simroth, our
common black slug, _Arion ater_, is a northern type, which in more
southern latitudes assumes the form known as _A. rufus_. Similarly
_Limax maximus_ “in its northern form _cinereo-niger_ is almost wholly
black, but in the more genial climate of Italy develops a series of
brilliantly coloured and strikingly marked variations which have
received numerous distinctive names from Italian limacologists.”[187]
According to Scharff, however[188] (who regards the colours of slugs as
in the main protective), these dark forms are by no means exclusively
northern, being found equally on the parched plains of Spain and
Portugal, and in the bleak climate of Norway. The same authority
observes that similar forms occur both in the dry regions of E.
Germany, and in the very humid district of western Ireland.

It appears unquestionable that marine genera from high northern
latitudes are provided with shells of uniform colour, or whitish with a
pale brown epidermis; spots, bands, or stripes seldom occur. The arctic
forms of _Buccinum_, _Trophon_, _Chrysodomus_, _Margarita_, _Crenella_,
_Leda_, _Yoldia_, _Astarte_ illustrate this fact. In the more temperate
seas of Europe, colours tend on the whole to increase, although there
are certain genera (_e.g._ _Pecten_) which are not more brightly
coloured in Mediterranean than in Icelandic waters.

Land Mollusca inhabiting the mainland of a continent not unfrequently
become smaller when they have spread to adjacent islands where perhaps
the rainfall is less abundant or the soil and food-supply less nicely
adjusted to their wants. _Orthalicus undatus_ is decidedly larger on
the mainland of S. America than on the adjacent islands of Trinidad and
Grenada. Specimens of _Bulimulus exilis_ from Barbados are invariably
broader and more obese than those from S. Thomas, while those from the
volcanic island of S. Lucia, where lime is deficient, are small and
very slender. _Streptaxis deformis_, as occurring at Trinidad, is only
half the size of specimens from Georgetown, Demerara.[189]

Certain localities appear, for some unexplained reason, to be
particularly favourable to the production of albino varieties. The
neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen
of these forms of land Mollusca and five of fresh-water.[190]

Our common _Helix aspersa_, as found near Bristol, is said to be ‘dark
coloured’; about Western-super-mare ‘brown, with black markings’;
near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and
large.’[191] Sometimes the same kind of variation is exhibited
by different species in the same locality. Thus specimens of _H.
aspersa_, _H. nemoralis_, and _H. hortensis_, taken from the same
bank at Torquay, presented a straw-coloured tinge of ground colour,
with red-brown bands or markings. Trochiform _H. nemoralis_ and _H.
arbustorum_, sinistral _H. hortensis_ and _H. aspersa_, sinistral _H.
aspersa_ and _H. virgata_, and similarly banded forms of _H. caperata_
and _H. virgata_, have been taken together.[192]

The immediate neighbourhood of the sea appears frequently to have the
effect of dwarfing land Mollusca. Thus the var. _conoidea_ of _Helix
aspersa_, which is small, conical, with a compressed mouth, occurs
‘on sandhills and cliffs at the seaside.’ The varieties _conica_ and
_nana_ of _Helix hispida_ are found ‘near the sea.’ _Helix virgata_
is exceedingly small in similar localities, and tends to become
unicoloured. _H. caperata_ var. _Gigaxii_, a small depressed form,
occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact
opposite is the case, for _H. nemoralis_ var. _major_, which is
‘much larger’ than the type, occurs on ‘sandhills and downs’ and is
‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of
_Limnaea peregra_ known as _maritima_ appears to be confined to the
neighbourhood of the sea.

Dwarfing of the shell seems frequently to be the result of an elevated
locality, not perhaps so much as the direct consequence of purer air
and less barometric pressure, as of changes in the character of the
food supply and in the humidity of the air. Several species of _Helix_
have a variety _minor_ which is characteristic of an Alpine habitat.
_Helix arbustorum_ var. _alpestris_, which is scarcely two-thirds the
size of the type, occurs on the Swiss Alps in the region of perpetual
snow. Sometimes a very slight elevation is sufficient to produce the
dwarfed form. At Tenby the type form of _Helix pisana_ is scattered in
countless numbers over the sandhills just above high-water mark. At the
extreme western end of these sandhills rises abruptly to a height of
over 100 feet the promontory known as Giltar Head, the vegetation of
which is entirely distinct from that of the burrows below. There is a
colony of _H. pisana_ at the end of Giltar, all of which are devoid of
the characteristic markings of the typical form, and most are dwarfed
and stunted in growth.

Occasionally the same variety will be found to be produced by
surroundings of very different nature. Thus the var. _alpestris_, of
_H. arbustorum_ mentioned above, besides being characteristic of high
Alpine localities, also occurs abundantly in low marshes at Hoddesdon
on the River Lea. _Helix pulchella_ var. _costata_, according to
Jeffreys, is found in dry and sandy places, often under loose stones
and bricks on walls, while other authorities have noticed it in wet and
dry localities quite indifferently.

Sometimes the production of a variety may be traced to the intrusion
of some other organism. According to Brot, nine-tenths of the _Limnaea
peregra_ inhabiting a certain pond near Geneva, were, during one
season, afflicted with a malformation of the base of the columella.
This deformity coincided with the appearance, in the same waters, of
extraordinary numbers of _Hydra viridis_. The next season, when the
_Hydra_ disappeared, the next generation of _Limnaea_ was found to
have resumed its normal form.

It has been noticed that a form of _Helix caperata_ with a flattened
spire and wide umbilicus is restricted to tilled fields, especially the
borders of clover fields, while a form with a more elevated spire and
more compact whorls occurs exclusively in open downs and uncultivated
places. The Rev. S. S. Pearce accounts[194] for this divergence by the
explanation that the flatter spire enables the shell of the fields
to creep about more easily under the leaves or matted weeds, seldom
requiring to crawl up a stalk or stem, while on the short turf of the
downs and pastures the smaller and more rounded shell enables the
animal to manoeuvre in and out of the blades of grass, and even to
crawl up them with considerable activity. The same writer endeavours
to explain the causes which regulate the distribution of _H. caperata_
var. _ornata_. He found that this variety (dark bands on a white
ground) occurred almost exclusively on downs which were fed upon by
sheep, associated with the common or mottled form, while the latter
form alone occurred in localities where sheep were not accustomed to
feed. Assuming then, as is probably the case, that sheep, in the course
of their close pasturing, devour many small snails, he believes that
individuals of the more conspicuous form _ornata_ were more likely
to be noticed, and therefore avoided, by the sheep, than the mottled
form, which would more easily escape their observation. Hence the var.
_ornata_ is due to the advantage which strikingly coloured individuals
obtained owing to their conspicuous habit, as compared with the typical
form, which would be less readily detected.

(_b_) _Changes in Soil, Station, Character of Water, etc._--A
deficiency of lime in the composition of the soil of any particular
locality produces very marked effects upon the shells of the Mollusca
which inhabit it; they become small and very thin, occasionally almost
transparent. The well-known var. _tenuis_ of _Helix aspersa_ occurs
on downs in the Channel Islands where calcareous material is scarce.
For similar reasons, _H. arbustorum_ develops a var. _fusca_, which is
depressed, very thin, and transparent, at Scilly, and also at Lunna I.,
E. Zetland.

  [Illustration: FIG. 35.--19 specimens of _Purpura lapillus_ L.,
  Great Britain, illustrating variation.

   (1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined
   and coloured rock; (4) Herm, rather exposed; (5) Solent, very
   sheltered; (6) Land’s End, exposed rocks, small food supply;
   (7) Scilly, exposed rocks, fair food supply; (8) St. Leonards,
   flat mussel beds at extreme low water; (9) Robin Hood’s Bay,
   sheltered under boulders, good food supply; (10) Rhoscolyn, on
   oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed
   rocks; (12) Estuary of Conway, very sheltered, abundant food
   supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor
   food supply; (14) slightly monstrous; (15), (16), (17) Morthoe,
   rather exposed rocks, but abundant food supply; (18) St. Bride’s
   Bay; (19) L. Swilly, sheltered, but small food supply. All from
   the author’s collection, except (10).]

The common dog-whelk (_Purpura lapillus_) of our own coasts is an
exceedingly variable species, and in many cases the variations may be
shown to bear a direct relation to the manner of life (Fig. 35). Forms
occurring in very exposed situations, _e.g._ Land’s End, outer rocks of
the Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a
short spire and relatively large mouth, the latter being developed in
order to increase the power of adherence to the rock and consequently
of resistance to wave force. On the other hand, shells occurring in
sheltered situations, estuaries, narrow straits, or even on open coasts
where there is plenty of shelter from the waves, are comparatively
of great size, with a well-developed, sometimes produced spire, and
a mouth small in proportion to the area of shell surface. In the
accompanying figure, the specimens from the Conway estuary and the
Solent (12, 5) well illustrate this latter form of shell, while that
from exposed rocks is illustrated by the specimens from Robin Hood’s
Bay (13, 14). Had these specimens occurred alone, or had they been
brought from some distant and unexplored region, they must inevitably
have been described as two distinct species.

  [Illustration: FIG. 36.--Valves of _Cardium edule_ from the four
  upper terraces of Shumish Kul, a dry salt lake adjacent to the
  Aral Sea. (After Bateson.)]

Mr. W. Bateson has made[195] some observations on the shells of
_Cardium edule_ taken from a series of terraces on the border of
certain salt lakes which once formed a portion of the Sea of Aral.
As these lakes gradually became dry, the water they contained became
salter, and thus the successive layers of dead shells deposited on
their borders form an interesting record of the progressive variation
of this species under conditions which, in one respect at least, can
be clearly appreciated. At the same time the diminishing volume of
water, and the increasing average temperature, would not be without
their effect. It was found that the principal changes were as follows:
the thickness, and consequently the weight, of the shells became
diminished, the size of the beaks was reduced, the shell became highly
coloured, and diminished considerably in size, and the breadth of the
shells increased in proportion to their length (Fig. 36). Shells of
the same species of _Cardium_, occurring in Lake Mareotis, were found
to exhibit very similar variations as regards colour, size, shape, and
thickness.

_Unio pictorum_ var. _compressa_ occurs near Norwich at two similar
localities six or seven miles distant from one another, under
circumstances which tend to show that similar conditions have produced
similar results. The form occurs where the river, by bending sharply
in horse-shoe shape, causes the current to rush across to the opposite
side and form an eddy near the bank on the outside of the bend. Just
at the edge of the sharp current next the eddy the shells are found,
the peculiar form being probably due to the current continually washing
away the soft particles of mud and compelling the shell to elongate
itself in order to keep partly buried at the bottom.[196]

The rivers Ouse and Foss, which unite just below York, are rivers
of strikingly different character, the Ouse being deep, rapid, with
a bare, stony bottom, and little vegetable growth, and receiving a
good deal of drainage, while the Foss is shallow, slow, muddy, full
of weeds and with very little drainage. In the Foss, fine specimens
of _Anodonta anatina_ occur, lustrous, with beautifully rayed shells.
A few yards off, in the Ouse, the same species of _Anodonta_ is dull
brown in colour, its interior clouded, the beaks and epidermis often
deeply eroded. Precisely the same contrast is shown in specimens of
_Unio tumidus_, taken from the same rivers, Ouse specimens being also
slightly curved in form. Just above Yearsley Lock in the Foss, _Unio
tumidus_ occurs, but always dwarfed and malformed, a result probably
due to the effect of rapidly running water upon a species accustomed to
live in still water.[197] Simroth records the occurrence of remarkably
distorted varieties in two species of _Aetheria_ which lived in swift
falls of the River Congo.[198]

A variety of _Limnaea peregra_ with a short spire and rather strong,
stoutly built shell occurs in Lakes Windermere, Derwentwater, and
Llyn-y-van-fach. It lives adhering to stones in places where there are
very few weeds, its shape enabling it to withstand the surf of these
large lakes, to which the ordinary form would probably succumb.[199]

Scalariform specimens of _Planorbis_ are said to occur most commonly in
waters which are choked by vegetation, and it has been shown that this
form of shell is able to make its way through masses of dense weed much
more readily than specimens of normal shape.

Continental authorities have long considered _Limnaea peregra_ and
_L. ovata_ as two distinct species. Hazay, however, has succeeded in
rearing specimens of so-called _peregra_ from the ova of _ovata_, and
so-called _ovata_ from the ova of _peregra_, simply by placing one
species in running water, and the other in still water.

According to Mr. J. S. Gibbons[200] certain species of _Littorina_, in
tropical and sub-tropical regions, are confined to water more or less
brackish, being incapable of living in pure salt water. “I have met,”
says Mr. Gibbons, “with three of these species, and in each case they
have been distinguished from the truly marine species by the extreme
(comparative) thinness of their shells, and by their colouring being
richer and more varied; they are also usually more elaborately marked.
They are to be met with under three different conditions--(1) in
harbours and bays where the water is salt with but a slight admixture
of fresh water; (2) in mangrove swamps where salt and fresh water mix
in pretty equal volume; (3) on dry land, but near a marsh or the dry
bed of one.

“_L. intermedia_ Reeve, a widely diffused E. African shell, attaches
itself by a thin pellicle of dried mucus to grass growing by the margin
of slightly brackish marshes near the coast, resembling in its mode
of suspension the Old World _Cyclostoma_. I have found it in vast
numbers in situations where, during the greater part of the year,
it is exposed to the full glare of an almost vertical sun, its only
source of moisture being a slight dew at night-time. The W. Indian _L.
angulifera_ Lam., and a beautifully coloured E. African species (_? L.
carinifera_), are found in mangrove swamps; they are, however, less
independent of salt water than the last.”

Mr. Gibbons goes on to note that brackish water species (although not
so solid as truly marine species) tend to become more solid as the
water they inhabit becomes less salt. This is a curious fact, and the
reverse of what one would expect. Specimens of _L. intermedia_ on
stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are
much smaller, darker, and more fragile, than those living on grass a
few hundred yards away. _L. angulifera_ is unusually solid and heavy
at Puerto Plata (S. Domingo) among mangroves, where the water is in a
great measure fresh; at Havana and at Colon, where it lives on stakes
in water but slightly brackish, it is thinner and smaller and also
darker coloured.

(_c_) _Changes in the Volume of Water._--It has long been known that
the largest specimens, _e.g._ of _Limnaea stagnalis_ and _Anodonta
anatina_, only occurred in pieces of water of considerable size.
Recent observation, however, has shown conclusively that the volume of
water in which certain species live has a very close relation to the
actual size of their shells, besides producing other effects. _Lymnaea
megasoma_, when kept in an aquarium of limited size, deposited eggs
which hatched out; this process was continued in the same aquarium for
four generations in all, the form of the shell of the last generation
having become such that an experienced conchologist gave it as his
opinion that the first and last terms of the series could have no
possible specific relation to one another. The size of the shell
became greatly diminished, and in particular the spire became very
slender.[201]

The same species being again kept in an aquarium under similar
conditions, it was found that the third generation had a shell only
four-sevenths the length of their great grandparents. It was noticed
also that the sexual capacities of the animals changed as well. The
liver was greatly reduced, and the male organs were entirely lost.[202]

K. Semper conducted some well-known experiments bearing on this point.
He separated[203] specimens of _Limnaea stagnalis_ from the same mass
of eggs as soon as they were hatched, and placed them simultaneously in
bodies of water varying in volume from 100 to 2000 cubic centimetres.
All the other conditions of life, and especially the food supply, were
kept at the known optimum. He found, in the result, that the size of
the shell varied directly in proportion to the volume of the water
in which it lived, and that this was the case, whether an individual
specimen was kept alone in a given quantity of water, or shared it with
several others. At the close of 65 days the specimens raised in 100
cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9
mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in
2000 cubic cm. attained a length of 18 mm. (Fig. 37).

An interesting effect of a sudden fall of temperature was noticed by
Semper in connection with the above experiments. Vessels of unequal
size, containing specimens of the _Limnaea_, happened to stand before a
window at a time when the temperature suddenly fell to about 55° F. The
sun, which shone through the window, warmed the water in the smaller
vessels, but had no effect upon the temperature of the larger. The
result was, that the _Limnaea_ in 2000 cubic cm., which ought to have
been 10 mm. long when 25 days old, were scarcely longer, at the end of
that period, than those which had lived in the smaller vessels, but
whose water had been sufficiently warm.

   [Illustration: FIG. 37.--Four equally old shells of _Limnaea
  stagnalis_, hatched from the same mass of ova, but reared in
  different volumes of water: =A= in 100, =B= in 250, =C= in 600,
  and =D= in 2000 cubic centimetres. (After K. Semper.)]




                              CHAPTER IV

   USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION OF THE
    OYSTER, MUSSEL, AND SNAIL--SNAILS AS MEDICINE--PRICES GIVEN FOR
                                SHELLS


The employment of shells as a medium of exchange was exceedingly common
amongst uncivilised tribes in all parts of the world, and has by no
means yet become obsolete. One of the commonest species thus employed
is the ‘money cowry’ (_Cypraea moneta_, L.), which stands almost alone
in being used entire, while nearly all the other forms of shell money
are made out of portions of shells, thus requiring a certain amount of
labour in the process of formation.

One of the earliest mentions of the cowry as money occurs in an ancient
Hindoo treatise on mathematics, written in the seventh century A.D.
A question is propounded thus: ‘the ¼ of 1/16 of ⅕ of ¾ of ⅔ of ½ a
dramma was given to a beggar by one from whom he asked an alms; tell
me how many cowry shells the miser gave.’ In British India about 4000
are said to have passed for a shilling, but the value appears to differ
according to their condition, poor specimens being comparatively
worthless. According to Reeve[204] a gentleman residing at Cuttack is
said to have paid for the erection of his bungalow entirely in cowries.
The building cost him 4000 Rs. sicca (about £400), and as 64 cowries =
1 pice, and 64 pice = 1 rupee sicca, he paid over 16,000,000 cowries in
all.

Cowries are imported to England from India and other places for the
purposes of exportation to West Africa, to be exchanged for native
products. The trade, however, appears to be greatly on the decrease. At
the port of Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]

A banded form of _Nerita polita_ was used as money in certain parts of
the South Pacific. The sandal-wood imported into the China market is
largely obtained from the New Hebrides, being purchased of the natives
in exchange for _Ovulum angulosum_, which they especially esteem as an
ornament. Sometimes, as in the Duke of York group, the use of shell
money is specially restricted to certain kinds of purchase, being
employed there only in the buying of swine.

Among the tribes of the North-West coasts of America the common
_Dentalium indianorum_ used to form the standard of value, until it
was superseded, under the auspices of the Hudson’s Bay Company, by
blankets. A slave was valued at a fathom of from 25 to 40 of these
shells, strung lengthwise. Inferior or broken specimens were strung
together in a similar way, but were less highly esteemed; they
corresponded more to our silver and copper coins, while the strings of
the best shells represented gold.

The _wampum_ of the eastern coast of North America differed from all
these forms of shell money, in that it required a laborious process
for its manufacture. Wampum consisted of strings of cylindrical beads,
each about a quarter of an inch in length and half that breadth. The
beads were of two colours, white and purple, the latter being the more
valuable. Both were formed from the common clam, _Venus mercenaria_,
the valves of which are often stained with purple at the lower margins,
while the rest of the shell is white. Cut small, ground down, and
pierced, these shells were converted into money, which appears to have
been current along the whole sea-board of North America from Maine to
Florida, and on the Gulf Coast as far as Central America, as well as
among the inland tribes east of the Mississippi. Another kind of wampum
was made from the shells of _Busycon carica_ and _B. perversum_. By
staining the wampum with various colours, and disposing these colours
in belts in various forms of arrangement, the Indians were able to
preserve records, send messages, and keep account of any kind of event,
treaty, or transaction.

Another common form of money in California was _Olivella biplicata_,
strung together by rubbing down the apex. Button-shaped disks cut from
_Saxidomus arata_ and _Pachydesma crassatelloides_, as well as oblong
pieces of _Haliotis_, were employed for the same purpose, when strung
together in lengths of several yards.

“There is a curious old custom,” writes Mr. W. Anderson Smith,[206]
“that used formerly to be in use in this locality [the western coast
of Scotland], and no doubt was generally employed along the sea-board,
as the most simple and ready means of arrangement of bargains by a
non-writing population. That was, when a bargain was made, each party
to the transaction got one half of a bivalve shell--such as mussel,
cockle, or oyster--and when the bargain was implemented, the half that
fitted exactly was delivered up as a receipt! Thus a man who had a box
full of unfitted shells might be either a creditor or a debtor; but the
box filled with fitted shells represented receipted accounts. Those who
know the difficulty of fitting the valves of some classes of bivalves
will readily acknowledge the value of this arrangement.”

Shells are employed for use and for ornament by savage--and even by
civilised--tribes in all parts of the world. The natives of Fiji
thread the large _Turbo argyrostoma_ and _crenulatus_ as weights at
the edge of their nets, and also employ them as sinkers. A _Cypraea
tigris_ cut into two halves and placed round a stone, with two or three
showy _Oliva_ at the sides, is used as a bait for cuttles. _Avicula
margaritifera_ is cut into scrapers and knives by this and several
other tribes. Breast ornaments of _Chama_, grouped with _Solarium
perspectivum_ and _Terebra duplicata_ are common among the Fijians, who
also mount the _Avicula_ on a backing of whales’ teeth sawn in two,
for the same purpose. The great Orange Cowry (_Cypraea aurantiaca_)
is used as a badge of high rank among the chieftains. One of the
most remarkable Fijian industries is the working of whales’ teeth to
represent this cowry, as well as the commoner _C. talpa_, which is more
easily imitated.

Among the Solomon islanders, cowries are used to ornament their shields
on great field days, and split cowries are worn as a necklace, to
represent human teeth. Small bunches of _Terebellum subulatum_ are
worn as earrings, and a large valve of _Avicula_ is employed as a head
ornament in the centre of a fillet. The same islanders ornament the
raised prows of their canoes, as well as the inside of the stern-post,
with a long row of single _Natica_.

The native Papuans employ shells for an immense variety of purposes.
Circlets for the head are formed of rows of _Nassa gibbosula_, rubbed
down till little but the mouth remains. Necklaces are worn which
consist of strings of _Oliva_, young _Avicula_, _Natica melanostoma_,
opercula of _Turbo_, and valves of a rich brown species of _Cardium_,
pendent at the end of strings of the seeds known as Job’s tears.
_Struthiolaria_ is rubbed down until nothing but the mouth is left,
and worn in strings round the neck. This is remarkable, since
_Struthiolaria_ is not a native Papuan shell, and indeed occurs no
nearer than New Zealand. Sections of _Melo_ are also worn as a breast
ornament, dependent from a necklace of cornelian stones. _Cypraea
erosa_ is used to ornament drinking bowls, and _Ovulum ovum_ is
attached to the native drums, at the base of a bunch of cassowary
feathers, as well as being fastened to the handle of a sago-beater.

In the same island, the great _Turbo_ and _Conus millepunctatus_ are
ground down to form bracelets, which are worn on the biceps. The
crimson lip of _Strombus luhuanus_ is cut into beads and perforated
for necklaces. Village elders are distinguished by a single _Ovulum
verrucosum_, worn in the centre of the forehead. The thick lip of
_Cassis cornuta_ is ground down to form nose pieces, 4½ inches long.
Fragments of a shell called _Kaïma_ (probably valves of a large
_Spondylus_) are worn suspended from the ears, with little wisps of
hair twisted up and thrust through a hole in the centre. For trumpets,
_Cassis cornuta_, _Triton tritonis_, and _Ranella lampas_ are used,
with a hole drilled as a mouthpiece in one of the upper whorls. Valves
of _Batissa_, _Unio_, and _Mytilus_ are used as knives for peeling
yams. Spoons for scooping the white from the cocoa-nut are made from
_Avicula margaritifera_. _Melo diadema_ is used as a baler in the
canoes.[207]

In the Sandwich Islands _Melampus luteus_ is worn as a necklace, as
well as in the Navigator Islands. A very striking necklace, in the
latter group, is formed of the apices of a _Nautilus_, rubbed down to
show the nacre. The New Zealanders use the green opercula of a Turbo,
a small species of _Venus_, and _Cypraea asellus_ to form the eyes of
their idols. Fish-hooks are made throughout the Pacific of the shells
of _Avicula_ and _Haliotis_, and are sometimes strengthened by a
backing made of the columella of _Cypraea arabica_. Small axe-heads are
made from _Terebra crenulata_ ground down (Woodlark I.), and larger
forms are fashioned from the giant _Tridacna_ (Fiji).

Shells are used to ornament the elaborate cloaks worn by the women of
rank in the Indian tribes of South America. Specimens of _Ampullaria_,
_Orthalicus_, _Labyrinthus_, and _Bulimulus_ depend from the bottom
and back of these garments, while great _Bulimi_, 6 inches long, are
worn as a breast ornament, and at the end of a string of beads and
teeth.[208]

The chank-shell (_Turbinella rapa_) is of especial interest from
its connexion with the religion of the Hindoos. The god Vishnu is
represented as holding this shell in his hand, and the sinistral form
of it, which is excessively rare, is regarded with extraordinary
veneration. The chank appears as a symbol on the coins of some of the
ancient Indian Empires, and is still retained on the coinage of the
Rajah of Travancore.

The chief fishery of the chank-shell is at Tuticorin, on the Gulf of
Manaar, and is conducted during the N. E. monsoon, October-May. In
1885–86 as many as 332,000 specimens were obtained, the net amount
realised being nearly Rs.24,000. In former days the trade was much
more lucrative, 4 or 5 millions of specimens being frequently shipped.
The government of Ceylon used to receive £4000 a year for licenses to
fish, but now the trade is free. The shells are brought up by divers
from 2 or 3 fathoms of water. In 1887 a sinistral specimen was found at
Jaffna, which sold for Rs.700.[209] Nearly all the shells are sent to
Dacca, where they are sliced into bangles and anklets to be worn by the
Hindoo women.

Perhaps the most important industry which deals only with the shells of
Mollusca is that connected with the ‘pearl-oyster.’ The history of the
trade forms a small literature in itself. It must be sufficient here
to note that the species in question is not an ‘oyster,’ properly so
called, but an _Avicula_ (_margaritifera_ Lam.). The ‘mother-of-pearl,’
which is extensively employed for the manufacture of buttons, studs,
knife-handles, fans, card-cases, brooches, boxes, and every kind of
inlaid work, is the internal nacreous laminae of the shell of this
species. The most important fisheries are those of the Am Islands, the
Soo-loo Archipelago, the Persian Gulf, the Red Sea, Queensland, and the
Pearl Islands in the Bay of Panama. The shell also occurs in several
of the groups of the South Pacific--the Paumotu, Gambier and Navigator
Islands, Tahiti being the centre of the trade--and also on the coasts
of Lower California.[210]

Pearls are the result of a disease in the animal of this species of
_Avicula_ and probably in all other species within which they occur.
When the _Avicula_ is large, well formed, and with ample space for
individual development, pearls scarcely occur at all, but when the
shells are crowded together, and become humped and distorted, as
well as affording cover for all kinds of marine worms and parasitic
creatures, then pearls are sure to be found. Pearls of inferior value
and size are also produced by _Placuna placenta_, many species of
_Pinna_, the great _Tridacna_, the common _Ostrea edulis_, and several
other marine bivalves. They are not uncommon in _Unio_ and _Anodonta_,
and the common _Margaritana margaritifera_ of our rapid streams is
still said to be collected, in some parts of Wales, for the purpose of
extracting its small ‘seed-pearls.’ Pink pearls are obtained from the
giant conch-shell of the West Indies (_Strombus gigas_), as well as
from certain _Turbinella_.

In Canton, many houses are illuminated almost entirely by skylights and
windows made of shells, probably the semitransparent valves of _Placuna
placenta_. In China lime is commonly made of ground cockle-shells,
and, when mixed with oil, forms an excellent putty, used for cementing
coffins, and in forming a surface for the frescoes with which the
gables of temples and private houses are adorned. Those who suffer from
cutaneous diseases, and convalescents from small-pox, are washed in
Canton with the water in which cockles have been boiled.[211]

A recent issue of the Peking Gazette contains a report from the
outgoing Viceroy of Fukhien, stating that he had handed over the
insignia of office to his successor, including _inter alia_ the
conch-shell bestowed by the Throne. A conch-shell with a whorl turning
to the _right_, _i.e._ a sinistral specimen, is supposed when blown to
have the effect of stilling the waves, and hence is bestowed by the
Emperor upon high officers whose duties oblige them to take voyages by
sea. The Viceroy of Fukhien probably possesses one of these shells in
virtue of his jurisdiction over Formosa, to which island periodical
visits are supposed to be made.[212]

Shells appear to be used occasionally by other species besides man.
Oyster-catchers at breeding time prepare a number of imitation nests
in the gravel on the spit of land where they build, putting bits of
white shell in them to represent eggs.[213] This looks like a trick
in order to conceal the position of the true nest. According to
Nordenskjöld, when the eider duck of Spitzbergen has only one or two
eggs in its nest, it places a shell of _Buccinum glaciale_ beside
them. The appropriation of old shells by hermit-crabs is a familiar
sight all over the world. Perhaps it is most striking in the tropics,
where it is really startling, at first experience, to meet--as I have
done--a large _Cassis_ or _Turbo_, walking about in a wood or on a hill
side at considerable distances from the sea. A Gephyrean (_Phascolion
strombi_) habitually establishes itself in the discarded shells of
marine Mollusca. Certain Hymenoptera make use of dead shells of _Helix
hortensis_ in which they build their cells.[214] Magnus believes that
in times when heavy rains prevail, and the usual insects do not venture
out, certain flowers are fertilised by snails and slugs crawling over
them, _e.g._ _Leucanthemum vulgare_ by _Limax laevis_.[215]


=Mollusca as Food for Man.=--Probably there are few countries in the
world in which less use is made of the Mollusca as a form of food than
in our own. There are scarcely ten native species which can be said to
be at all commonly employed for this purpose. Neighbouring countries
show us an example in this respect. The French, Italians, and Spanish
eat _Natica_, _Turbo_, _Triton_, and _Murex_, and, among bivalves,
_Donax_, _Venus_, _Lithodomus_, _Pholas_, _Tapes_, and _Cardita_,
as well as the smaller Cephalopoda. Under the general designation
of _clam_ the Americans eat _Venus mercenaria_, _Mya arenaria_,
and _Mactra solidissima_. In the Suez markets are exposed for sale
_Strombus_ and _Melongena_, _Avicula_ and _Cytherea_. At Panama _Donax_
and _Solen_ are delicacies, while the natives also eat the great
_Murex_ and _Pyrula_, and even the huge _Arca grandis_, which lives
embedded in the liquid river mud.

The common littoral bivalves seem to be eaten in nearly all countries
except our own, and it is therefore needless to enumerate them. The
Gasteropoda, whose habits are scarcely so cleanly, seem to require a
bolder spirit and less delicate palate to venture on their consumption.

The Malays of the East Indian islands eat _Telescopium fuscum_ and
_Pyrazus palustris_, which abound in the mangrove swamps. They throw
them on their wood fires, and when they are sufficiently cooked, break
off the top of the spire and suck the animal out through the opening.
_Haliotis_ they take out of the shell, string together, and dry in the
sun. The lower classes in the Philippines eat _Arca inaequivalvis_,
boiling them as we do mussels.[216] In the Corean islands a species
of _Monodonta_ and another of _Mytilus_ are quite peppery, and bite
the tongue; our own _Helix revelata_, as I can vouch from personal
experience, has a similar flavour. _Fusus colosseus_, _Rapana bezoar_,
and _Purpura luteostoma_ are eaten on the southern coasts of China;
_Strombus luhuanus_, _Turbo chrysostomus_, _Trochus niloticus_, and
_Patella testudinaria_, by the natives of New Caledonia; _Strombus
gigas_ and _Livona pica_ in the West Indies; _Turbo niger_ and
_Concholepas peruvianus_ on the Chilian coasts; four species of
_Strombus_ and _Nerita_, one each of _Purpura_ and _Turbo_, besides two
_Tridacna_ and one _Hippopus_, by the natives of British New Guinea.
West Indian negroes eat the large Chitons which are abundant on their
rocky coasts, cutting off and swallowing raw the fleshy foot, which
they call ‘beef,’ and rejecting the viscera. Dried cephalopods are a
favourite Chinese dish, and are regularly exported to San Francisco,
where the Chinamen make them into soup. The ‘Challenger’ obtained two
species of _Sepia_ and two of _Loligo_ from the market at Yokohama.

The insipidity of fresh-water Mollusca renders them much less desirable
as a form of food. Some species of Unionidae, however, are said to
be eaten in France. _Anodonta edulis_ is specially cultivated for
food in certain districts of China, and the African _Aetheriae_ are
eaten by negroes. _Navicella_ and _Neritina_ are eaten in Mauritius,
_Ampullaria_ and _Neritina_ in Guadeloupe, and _Paludina_ in Cambodia.

The vast heaps of empty shells known as ‘kitchen-middens,’ occur in
almost every part of the world. They are found in Scotland, Denmark,
the east and west coasts of North America, Brazil, Tierra del Fuego,
Australia and New Zealand, and are sometimes several hundred yards
in length. They are invariably composed of the edible shells of the
adjacent coast, mixed with bones of Mammals, birds, and fish. From
their great size, it is believed that many of them must have taken
centuries to form.

Pre-eminent among existing shell-fish industries stands the cultivation
of the oyster and the mussel, a more detailed account of which may
prove interesting.

The cultivation of the oyster[217] as a luxury of food dates at least
from the gastronomic age of Rome. Every one has heard of the epicure
whose taste was so educated that

                                    “he could tell
    At the first mouthful, if his oysters fed
    On the Rutupian or the Lucrine bed
    Or at Circeii.”[218]

The first artificial oyster-cultivator on a large scale appears to have
been a certain Roman named Sergius Orata, who lived about a century
B.C. His object, according to Pliny the elder,[219] was not
to please his own appetite so much as to make money by ministering
to the appetites of others. His _vivaria_ were situated on the
Lucrine Lake, near Baiae, and the Lucrine oysters obtained under his
cultivation a notoriety which they never entirely lost, although
British oysters eventually came to be more highly esteemed. He must
have been a great enthusiast in his trade, for on one occasion when he
became involved in a law-suit with one of the riparian proprietors,
his counsel declared that Orata’s opponent made a great mistake if he
expected to damp his ardour by expelling him from the lake, for, sooner
than not grow oysters at all, he would grow them upon the roof of his
house.[220] Orata’s successors in the business seem to have understood
the secret of planting young oysters in new beds, for we are told that
specimens brought from Brundisium and even from Britain were placed for
a while in the Lucrine Lake, to fatten after their long journey, and
also to acquire the esteemed “Lucrine flavour.”

Oysters are ‘in season’ whenever there is an ‘r’ in the month, in other
words, from September to April. ‘Mensibus erratis,’ as the poet has
it, ‘vos ostrea manducatis!’ It has been computed that the quantity
annually produced in Great Britain amounts to no less than sixteen
hundred million, while in America the number is estimated at five
thousand five hundred million, the value being over thirteen million
dollars, and the number of persons employed fifty thousand. Arcachon,
one of the principal French oyster-parks, has nearly 10,000 acres of
oyster beds, the annual value being from eight to ten million francs;
in 1884–85, 178,359,000 oysters were exported from this place alone. In
the season 1889–90, 50,000 tons of oysters were consumed in London.

Few will now be found to echo the poet Gay’s opinion:

    “That man had sure a palate covered o’er
    With brass or steel, that on the rocky shore
    First broke the oozy oyster’s pearly coat,
    And risq’d the living morsel down his throat.”

There were halcyon days in England once, when oysters were to be
procured at 8d. the bushel. Now it costs exactly that amount before
a bushel, brought up the Thames, can even be exposed for sale at
Billingsgate (4d. porterage, 4d. market toll), and prime Whitstable
natives average from 3½d. to 4d. each. The principal causes of this
rise in prices, apart from the increased demand, are (1) over-dredging;
(2) ignorant cultivation, and to these may be added (3) the effect of
bad seasons in destroying young oysters, or preventing the spat from
maturing. Our own principal beds are those at Whitstable, Rochester,
Colchester, Milton (famous for its ‘melting’ natives), Faversham,
Queenborough, Burnham, Poole, and Carlingford in Co. Down, and
Newhaven, near Edinburgh.

The oyster-farms at Whitstable, public and private, extend over an
area of more than 27 square miles. The principal of these is a kind of
joint-stock company, with no other privilege of entrance except birth
as a free dredgeman of the town. When a holder dies, his interest dies
with him. Twelve directors, known as “the Jury,” manage the affairs of
the company, which finds employment for several thousand people, and
sometimes turns over as much as £200,000 a year. The term ‘Natives,’
as applied to these Whitstable or to other English oysters, requires a
word of explanation. A ‘Native’ oyster is simply an oyster which has
been bred on or near the Thames estuary, but very probably it may be
developed from a brood which came from Scotland or some other place at
a distance. For some unexplained reason, oysters bred on the London
clay acquire a greater delicacy of flavour than elsewhere. The company
pay large sums for brood to stock their own grounds, since there can be
no certainty that the spat from their own oysters will fall favourably,
or even within their own domains at all. Besides purchases from other
beds, the parks are largely stocked with small oysters picked up along
the coast or dredged from grounds public to all, sometimes as much
as 50s. a bushel being paid for the best brood. It is probably this
system of transplanting, combined with systematic working of the beds,
which has made the Whitstable oyster so excellent both as to quality
and quantity of flesh. The whole surface of the ‘layings’ is explored
every year by the dredge, successive portions of the ground being
gone over in regular rotation, and every provision being made for the
well-being of the crop, and the destruction of their enemies. For three
days of every week the men dredge for ‘planting,’ _i.e._ for the
transference of suitable specimens from one place to another, the
separation of adhering shells, the removal of odd valves and of every
kind of refuse, and the killing off of dangerous foes. On the other
three days they dredge for the market, taking care only to lift such a
number as will match the demand.

The Colne beds are natural beds, as opposed to the majority of the
great working beds, which are artificial. They are the property of
the town of Colchester, which appoints a water-bailiff to manage the
concern. Under his direction is a jury of twelve, who regulate the
times of dredging, the price at which sales are to be made, and are
generally responsible for the practical working of the trade. Here, and
at Faversham, Queenborough, Rochester, and other places, ‘natives’ are
grown which rival those of Whitstable.

There can be no question, however, that the cultivation of oysters by
the French is far more complete and efficient than our own, and has
reached a higher degree of scientific perfection combined with economy
and solid profits. And yet, between 40 and 50 years ago, the French
beds were utterly exhausted and unproductive, and showed every sign
of failure and decay. It was in 1858 that the celebrated beds on the
Ile de Ré, near Rochelle, were first started. Their originator was
a certain shrewd stone-mason, by name Boeuf. He determined to try,
entirely on his own account, whether oysters could not be made to grow
on the long muddy fore-shore which is left by the ebb of the tide.
Accordingly, he constructed with his own hands a small basin enclosed
by a low wall, and placed at the bottom a number of stones picked out
of the surrounding mud, stocking his ‘parc’ with a few bushels of
healthy young brood. The experiment was entirely successful, in spite
of the jeers of his neighbours, and Boeuf’s profits, which soon began
to mount up at an astonishing rate, induced others to start similar
or more extensive farms for themselves. The movement spread rapidly,
and in a few years a stretch of miles of unproductive mud banks was
converted into the seat of a most prosperous industry. The general
interests of the trade appear to be regulated in a similar manner to
that at Whitstable; delegates are appointed by the various communities
to watch over the business as a whole, while questions affecting the
well-being of oyster-culture are discussed in a sort of representative
assembly.

At the same time as Boeuf was planting his first oysters on the shores
of the Ile de Ré, M. Coste had been reporting to the French government
in favour of such a system of ostreiculture as was then practised
by the Italians in the old classic Lakes Avernus and Lucrinus. The
principle there adopted was to prevent, as far as possible, the escape
of the spat from the ground at the time when it is first emitted by
the breeding oyster. Stakes and fascines of wood were placed in such
a position as to catch the spat and give it a chance of obtaining a
hold before it perished or was carried away into the open sea. The old
oyster beds in the Bay of St. Brieuc were renewed on this principle,
banks being constructed and overlaid with bundles of wood to prevent
the escape of the new spat. The attempt was entirely successful, and
led to the establishment or re-establishment of those numerous parcs,
with which the French coast is studded from Brest to the Gironde. The
principal centres of the industry are Arcachon, Auray, Cancale, and la
Teste.

It is at Marennes, in Normandy, that the production of the celebrated
‘green oyster’ is carried out, that especial luxury of the French
epicure. Green oysters are a peculiarly French taste, and, though they
sometimes occur on the Essex marshes, there is no market for them in
England. The preference for them, on the continent, may be traced back
as early as 1713, when we find a record of their having been served
up at a supper given by an ambassador at the Hague. Green oysters are
not always green, it is only after they are placed in the ‘claires,’
or fattening ponds, that they acquire the hue; they never occur in
the open sea. The green colour does not extend over the whole animal,
but is found only in the branchiae and labial tentacles, which are
of a deep blue-green. Various theories have been started to explain
the ‘greening’ of the mollusc; the presence of copper in the tanks,
the chlorophyll of marine algae, an overgrowth of some parasite, a
disease akin to liver complaint, have all found their advocates. Prof.
Lankester seems to have established[221] the fact,--which indeed had
been observed 70 years before by a M. Gaillon,--that the greening is
due to the growth of a certain diatom (_Navicula ostrearia_) in the
water of the tanks. This diatom, which is of a deep blue-green colour,
appears from April to June, and in September. The oyster swallows
quantities of the _Navicula_; the pigment enters the blood in a
condition of chemical modification, which makes it colourless in all
the other parts of the body, but when the blood reaches the gills the
action of the secretion cells causes the blue tint to be restored. The
fact that the colour is rather green than blue in the gills, which are
yellowish brown, is due to certain optical conditions.

Not till the young white oyster has been steeped for several years in
the muddy waters of the ‘claires’ does it acquire the proper tint to
qualify it for the Parisian restaurant. The ‘claires’ are each about
100 feet square, surrounded by low broad banks of earth, about 3 feet
high and 6 feet thick at the base. Before the oysters are laid down,
the gates which admit the tide are carefully opened and shut a great
many times, in order to collect a sufficient amount of the _Navicula_.
When this is done, the beds are formed, and are not again overflowed
by the sea, except at very high tides. The oysters are shifted from
one ‘claire’ to another, in order to perfect the ‘greening’ process.
About fifty million of these ‘huitres de Marennes’ are produced
annually, yielding a revenue of 2,500,000 francs.

It appears, from the experience of one of the most enthusiastic of
French oyster-growers (Dr. Kemmerer), that oysters _grow_ best in
muddy water, and _breed_ best in clear water. Thus the open sea is
the place where the spat should fall and be secured, and, as soon as
it is of a suitable size, it should be transferred to the closed tank
or reservoir, where it will find the quiet and the food (confervae,
infusoria, minute algae) which are so requisite for its proper growth.
In muddy ground the animal and phosphorous matter increases, and the
flesh becomes fatter and more oily. A sudden change from the clear
sea-water to the muddy tank is inadvisable, and thus a series of
shiftings through tanks with water of graduated degrees of nourishment
is the secret of proper oyster cultivation.

The American oyster trade is larger even than the French. The Baltimore
oyster beds in the Chesapeake River and its tributaries cover 3000
acres, and produce an annual crop of 25 million bushels, as many as
100,000 bushels being sometimes taken from Chesapeake Bay in a single
day. Baltimore is the centre of the tinned oyster trade, while that
in raw oysters centres in New York. Most of the beds whose produce is
carried to New York are situated in New Jersey, Connecticut, Delaware,
or Virginia. The laws of these states do not allow the beds to be owned
by any but resident owners, and the New York dealers have consequently
to form fictitious partnerships with residents near the various oyster
beds, supply them with money to buy the beds and plant the oysters,
and then give them a share in the profits. It has been estimated that
from the Virginia beds 4,000,000 bushels of oysters are carried every
year to Fair Haven in New England, 4,000,000 to New York, 3,000,000 to
Providence, and 2,000,000 each to Boston, Philadelphia, and Baltimore.
The American ‘native’ (_O. virginica_) is a distinct species from our
own, being much larger and longer in proportion to its breadth; it is
said to be also much more prolific.

According to Milne-Edwards,[222] in the great oyster parks on the
coasts of Calvados, the oysters are educated to keep their shells
closed when out of water, and so retain water enough inside to keep
their gills moist, and arrive at their destination in good condition.
As soon as an oyster is taken out of the sea, it closes its shells,
and keeps them closed until the shock of removal has passed away, or
perhaps until the desirability of a fresh supply of water suggests
itself. The men take advantage of this to exercise the oysters,
removing them from the sea for longer and longer periods. In time this
has the desired effect; the well-educated mollusc learns that it is
hopeless to ‘open’ when out of the water, and so keeps his shell closed
and his gills moist, and his general economy in good condition.

Oysters have been known to live entirely out of water for a
considerable time. Prof. Verrill once noticed[223] a large cluster
of oysters attached to an old boot, hanging outside a fish-shop
in Washington. They had been taken out of the water on about 10th
December, and on 25th February following some of the largest were still
alive. It was noticed that all those which survived had the hinge
upward and the ventral edge downward, this being the most favourable
position possible for the retention of water within the gill-cavity,
since the edge of the mantle would pack against the margins of the
shell, and prevent the water from leaking away.

Such a succulent creature as the oyster has naturally many enemies.
One of the worst of these is the ravenous Starfish, or Five-finger.
His omnivorous capacities are well described by a clever writer and
shrewd observer of nature: “Here is one doubled up like a sea-urchin,
brilliant of hue, and when spread out quite 16 inches in diameter;
where, and oh where, can you obtain a prey? The hoe we carry is thrust
out and the mass dragged shorewards, when the rascal disgorges two
large dog-whelks he has been in the process of devouring. We feel
a comfortable glow of satisfaction to think that this enemy of our
oyster-beds is also the enemy of our other enemy, this carnivorous
borer. Here, quite close alongside, is another, only inferior in size,
and we drag him ashore likewise, to find that the fellow has actually
had the courage and audacity to suck the contents out of a large
horse-mussel (_Modiola_), the strong muscle alone remaining undevoured.
We proceed along but a short way when we meet with still another in
the curled-up condition in which they gorge themselves, and as we drag
it shorewards the shell of a _Tapes pullastra_ drops from the relaxing
grasp of the ogre. Slowly the extended stomach returns to its place,
and the monster settles back to an uncomfortable after-dinner siesta
on an exposed boulder; for the starfish wraps its turned-out stomach
around the prey it has secured, in place of attempting to devour the
limey covering in which most of its game is protected. Once the mouth
of the shell is enclosed in the stomach of the starfish, the creature
soon sickens, the hinge-spring relaxes its hold, and the shell opening
permits the starfish to suck out the gelatinous contents, and cast free
the calcareous skeleton.”[224]

According to other observers the starfish seizes the oyster with two
of his fingers, while with the other three he files away the edge of
the flat or upper valve until the points of contact with the round
valve are reduced almost to nothing; then he can introduce an arm, and
the rest is easy work. Others suggest that the starfish suffocates the
oyster by applying two of its fingers so closely to the edge of the
valves that the oyster is unable to open them; after a while the vital
powers relax and the shell gapes. The Rev. J. G. Wood holds[225] that
the starfish pours a secretion from its mouth which “paralyses the
hinge muscle and causes the shell to open.” Sometimes in a single night
a whole bed of oysters will be totally destroyed by an invasion of
starfish. Another dreaded enemy is the ‘whelk,’ a term which includes
_Purpura lapillus_, _Murex erinaceus_, _Buccinum undatum_, and probably
also _Nassa reticulata_. All these species perforate the shell with the
end of their radula, and then suck out the contents through the neatly
drilled hole. Skate fish are the cause of terrible destruction in the
open beds, and a scarcely less dangerous visitant is the octopus. Crabs
crush the young shells with their claws, and are said to gather in
bands and scratch sand or mud over the larger specimens, which makes
them open their shells. Yet another, and perhaps unconscious, foe is
found in the common mussel, which takes up room meant for the young
oysters, grows over the larger individuals, and harbours all sorts
of refuse between and under its closely packed ranks. _Cliona_, a
parasitic sponge, bores in between the layers of the oyster’s shell,
pitting them with tiny holes (corresponding to its _oscula_), and
disturbing the inmate, who has constantly to construct new layers of
shell from the inside. Weed, annelids, ‘blubber,’ shifting sand or
mud, sewage or any poisoning of the water, are seriously harmful to
the oyster’s best interests. A very severe winter is often the cause
of wholesale destruction in the beds. According to the _Daily News_ of
26th March 1891, the Whitstable oyster companies lost property to the
value of £30,000 in the exceptionally cold winter of 1890–91, when, on
the coast of Kent, the surface temperature of the sea sank below 32°,
and the advancing tide pushed a small ice-floe before it. Two million
oysters were laid down in one week of the following spring, to make up
for the loss. During the severe winter of 1892–93 extraordinary efforts
were made at Hayling I. to protect the oysters from the frost. Twenty
million oysters were placed in ponds for the winter, and a steam-engine
was for days employed to keep the ponds thawed and supplied with water,
while large coal and coke fires were kept burning at the edge of the
ponds.[226] On the other hand, the unusually warm and sunny summer
of 1893 is said to have resulted in the finest fall of spat known in
Whitstable for fifty years.[227]

The _reproductive activity_ of the oyster is supposed to commence about
the third year. Careful research has shown[228] that the sexes in the
English oyster are not separate, but that each individual is male as
well as female, producing spermatozoa as well as ova in the same gland.
Here, however, two divergent views appear. Some authorities hold that
the oyster does not fecundate its own eggs, but that this operation is
performed by spermatozoa emitted by other specimens. It is believed
that, in each individual, the spermatozoa arrive at maturity first, and
that the ova are not produced until after the spermatozoa have been
emitted; thus the oyster is first male and then female, morphologically
hermaphrodite, but physiologically unisexual. Others are of opinion
that the oyster does fecundate its own eggs, ova being first produced,
and passed into the infrabranchial chamber--the ‘white-sick’ stage--and
then, after an interval, spermatozoa being formed and fecundating
these ova--the ‘black-sick’ stage. In this latter view the oyster
is first female and then male, and is, both morphologically and
physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters
are the male, and ‘white-sick’ the female, is therefore quite incorrect.

The ova, in their earliest stage, consist of minute oval clusters of
globules floating in a transparent mucus. They pass from the ovary
into the gills and folds of the mantle, and are probably fecundated
within the excretory ducts of the ovary, before arriving in the mantle
chamber. In this stage the oyster is termed ‘white-sick.’ In about a
fortnight, as the course of development proceeds, the fertilised ova
become ciliated at one end (the so-called _veliger_ stage, p. 131),
and soon pigment appears in various parts of the embryos, giving them
a darker colour, which varies from grayish to blue, and thus the
white-sick oyster becomes ‘black-sick.’ When the black spat emerge,
they are still furnished with cilia for their free-swimming life. This
is of very short duration, for unless the embryo finds some suitable
ground on which to affix itself within forty-eight hours, it perishes.
As the spat escapes from the parent oyster, which slightly opens its
valves and blows the spat out in jets, it resembles a thick cloud in
the water, and is carried about at the mercy of wind and tide. April to
August are the usual spawning months, warm weather being apparently an
absolute necessity to secure the adhering of the spat. A temperature
of 65° to 72° F. seems requisite for their proper deposit. Thus on a
fine, warm day, with little wind or tide running, the spat will fall
near the parents and be safely secured, while in cold, blustering
weather it will certainly be carried off to a distance, and probably
be altogether lost. The number of young produced by each individual
has been variously estimated at from 300,000 to 60,000,000. Either
extreme seems enormous, but it must be remembered that besides climatal
dangers, hosts of enemies--other Mollusca, fish, and Crustacea--beset
the opening career of the young oyster.

As soon as the spat has safely ‘fallen,’ it adheres to some solid
object, and loses the cilia which were necessary for its swimming life.
It begins to grow rapidly, increasing from about 1/20 inch in diameter
to about the size of a threepenny piece in five or six months, and in
a year to one inch in diameter. Roughly speaking, the best guide to an
oyster’s age is its size; it is as many years old as it measures inches
across.

The oyster is at its prime at the age of five; its natural life is
supposed to be about ten years. The rings, or ‘shoots’ on a shell are
not--as is frequently supposed--marks of annual growth; cases have been
noticed where as many as three ‘shoots’ were made during the year.

An oyster is furnished, on the protruding edges of the mantle, with
pigmented spots which may be termed ‘visual organs,’ though they hardly
rise to the capacities and organisation of real ‘eyes.’ But there is no
doubt that they are sufficiently sensitive to the action of light to
enable the oyster to apprehend the approach of danger, and close his
doors accordingly. ‘How sensitive,’ notes Mr. W. Anderson Smith,[229]
‘the creatures are to the light above them; the shadow of the iron as
it passes overhead is instantaneously noted, and snap! the lips are
firmly closed.’

The geographical distribution of _Ostrea edulis_ extends from Tränen,
in Norway, close to the Arctic circle, to Gibraltar and certain parts
of the Mediterranean, Holland, and N. Germany to Heligoland, and the
western shores of Sleswick and Jutland. It occurs in Iceland, but does
not enter the Baltic, where attempts to colonise it have always failed.
Some authorities regard the Mediterranean form as a distinct species.

The literature of oyster-cookery may be passed over in silence. The
curious may care to refer to M. S. Lovell’s _Edible British Mollusks_,
where no less than thirty-nine different ways of dressing oysters
are enumerated. It may, however, be worth while to add a word on
the subject of _poisonous_ oysters. Cases have been known where a
particular batch of oysters has, for some reason, been fatal to those
who have partaken of them. It is possible that this may have been due,
in certain instances, to the presence of a superabundance of copper
in the oysters, and there is no doubt that the symptoms detailed have
often closely resembled those of copper poisoning. Cases of poisoning
have occurred at Rochefort through’ the importation of ‘green oysters’
from Falmouth. It would no doubt be dangerous ever to eat oysters
which had grown on the copper bottom of a ship. But copper is present,
in more or less minute quantities, in very many Mollusca, and it
is more probable that a certain form of slow decomposition in some
shell-fish develops an alkaloid poison which is more harmful to some
people than to others, just as some people can never digest any kind
of shell-fish.[230] These alkaloid developments from putrescence are
called ptomaines. In confirmation of this view, reference may be
made to a case, taken from an Indian Scientific Journal, in which an
officer, his wife, and household ate safely of a basket of oysters for
three days at almost every meal. The basket then passed out of their
hands, not yet exhausted of its contents, and a man who had already
eaten of these oysters at the officer’s table was afterwards poisoned
by some from the same basketful.

The cultivation of the common mussel (_Mytilus edulis_ L.) is not
practised in this country, although it is used as food in the natural
state of growth all round our coasts. The French appear to be the only
nation who go in for extensive mussel farming. The principal of these
establishments is at a little town called Esnaudes, not far from La
Rochelle, and within sight of the Ile de Ré and its celebrated oyster
parks. The secret of the cultivation consists in the employment of
‘bouchots,’ or tall hurdles, which are planted in the mud of the
fore-shore, and upon which the mussel (_la moule_, as the French call
it) grows. The method is said[231] to have been invented as long ago as
1235 by a shipwrecked Irishman named Walton. He used to hang a purse
net to stakes, in the hope of capturing sea birds. He found, however,
that the mussels which attached themselves to his stakes were a much
more easily attainable source of food, and he accordingly multiplied
his stakes, out of which the present ‘_bouchot_’ system has developed.
The shore is simply a stretch of liquid mud, and the bouchots are
arranged in shape like a single or double V, with the opening looking
towards the sea. The fishermen, in visiting the bouchots, glide about
over the mud in _piroques_ or light, flat-bottomed boats, propelling
them by shoving the mud with their feet. Each bouchot is now about 450
yards long, standing 6 feet out of the mud, making a strong wall of
solid basket-work, and as there are altogether at least 500 bouchots,
the total mussel-bearing length of wall is nearly 130 miles.

The mussel-spat affixes itself naturally to the bouchots nearest the
sea, in January and February. Towards May the planting begins. The
young mussels are scraped off these outermost bouchots, and placed
in small bags made of old canvas or netting, each bag holding a good
handful of the mussels. The bags are then fastened to some of the inner
bouchots, and the mussels soon attach themselves by their byssus, the
bag rotting and falling away. They hang in clusters, increasing rapidly
in size, and at the proper time are transplanted to bouchots farther
and farther up the tide level, the object being to bring the matured
animal as near as possible to the land when it is time for it to be
gathered. This process, which aims at keeping the mussel out of the
mud, while at the same time giving it all the nutrition that comes
from such a habitat, extends over about a year in the case of each
individual. Quality, rather than quantity, is the aim of the Esnaudes
boucholiers. The element of quantity, however, seems to come in when
we are told that each yard of the bouchots is calculated to yield a
cartload of mussels, value 6 francs, and that the whole annual revenue
is at least £52,000.

In this country, and especially in Scotland, mussels are largely used
as bait for long-line fishing. Of late years other substances have
rather tended to take the place of mussels, but within the last twenty
years, at Newhaven on the Firth of Forth, three and a half million
mussels were required annually to supply bait for four deep-sea craft
and sixteen smaller vessels. According to Ad. Meyer,[232] boughs of
trees are laid down in Kiel Bay, and taken up again, after three, four,
or five years, between December and March, when they are found covered
with fine mussels. The boughs are then sold, just as they are, by
weight, and the shell-fish sent into the interior of Germany.

Mussels are very sensitive to cold weather. In 1874, during an
easterly gale, 195 acres of mussels at Boston, in Lincolnshire, were
killed in a single night. They soon affix themselves to the bottom
of vessels that have lain for any length of time in harbour or near
the coast. The bottom of the _Great Eastern_ steamship was at one
time so thickly coated with mussels that it was estimated that a
vessel of 200 tons could have been laden from her. In some of our
low-lying coast districts mussels are a valuable protection against
inundation. “An action for trespass was brought some time ago for the
purpose of establishing the right of the lord of the manor to prevent
the inhabitants of Heacham from taking mussels from the sea-shore.
The locality is the fore-shore of the sea, running from Lynn in a
north-westerly direction towards Hunstanton in Norfolk; and the
nature of the shore is such that it requires constant attention, and
no little expenditure of money, to maintain its integrity, and guard
against the serious danger of inundations of the sea. Beds of mussels
extend for miles along the shore, attaching themselves to artificial
jetties running into the sea, thereby rendering them firm, and thus
acting as barriers against the sea [and as traps to catch the silt,
and thus constantly raise the level of the shore]. Therefore, while it
is important for the inhabitants, who claim a right by custom, to take
mussels and other shell-fish from the shore, it is equally important
for the lord of the manor to do his utmost to prevent these natural
friends of his embankments and jetties from being removed in large
quantities.”[233]

The fable that Bideford Bridge is held together by the byssi of
_Mytilus_, which prevent the fabric from being carried away by the
tide, has so often been repeated that it is perhaps worth while to give
the exact state of the case, as ascertained from a Town Councillor.
The mussels are supposed to be of some advantage to the bridge,
consequently there is a by-law forbidding their removal, but the
corporation have not, and never had, any boat or men employed in any
way with regard to them.

Poisoning by mussels is much more frequent than by oysters. At
Wilhelmshaven,[234] in Germany, in 1885, large numbers of persons were
poisoned, and some died, from eating mussels taken from the harbour.
It was found that when transferred to open water these mussels became
innocuous, while, on the other hand, mussels from outside, placed in
the harbour, became poisonous. The cause obviously lay in the stagnant
and corrupted waters of the harbour, which were rarely freshened by
tides. It was proved to demonstration that the poison was not due to
decomposition; the liver of the mussels was the poisonous part. In the
persons affected, the symptoms were of three kinds, exanthematous
(skin eruptions), choleraic, and paralytic. Cases of similar poisoning
are not unfrequent in our own country, and the circumstances tend to
show that, besides the danger from mussels bred in stagnant water,
there is also risk in eating them when ‘out of season’ in the spawning
time.

_Whelks_ are very largely employed for bait, especially in the cod
fishery. The whelk fishery in Whitstable Bay, both for bait and for
human food, yields £12,000 a year. Dr. Johnston, of Berwick, estimated
that about 12 million limpets were annually consumed for bait in that
district alone. The cockle fishery in Carmarthen Bay employs from 500
to 600 families, and is worth £15,000 a year, that in Morecambe Bay is
worth £20,000.

=Cultivation of Snails for Food; Use as Medicine.=--It was a certain
Fulvius Hirpinus who, according to Pliny the elder,[235] first
instituted snail preserves at Tarquinium, about 50 B.C. He appears to
have bred several species in his ‘cochlearia,’ keeping them separate
from one another. In one division were the _albulae_, which came from
Reate; in another the ‘very big snails’ (probably _H. lucorum_), from
Illyria; in a third the African snails, whose characteristic was their
fecundity; in a fourth those from Soletum, noted for their ‘nobility.’
To increase the size of his snails, Hirpinus fed them on a fattening
mixture of meal and new wine, and, says the author in a burst of
enthusiasm, ‘the glory of this art was carried to such an extent that
a single snail-shell was capable of holding eighty sixpenny pieces.’
Varro[236] recommends that the snaileries be surrounded by a ditch, to
save the expense of a special slave to catch the runaways. Snails were
not regarded by the Romans as a particular luxury. Pliny the younger
reproaches[237] his friend Septicius Clarus for breaking a dinner
engagement with him, at which the _menu_ was to have been a lettuce,
three snails and two eggs apiece, barley water, mead and snow, olives,
beetroot, gourds and truffles, and going off somewhere else where he
got oysters, scallops, and sea-urchins. In Horace’s time they were used
as a gentle stimulant to the appetite, for

    “’Tis best with roasted shrimps and Afric snails
    To rouse your drinker when his vigour fails.”[238]

Escargotières, or snail-gardens, still exist in many parts of Europe,
_e.g._ at Dijon, at Troyes and many other places in central and
southern France, at Brunswick, Copenhagen, and Ulm. The markets at
Paris, Marseilles, Bordeaux, Toulouse, Nantes, etc., are chiefly
supplied by snails gathered from the open country, and particularly
from the vineyards, in some of which _Helix pomatia_ abounds. In the
_Morning Post_ of 8th May 1868 there is an account of the operation of
clearing the celebrated Clos de Vougeot vineyard of these creatures. No
less than 240 gallons were captured, at a cost in labour of over 100
francs, it being estimated that these snails would have damaged the
vines to an extent represented by the value of 15 to 20 pipes of wine,
against which may be set the price fetched by the snails when sold in
the market.

It is generally considered dangerous to eat snails at once which have
been gathered in the open country. Cases have occurred in which death
by poisoning has resulted from a neglect of this precaution, since
snails feed on all manner of noxious herbs. Before being sent to table
at the restaurants in the great towns, they are fattened by being fed
with bran in the same way as oysters.

The Roman Catholic Church permits the consumption of snails during
Lent. Very large numbers are eaten in France and Austria at this time.
At the village of Cauderon, near Bordeaux, it is the proper thing to
end Carnival with especial gaiety, but to temper the gaiety with a dish
of snails, as a foretaste of Lenten mortification.

The following species appear to be eaten in France at the present
day: _H. pomatia_, _aspersa_, _nemoralis_, _hortensis_, _aperta_,
_pisana_, _vermiculata_, _lactea_. According to Dr. Gray, the glassmen
at Newcastle used to indulge in a snail feast once a year, and a recent
writer informs us that _H. aspersa_ is still eaten by working people in
the vicinity of Pontefract and Knottingley.[239] But in this country
snails appear to be seldom consciously used as an article of food;
the limitation is necessary, for Lovell tells us that they are much
employed in the manufacture of cream, and that a retired (!) milkman
pronounced it the most successful imitation known.

Preparations made from snails used to be highly esteemed as a cure
for various kinds of diseases and injuries. Pliny the elder recommends
them for a cough and for a stomach-ache, but it is necessary “to take
an uneven number of them.”[240] Five African slugs, roasted and beaten
to a powder, with half a drachm of acacia, and taken with myrtle wine,
is an excellent remedy for dysentery. Treated in various ways, snails
have been considered, in modern times, a cure for ague, corns, web
in the eye, scorbutic affections, hectic fevers, pleurisy, asthma,
obstructions, dropsy, swelling of the joints, headache, an impostume
(whitlow), and burns. One of Pliny’s remedies for headache, which
competes with the bones of a vulture’s head or the brain of a crow or
an owl, is a plaister made of slugs with their heads cut off, which is
to be applied to the forehead. He regards slugs as immature snails,
whose growth is not yet complete (_nondum perfectae_). Lovell
states that “a large trade in snails is carried on for Covent Garden
market in the Lincolnshire fens, and that they are sold at 6d. per
quart, being much used for consumptive patients and weakly children.”

The custom still seems to linger on in some parts of the country. Mr.
E. Rundle, of the Royal Cornwall Infirmary, gives his experience in
the following terms: “I well remember, some twelve years since, an
individual living in an adjoining parish [near Truro] being pointed
out to me as ‘a snail or slug eater.’ He was a delicate looking man,
and said to be suffering from consumption. Last summer I saw this man,
and asked him whether the statement that he was a ‘snail eater’ was
true: he answered, ‘Yes, that he was ordered small white slugs--not
snails--and that up till recently he had consumed a dozen or more every
morning, and he believed they had done him good.’ There is also another
use to which the country people here put snails, and that is as an eye
application. I met with an instance a few weeks since, and much good
seemed to have followed the use.”[241]

A reverend Canon of the Church of England, whose name I am not
permitted to disclose, informs me that there was a belief among the
youth of his native town (Pontypool, in Monmouthshire) that young
slugs were ‘good for consumption,’ and that they were so recommended
by a doctor who practised in the town. The slugs selected were about ¾
inch long, “such as may be seen crawling on the turf of a hedge-bank
after a shower of rain.” They were “placed upon the tongue without
any previous preparation, and swallowed alive.” My informant himself
indulged in this practice for some time, “not on account of any
gustatory pleasure it afforded, but from some vague notion that it
might do him good.”

A colleague of mine at King’s College tells me that the country people
at Ponteland, near Morpeth, habitually collect _Limax agrestis_
and boil it in milk as a prophylactic against consumption. He has
himself frequently devoured them alive, but they must be swallowed, not
scrunched with the teeth, or they taste somewhat bitter.

Snails have occasionally fallen, with other noxious creatures, under
the ban of the Church. In a prayer of the holy martyr Trypho of
Lampsacus (about 10th cent. A.D.) there is a form of exorcism
given which may be used as occasion requires. It runs as follows: “O
ye Caterpillars, Worms, Beetles, Locusts, Grasshoppers, Woolly-Bears,
Wireworms, Longlegs, Ants, Lice, Bugs, Skippers, Cankerworms,
Palmerworms, _Snails_, Earwigs, and all other creatures that cling
to and wither the fruit of the grape and all other herbs, I charge you
by the many-eyed Cherubim, and by the six-winged Seraphim, which fly
round the throne, and by the holy Angels and all the Powers, etc. etc.,
hurt not the vines nor the land nor the fruit of the trees nor the
vegetables of ---- the servant of the Lord, but depart into the wild
mountains, into the unfruitful woods, in which God hath given you your
daily food.”

=Prices given for Shells.=--Very high prices have occasionally been
given for individual specimens, particularly about thirty or forty
years ago, when the mania for collecting was at its height. In those
days certain families, such as the Volutidae, Conidae, and Cypraeidae,
were the especial objects of a collector’s ardour, and he spared no
expense to make his set of the favourite genus as complete as possible.
Thus at Stevens’ auction-rooms in Covent Garden, on 21st July 1854,
one specimen of _Conus cedo nulli_ fetched £9: 10s., and another £16,
a _C. omaicus_ 16 guineas, _C. victor_ £10, and _C. gloria maris_,
the greatest prize of all, £43: 1s. At the Vernède sale, on 14th
Dec. 1859 two _Conus omaicus_ fetched £15 and £22, and a _C. gloria
maris_ £34. At the great Dennison sale, in April 1865, the _Conidae_
fetched extravagant prices, six specimens averaging over £20 apiece.
_Conus cedo nulli_ went for £18 and £22, _C. omaicus_ for £12, _C.
malaccanus_ for 10 guineas (this and one of the _cedo nulli_ being the
actual specimens figured in Reeve’s _Conchologia Iconica_), _C. cervus_
for £19 and _C. gloria maris_ for £42. On 9th May 1866 a _Cypraea
Broderipii_ was sold at Stevens’ auction-rooms for £13, and at the
Dennison sale a _Cypraea princeps_ fetched £40, and _C. guttata_ £42.
The _Volutidae_, although not quite touching these prices, have yet
done fairly well. Mr. Dennison’s _Voluta fusiformis_ sold for £6: 15s.,
_V. papillaris_ for £5, _V. cymbiola_ for £5: 15s., _V. reticulata_
for eight guineas, and two specimens of the rarest of all Volutas, _V.
festiva_, for £14 and £16, both being figured in the _Conchologia_. At
the same sale, two unique specimens of _Oniscia Dennisoni_ fetched £17
and £18 respectively, and, at the Vernède sale, _Ancillaria Vernèdei_
was bought for £6: 10s., and _Voluta piperata_ for £7: 10s.

A unique specimen of a recent _Pleurotomaria_ (_quoyana_ F. and B.) was
purchased by Miss de Burgh in 1873 for 25 guineas, and another species
of the same genus (_adansoniana_ Cr. and F.), of extraordinary size and
beauty, is now offered for sale for about £100.

Bivalves have never fetched quite such high prices as univalves, but
some of the favourite and showy genera have gone near to rival them.
On 22nd June 1869, at Stevens’, _Pecten solaris_ fetched £4: 5s.,
_P. Reevii_ £4: 8s., and _Cardita varia_ 5 guineas. Mr. Dennison’s
specimens of _Pecten subnodosus_ sold for £7, of _Corbula Sowerbyi_ for
£10, of _Pholadomya candida_ for £8 and £13, while at the Vernède sale
a _Chama damicornis_ fetched £7.




                               CHAPTER V

    REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE FERTILISED
         OVUM--DIFFERENCES OF SEX--DIOECIOUS AND HERMAPHRODITE
             MOLLUSCA--DEVELOPMENT OF FRESH-WATER BIVALVES


Reproduction in the Mollusca invariably takes place by means of
eggs, which, after being developed in the ovary of the female, are
fertilised by the spermatozoa of the male. As a rule, the eggs are
‘laid,’ and undergo their subsequent development apart from the
parent. This rule, however, has its exceptions, both among univalve
and bivalve Mollusca, a certain number of which hatch their young
from the egg before expelling them. Such ovoviviparous genera are
_Melania_, _Paludina_, _Balea_, and _Coeliaxis_ among land and
fresh-water Mollusca, and _Cymba_ and many _Littorina_ amongst marine.
The young of _Melania tuberculata_, in Algeria, have been noticed to
return, as if for shelter, to the branchial cavity of the mother, some
days after first quitting it. Isolated species among Pulmonata are
known to be ovoviviparous, _e.g._ _Patula Cooperi_, _P. Hemphilli_,
and _P. rupestris_, _Acanthinula harpa_, _Microphysa vortex_,
_Pupa cylindracea_ and _muscorum_, _Clausilia ventricosa_, _Opeas
dominicensis_, _Rhytida inaequalis_, etc. All fresh-water Pelecypoda
yet examined, except _Dreissensia_, are ovoviviparous.

The number of eggs varies greatly, being highest in the Pelecypoda. In
_Ostrea edulis_ it has been estimated at from 300,000 to 60,000,000;
in _Anodonta_ from 14,000 to 20,000; in _Unio pictorum_ 200,000. The
eggs of _Doris_ are reckoned at from 80,000 to 600,000, of _Loligo_
and _Sepia_ at about 30,000 to 40,000. Pulmonata lay comparatively few
eggs. _Arion ater_ has been observed to lay 477 in forty-eight days (p.
42). Nests of _Helix aspersa_ have been noticed, in which the number
of eggs varied from about 40 to 100. They are laid in little cup-shaped
hollows at the roots of grass, with a little loose earth spread over
them. The eggs of _Testacella_ are rather large, and very elastic; if
dropped on a stone floor they will rebound sharply several inches. The
_Cochlostyla_ of the Philippines lay their eggs at the tops of the
great forest trees, folding a leaf together to serve as a protection.

The eggs of the great tropical _Bulimus_ and _Achatina_, together with
those of the _Macroön_ group of _Helix_ (_Helicophanta_, _Acavus_,
_Panda_) are exceedingly large, and the number laid must be decidedly
less than in the smaller Pulmonata. _Bulimus oblongus_, for instance,
from Barbados, lays an egg about the size of a sparrow’s (Fig. 38),
_Achatina sinistrorsa_ as large as a pigeon’s. The Cingalese _Helix
Waltoni_ when first hatched is about the size of a full-grown _H.
hortensis_. There is, in the British Museum, a specimen of the egg of a
_Bulimus_ from S. America (probably _maximus_ or _popelairanus_) which
measures exactly 1¾ inch in length.

The _Limnaeidae_ deposit their eggs in irregular gelatinous masses
on the under side of the leaves of water-plants, and on all kinds of
débris.

  [Illustration: FIG. 38.--Newly-hatched young and egg of _Bulimus
  oblongus_ Müll., Barbados. _Natural size._]

The _Rachiglossa_ or marine carnivorous families lay their eggs in
tough leathery or bladdery capsules, which are frequently joined
together in shapes which differ with the genus. Each capsule contains
a varying number of ova. The cluster of egg-capsules of _Buccinum
undatum_ is a familiar object on all our sandy coasts. The capsules
of _Purpura lapillus_ are like delicate pink grains of rice, set
on tiny stalks. They are not attached to one another, but are set
closely together in groups in sheltered nooks of the rocks. A single
_Purpura_ has been observed to produce 245 capsules! _Busycon_ lays
disc-shaped capsules which are all attached at a point in the edge to
a cartilaginous band nearly 3 feet in length, looking like a number of
coins tied to a string at equal distances from one another. In _Murex
erinaceus_ the egg-capsules are triangular, with a short stalk. They
are deposited separately in clusters of from 15 to 150, there being
about 20 ova in each capsule. It appears that all the species of the
same genus have by no means the same method of depositing their eggs,
nor do they always produce eggs of at all similar size or shape. Thus,
of two British species of _Nassa_, _N. reticulata_ lays egg-capsules in
shape like flattened pouches with a short stalk, and fastens them in
rows to the leaves of _Zostera_; _M. incrassata_, on the other hand,
deposits solitary capsules, which are shaped like rounded oil-flasks.
_Neptunea antiqua_ lays its eggs in bunched capsules, like _Bucc.
undatum_ (Fig. 40), but the capsules of _N. gracilis_ are solitary.

  [Illustration: FIG. 39.--Various forms of spawn in
  _Prosobranchiata_: =A= and =D=, _Pyrula_ or _Busycon_; =B=,
  _Conus_; =C=, _Voluta musica_; =E=, _Ampullaria_ (from specimens
  in the British Museum); all × ⅔.]

In _Natica_ the eggs are deposited in what looks like a thick piece of
sand-paper, curled in a spiral form (Fig. 41). The sand is agglutinated
by copious mucus into a sort of sheet, and the eggs are let into this,
sometimes (_N. heros_) in regular quincunx form. _Ianthina_ attaches
its eggs to the under side of its float (Fig. 42). The Trochidae
deposit their eggs on the under side of stones and sea-weeds, each
ovum being contained in a separate capsule, and all the capsules
glued together into an irregular mass of varying size. The female of
_Galerus chinensis_ hatches her eggs by keeping them between her foot
and the stone she adheres to. They are laid in from 6 to 10 capsules,
connected by a pedicle and arranged like the petals of a rose, with
10 to 12 eggs in each capsule. Those _Littorina_ which are not
ovoviviparous deposit their spawn on sea-weeds, rocks, and stones. The
eggs are enveloped in a glairy mass which is just firm enough to retain
its shape in the water; each egg has its own globule of jelly and is
separated from the others by a very thin transparent membrane.[242]

  [Illustration: FIG. 40.--Egg-capsules of, =A=, _Nassa reticulata_
  L. × ⅔; =B=, _Buccinum undatum_ L. × ⅔; =C=, _Neptunea antiqua_
  L. × ⅓.]

  [Illustration: FIG. 41.--Spawn of a species of _Natica_ (from a
  specimen in the British Museum) × ½.]

  [Illustration: FIG. 42.--_Ianthina fragilis_ Lam. =FL=, float;
  =O=, ova; =Pr=, proboscis; =Br=, branchiae; =F=, foot. (Quoy and
  Gaimard.)]

_Chiton marginatus_, when kept in captivity, has been noticed[243] to
elevate the posterior part of the girdle, and to pour out a continuous
stream of flaky white matter like a fleecy cloud, which proved to be of
a glutinous nature. It then discharged ova, at the rate of one or two
every second, for at least fifteen minutes, making a total of 1300 to
1500, each being about 1/100 inch diameter. The ova were shot into the
glutinous cloud, which seemed to serve as a sort of nidus to entangle
the ova and prevent them being carried away. The subsequent development
was rapid, and in seven days the young _Chiton_ was hatched, being then
about 1/20 inch long. Lovén has described the same species as laying
its eggs, loosely united in clusters of seven to sixteen, upon small
stones. There is probably some mistake about the identification, but
the observation illustrates the varying methods of oviposition among
allied forms.

  [Illustration: FIG. 43.--Egg-capsules of =A=, _Sepia elegans_
  Orb., and =B=, _Octopus vulgaris_ Lam.]

Not very much is known with regard to the ovipositing of the
_Cephalopoda_, especially those which inhabit deep water. Masses of
ova arranged in very various forms have occasionally been met with
floating in the ocean, but it is next to impossible to determine to
what species, or even genus, they belong.[244]

In _Loligo punctata_ the ova are contained in small cylindrical cases
measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each
case. Hundreds of these cases are attached together like a bundle of
sausages or young carrots, and the movements of the embryos within can
be distinctly noted. _Sepia officinalis_ lays large black pear-shaped
capsules, each of which is tied to some place of attachment by a kind
of ribbon at the upper end of the capsule, the whole forming a large
group like a bunch of grapes. _Octopus vulgaris_ deposits thousands
of small berry-shaped ova, attached to a string which runs along the
centre of the mass (Fig. 43).

The so-called shell of the female _Argonauta_ is nothing more than a
form of protection for the ova, and is in no sense homologous to the
ordinary molluscan shell. The ova consist of a large granulated mass,
attached to a many branched stem; they are contained in the spire
of the shell, in contact with the posterior part of the body of the
mother, but sometimes project externally beyond the coil of the spire.

Certain species possess the curious property of laying their eggs
on the outside of their own shells. _Buccinopsis Dalei_ is not
unfrequently found decorated with its own egg-capsules. Possibly this
species, which lives on oozy ground, finds this the only secure place
of attachment for its progeny. _Neritina fluviatilis_ has a similar
habit, and so have many other species of _Neritina_ and _Navicella_.
It is not quite clear, in the latter cases, whether the eggs are laid
by the specimens on whose shell they are found, or whether they are
deposited by others. In either case, perhaps the shell is the safest
place for them in the rapid streams which both genera frequent.
Specimens of _Hydrobia ulvae_ taken on the wet sands at the mouth
of the Dee, are found to have several little rounded excrescences
scattered over the surface of the shell. These, on examination, are
found to be little masses of small sand-grains, in the centre of which
is a clear jelly containing segmenting ova or young embryos. Here
again, in all probability, the shell is the only comparatively stable
object, in the expanse of shifting sands, on which the eggs can be
laid.[245]

The pulmonate genus _Libera_, which occurs on a few of the island
groups in the Central Pacific, is remarkable for the habit of laying
its eggs within its own cavernous umbilicus, which is narrowed at the
lower part. The eggs number from four to six, or the same number of
very young shells may be seen closely packed in the cavity, each being
in shape exactly like a young _Planorbis_. This constriction of the
umbilicus does not occur till the formation of the last two whorls,
_i.e._ till the animal is sexually mature. Some species, but not all,
provide for the safety of their eggs more completely by forming a very
thin shelly plate, which nearly closes the umbilical region, and breaks
away or is absorbed to facilitate the escape of the young shells.[246]


=Union of Limax.=--With regard to the act of union itself, the method
in certain species of _Limax_ deserves special notice. _L. maximus_
has been observed at midnight to ascend a wall or some perpendicular
surface. A pair then crawl round and round one another emitting a
quantity of mucus which at length forms a patch, 2 to 2½ inches in
diameter. When this acquires consistency the pair begin to twist round
each other in corkscrew form, and detach themselves from the wall,
hanging by a cord of the thickened mucus, about 8–15 inches long, and
still twisting round each other. The external generative organs are
then protruded and copulation takes place, after which the bodies
untwist, separate, and crawl up the cord again to the wall.[247]

=Periodicity in Breeding.=--In the marine Mollusca, the winter months
appear to be the usual time for the deposition of eggs. Careful
observations have been made on the Mollusca occurring at Naples,[248]
and the general result seems to be that for all Orders alike the
six winter months from November to April, roughly speaking, are the
breeding time. Scarcely any forms appear to breed habitually in August,
September, or October. On our own coasts, Nudibranchiata come in shore
to deposit their ova from January to April. _Purpura lapillus_ may
be observed depositing ova all the year round, but is most active
from January to April. _Buccinum undatum_ breeds from October to May;
_Littorina_ all the year round.

The land Mollusca exhibit rather more periodicity than the marine. In
temperate climates they breed exclusively in the summer months. In the
tropics their periods are determined by the dry and rainy seasons,
where such occur, otherwise they cohabit all the year round. According
to Karl Semper, the snails of the warm Mediterranean region arrive at
sexual maturity when they are six months old, _i.e._ before they are
fully grown. After a rest of about three months during the heat of
summer, a second period of ovipositing occurs.[249] _Helix hortensis_
and _H. nemoralis_ ascend trees, sometimes to a height of forty feet,
when pairing.[250]

=Hybridism= as the result of union between different species of
Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at
Anduze (Gard) as many as twenty specimens of _Pupa cinerea_ united
with _Clausilia papillaris_. No offspring seem to have resulted from
what the professor calls ‘this innocent error,’ for the wall was
carefully scrutinised for a long time, and no hybrid forms were ever
detected.

The same observer noticed, in the Luxembourg garden at Paris, and
M. Gassies has noticed[252] at various occasions, union between
_Helix aspersa_ and _nemoralis_, _H. aspersa_ and _vermiculata_,
between _Stenogyra decollata_ and a _Helix_ (sp. not mentioned), _H.
variabilis_ and _pisana_, _H. nemoralis_ and _hortensis_. In the two
latter cases a hybrid progeny was the result. It has been noticed that
these unions generally took place when the air was in a very electric
condition, and rain had fallen, or was about to fall, abundantly.

Of marine species _Littorina rudis_ has been noticed[253] in union both
with _L. obtusata_ and with _L. littorea_, but no definite facts are
known as to the result of such unions.


=Self-impregnation= (see p. 44).

=Development of the Fertilised Ovum.=--The first stages in the
development of the Mollusca are identical with those which occur in
other classes of animals. The fertilised ovum consists of a vitellus
or yolk, which is surrounded with albumen, and is either contained in
a separate capsule, or else several, sometimes many, ova are found in
the same capsule, only a small proportion of which ultimately develop.
The germinal vesicle, which is situated at one side of the vitellus,
undergoes unequal segmentation, the result of which is usually the
formation of a layer of small ectoderm cells overlying a few much
larger cells which contain nearly the whole of the yolk. The large
cells are then invaginated, or are simply covered by the growth of the
ectoderm cells. The result in either case is the formation of an area,
the blastopore, where the inner cells are not covered by the ectoderm.
The blastopore gradually narrows to a circular opening, which, in
the great majority of cases, eventually becomes the mouth. The usual
differentiation of germinal layers takes place, the epiblast eventually
giving rise to the epidermis, nervous system, and special sense organs,
the hypoblast to the liver and to the middle region of the alimentary
tract, the mesoblast to the muscles, the body cavity, the vascular,
the excretory and reproductive systems. The next, or _trochosphere_
(_trochophora_) stage, involves the formation of a circlet of praeoral
cilia, dividing the still nearly spherical embryo into two unequal
portions, the smaller of which consists simply of the prostomium, or
part in front of the mouth, the larger bearing the mouth and anus.

So far the series of changes undergone by the embryo are not
peculiar to the Mollusca; we now come to those which are definitely
characteristic of that group. The stage next succeeding the development
of the trochosphere is the definitive formation of the _velum_, a
process especially characteristic of the Gasteropoda and Pelecypoda,
but apparently not occurring in the great majority of land Pulmonata.

  [Illustration: FIG. 44.--Veligers of _Dentalium entalis_ L.: =A=,
  longitudinal section of a larva 14 hours old, × 285; =B=, larva
  of 37 hours, × 165; =C=, longitudinal section of larva of 34
  hours, × 165; _m_, mouth; _v_, _v_, velum. (After Kowalewsky.)]

The circlet of cilia becomes pushed more and more towards the anterior
portion of the embryo, the cilia themselves become longer, while the
portion of the body from which they spring becomes elevated into a
ridge or ring, which, as a rule, develops on each side a more or less
pronounced lobe. The name _velum_ is applied to this entire
process of ciliated ring and lobes, and to the area which they enclose.

  [Illustration: FIG.45.--Veliger of _Patella vulgata_ L., 130
  hours old: _f_, rudimentary foot; _op_, operculum; _sh_, shell;
  _v_, _v_, velum. (After Patten, highly magnified.)]

  [Illustration: FIG. 46.--Developed larva of _Cyclas cornea_ L.:
  _br_, rudimentary branchiae; _by_, byssus; _f_, foot; _m.e_,
  mantle edge; _sh_, shell. (After Ziegler, highly magnified.)]

  [Illustration: FIG. 47.--=A=, Advanced veliger of _Dreissensia_:
  _f_, foot; _m_, mouth; _sh_, shell; _v_, _v_, velum. (After
  Korschelt and Heider, much enlarged.) =B=, Veliger of a Pteropod
  (_Tiedemannia_): _op_, operculum; _sh_, shell; _v_, velum. (After
  Krohn, much enlarged.)]

In this so-called _veliger_ stage, the velum serves, in the first
place, to cause rotation of the larva within the egg-capsules, and,
after hatching, as an organ of locomotion. As a rule, the velum
disappears entirely in the adult mollusc after the free-swimming stage
is over, but in the common _Limnaea stagnalis_ it persists, losing its
cilia, as the very prominent circum-oral lobes. Simultaneously with
the development of the velum, and in some cases earlier, appear the
rudiments of the shell-gland and of the foot, the latter being situated
on the ventral side, between the mouth and anus, the former on the
dorsal side, behind the velum, and above the surface of the eventual
visceral sac. Thus the prime characteristics of the veliger stage,
subsequent to the appearance of the velum itself, are the development
of the visceral sac and shell-gland on the upper, and of the foot on
the under side. According to Lankester the primitive shell-gland does
not, as a rule, directly give rise to the shell of the adult mollusc,
but becomes filled up by a horny substance, and eventually disappears;
the permanent shell then forms over the surface of the visceral hump
from the original centre of the shell-gland. It is only in _Chiton_,
and possibly in _Limax_, that the primitive shell-sac is retained and
developed into the final shell-forming area, which is much wider, and
extends to the edges of the mantle. Within the velar area first appear
the rudiments of the tentacles and eyes; when these become developed
the velum atrophies and disappears.

Several of these veligers when captured in the open sea have been
mistaken for perfect forms, and have been described as such. Thus the
larva of _Dolium_ has been described as _Macgillivrayia_, that of a
_Purpura_ as _Chelotropis_ and _Sinusigera_, that of _Aporrhais pes
pelecani_ as _Chiropteron_, that of _Marsenia conspicua_ as _Brownia_,
_Echinospira_, and _Calcarella_.

_Cephalopoda._--The embryonic development of the Cephalopoda is
entirely distinct from that of all other Mollusca. The segmentation of
the vitellus is partial, and the embryo is furnished with a vitelline
sac, which is very large in the majority of cases (Fig. 48). There
is no free-swimming stage, but the embryo emerges from the egg fully
developed.

=Differences of Sex.=--In the Mollusca there are two main types of
sexual difference: (i) sexes separate (_dioecious_ type), (ii) sexes
united in the same individual (_hermaphrodite_ type).

  [Illustration: FIG. 48.--Two stages in the development of _Loligo
  vulgaris_ Lam.: _a_{1}_, _a_{1}_, first, and _a_{2}_, _a_{2}_,
  second pairs of arms; _br_, branchiae, seen through _m_, mantle;
  _e_, _e_, eyes; _fi_, fins; _fu_, funnel; _v.s_, vitelline sac.
  (After Kowalewsky.)]

In some cases--_e.g._ certain Pelecypoda--what is practically a third
type occurs. The animal is hermaphrodite, but the male and female
elements are not developed simultaneously, _i.e._ the same individual
is at one time female, at another male.

1. The sexes are separate in

   All _Cephalopoda_.

   _Gasteropoda Amphineura_ (except _Neomeniidae_).

   _Gasteropoda Prosobranchiata_ (except _Valvata_ and
   some species of _Marsenia_).

   _Scaphopoda_.

   Many _Pelecypoda_.

2. The sexes are united in

   _Gasteropoda Opisthobranchiata_.

   _Gasteropoda Pulmonata_.

   Certain _Pelecypoda_.[254]

In the dioecious Mollusca, sexual union is the rule, but is by no means
universal. In some instances,--_e.g._ _Vermetus_, _Magilus_, _Patella_,
_Haliotis_, _Crepidula_, _Chiton_, the _Scaphopoda_--the form and
habits of the animal do not admit of it; in others (many _Trochus_)
a male copulative organ is wanting. When this is the case, the male
scatters the spermatozoa freely; the majority must perish, but some
will be carried by currents in the direction of the female.

When the sexes are separate, the female is frequently larger than the
male. This is markedly the case in _Littorina_, _Buccinum_, and all the
_Cephalopoda_; in _Argonauta_ the difference is extreme, the male not
being more than ¼ the size of the female.

Those hermaphrodite Mollusca which are capable of sexual union
(Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently
divided into two sections, according as (1) there are separate
orifices for the male and female organs, or (2) one orifice serves for
both. To the former section (_Digonopora_[2]) belong the Limnaeidae,
Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including
all the Pteropoda; to the latter (_Monogonopora_[255]) nearly all the
Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata.
In the latter case during union, mutual impregnation takes place, and
each of the two individuals concerned has been observed (compare p.
42) to deposit eggs. In the former, however, no such reciprocal act
can take place, but the same individual can play the part of male to
one and female to another, and we sometimes find a string of _Limnaea_
thus united, each being at once male and female to its two adjacent
neighbours.


=The Reproductive System.=--Broadly speaking, the complicated
arrangements which are found in Mollusca resolve themselves into
modifications of three important factors:--

(_a_) The _gonads_ or _germ-glands_, in which are developed the ova
and the spermatozoa. These glands are generally known as the _ovary_ in
the female, the sperm-gland or _testis_ in the male.

(_b_) The channels which provide for the passage of the seminal
products; namely, the _oviduct_ in the female, the _vas deferens_ or
sperm-duct in the male.

(_c_) The external generative organs.

  [Illustration: FIG. 49.--Generative and other organs of
  _Littorina obtusata_ L., _female_.

    =A=, anus.
    =Br=, branchia.
    =Buc=, buccal mass.
    =H=, heart.
    =Hep=, hepatic duct.
    =I=, continuation of oesophagus.
    =Ki=, kidney.
    =Li=, liver.
    =M=, muscle of attachment.
    =O´=, female orifice.
    =Od=, oviduct.
    =Oes=, oesophagus.
    =Ov=, ovary.
    =Ra=, radula.
    =St=, stomach.
    =U=, uterus.

  (After Souleyet.)]

  [Illustration: FIG. 50.--Generative and other organs of
  _Littorina obtusata_ L., _male_.

    =A=, anus.
    =Br=, branchia.
    =H=, heart.
    =I=, intestine.
    =Li=, liver.
    =M=, muscle of attachment.
    =Pe=, penis.
    =Te=, testis.
    =VD=, vas deferens.

  (After Souleyet.)]

=Dioecious Mollusca.=--The common _Littorina obtusata_ will serve as
a typical instance of a dioecious prosobranchiate, exhibiting the
simplest form of organs. In the female the _ovary_, a lobe-shaped
body, is embedded in the liver. An _oviduct_ with many convolutions
conveys the ova into the _uterus_, an oblong chamber which consists
simply of a dilatation of the oviduct. The ova descend into the uterus,
which is sometimes furnished with a seminal pouch. In this seminal
pouch, or above it, in the oviduct, the ova come into contact with
the spermatozoa. The lower part of the uterus secretes a gelatinous
medium (or capsule, as the case may be) in which the fertilised ova
become enclosed previous to exclusion. In position the oviduct abuts
on the kidney, while the uterus is in close proximity to the rectum,
and the female external orifice is found close to the anus, within the
branchial cavity.

The male organs of _Littorina_ are more simple. The _testis_ is lodged,
like the ovary, in the liver; the _vas deferens_ is, like the oviduct,
convoluted, and eventually traverses the right side of the neck,
emerging near the right tentacle, and terminating in the _penis_ or
external copulative organ (Fig. 50).

This system prevails, with but slight modifications in detail,
throughout the prosobranchiate Gasteropoda. The most important
modification is the passage of the seminal products in certain cases
(many of the Diotocardia) through the right kidney, with which the
oviduct and vas deferens always stand in close relation. The same
arrangement occurs in the Scaphoda and some Pelecypoda.

The penis varies greatly in form and size. In the Strombidae (see
Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in
_Littorina_ it is somewhat spinulose at one side; in _Paludina_ a
portion of it is lodged in the right tentacle, which becomes atrophied
and much more obtuse than the tentacle on the left side.

=Spermatozoa.=--The shape of the spermatozoa and of the ova in
Mollusca is of the usual type. In _Paludina Ampullaria_, and certain
species of _Murex_ two types of spermatozoa occur, one hair-like,
the other worm-like, three times as long as the former, and not
tapering at one end. The former type alone take part in fertilisation,
and penetrate the ovum. It has been suggested that these worm-like
spermatozoa are a kind of incipient ova, and indicate a possible
stage in commencing hermaphroditism. And, since the nearest allies of
the Prosobranchiata (in which these types occur) are hermaphrodite
(_i.e._ the Opisthobranchiata and Pulmonata), it is not unreasonable
to suppose that the Prosobranchiata should show some tendency towards
hermaphroditism in their genital glands.[256]

=Cephalopoda.=--The special characteristic of the reproductive organs
in female Cephalopoda is the development of various glands, some of
considerable size, in connexion with the ovary and oviduct. _Sepia_,
_Loligo_, and _Sepiola_ are furnished with two large _nidamental
glands_, which open into the mantle cavity independently of the
oviduct. Their purpose is to produce a viscid mucus, which envelops the
ova at the moment of their emission and eventually hardens into the
egg-capsules. A pair of accessory nidamental glands occur in _Sepia_,
as well as a pair of smaller glands situated on the oviduct itself.

In many of the male Cephalopoda the vas deferens is long and dilated
at its outer end into a glandular reservoir, within which are formed
the spermatophores, or narrow cylindrical packets which contain a very
large number of spermatozoa. When charged, the spermatophores pass
into what is known as Needham’s sac, where they remain until required
for use. These spermatophores are a very characteristic part of the
reproductive arrangements in the Cephalopoda. The male of _Sepia_ has
been noticed to deposit them, during union, upon the buccal membrane
of the female. During the emission of the ova by the female, the
spermatophores, apparently through the agency of a kind of spring
contained at one end, burst, and scatter the spermatozoa over the ova.

=The Hectocotylus Arm.=--Perhaps the most remarkable feature in the
sexual relations of all the Mollusca is the so-called _hectocotylus_
of the Cephalopoda. In the great majority of the male Cephalopoda, one
of the ‘arms,’ which is modified for the purpose in various ways and
to a greater or less extent, becomes charged with spermatophores, and
sometimes, during union, becomes detached and remains within the mantle
of the female, preserving for some considerable time its power of
movement.

The hectocotylus is confined to the dibranchiate Cephalopoda, and
its typical form, _i.e._ when part of the arm becomes disengaged and
left with the female, occurs only in three genera of the Octopodidae,
_viz._[_Argonauta_, _Ocythoe_ (_Philonexis_), and _Tremoctopus_. In
all of these, the male is many sizes smaller than the female. In
_Argonauta_ the third arm on the left side becomes hectocotylised. At
first it is entirely enveloped in a kind of cyst, in such a way that
only a small portion of the tip projects; subsequently the cyst parts
asunder, and allows the arm to become expanded to its full length,
which considerably exceeds that of the other arms. At a certain point
the acetabula or suckers terminate, and the remainder of the arm
consists of a very long, tapering, sometimes thread-like filament,
which is pointed at the extreme tip. It is not yet known how the
spermatophores find their way into the hectocotylus, or how the
hectocotylus impregnates the ova of the female. The arm thus affected
is not always the same. In _Tremoctopus_ it is the third of the right
side, in the _Decapoda_ the modification usually affects the fourth of
the left.

  [Illustration: FIG. 51.--Male of _Ocythoe tuberculata_ Raf. (=
  _Philonexis catenulatus_ Fér.), Mediterranean, showing three
  stages, =A=, =B=, and =C=, in the development of the hectocotylus
  arm: _h.cy_, hectocotylus still in the cyst; _c´y´_, spoon-shaped
  cyst at the end of the arm when freed; _th_, thread-like organ
  freed by the rupture of _c´y´_. Natural size. From specimens in
  the British Museum.]

This singular property of the male Cephalopoda has only recently
been satisfactorily explained. It is true that Aristotle, more than
twenty-two centuries ago, distinctly stated that certain of the arms
were modified for sexual purposes. Speaking of what he calls the
polypus (which appears to represent the _Octopus vulgaris_ of the
Mediterranean), he says: ‘It differs from the female in having what the
fishermen call the white sexual organ _on its arm_;’ again, ‘Some say
that the male has something of a sexual nature (αὶδοιῶδές τι) on one of
its arms, that on which the largest suckers occur; that this is a kind
of muscular appendage attached to the middle of the arm, and that it is
entirely introduced within the funnel of the female’. Unfortunately the
word translated by _introduced_ is corrupt, and can only be restored
conjecturally. He again remarks, ‘The last of the arms, which tapers to
a fine point and is the only whitish arm, it uses in sexual union.’[257]

The typical hectocotylus seems to have entirely escaped notice until
early in the present century, when both Delle Chiaje and Cuvier
described it, as detected within the female, as a _parasite_, the
latter under the name of _Hectocotylus octopodis_. Kölliker, in
1845–49, regarded the Hectocotylus of _Tremoctopus_ as the entire male
animal, and went so far as to discern in it an intestine, heart, and
reproductive system. It was not until 1851 that the investigations of
Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H.
Müller, in 1853, completed the discovery by describing the entire male
of _Argonauta_.

In all genera of dibranchiate Cephalopoda except _Argonauta_,
_Ocythoe_, and _Tremoctopus_, one of the arms is sexually modified
in various ways, but never becomes so much prolonged, and is never
detached and left with the female. In _Loligo Forbesii_ Stp. the fourth
arm on the left has 23 pairs of regularly developed acetabula, which
then lessen in size and disappear, being replaced by long pedunculated
papillae, of which there are about 40 pairs. In _Loligo vulgaris_ Lam.
and _L. Pleii_ Orb. 18 or 19 pairs of acetabula are regularly formed,
and then occur 40 pairs of papillae, as in _Forbesii_. In other species
of _Loligo_ (_gahi_ Orb., _brevis_ Bl., _brasiliensis_ Orb.) only the
outer row of suckers becomes modified into papillae after about the
20th to the 22nd pair. In _Sepioteuthis sepioides_ the modification
is the same as in the _Loligo_ last mentioned, but the corresponding
arm on the right side is so covered with acetabula towards its extreme
end, that it is thought that it in some way co-operates with the
hectocotylised left arm.

In _Octopus_, the third arm on the right side is subject to
modification. This arm is always shorter than the corresponding arm on
the other side, and carries fewer suckers, but is furnished at the
extreme tip with a peculiar kind of plate, which connects with the
membrane at the base of the arm by a channel of skin, which probably
conveys the spermatophores up to the tip.

In _Octopus vulgaris_, the species referred to by Aristotle, the
hectocotylised arm is short, thin in its outer half and pointed at the
extremity, while the fold of skin is very white, and gives the arm
an appearance of being divided by a cleft at the side. At the same
time, an unusual development of one or two suckers on the arm is not
uncommon.[259]

  [Illustration: FIG. 52.--_Octopus lentus_ Baird, N. Atlantic,
  showing the peculiar formation of the hectocotylus arm, _h.a_.
  (After Verrill, × ½.)]

It is believed that in the Tetrabranchiate Cephalopoda (_Nautilus_) a
union of the four inner ventral arms may correspond functionally to the
hectocotylising of the arm in the Dibranchiates.

=Hermaphrodite Mollusca.=--(_a_) _Monogonopora._--The reproductive
system in the hermaphrodite Mollusca is far more complicated than in
the dioecious, from the union of the male and female organs in the same
individual. As a type of the Monogonopora, in which a single orifice
serves for both male and female organs, may be taken the common garden
snail (_Helix aspersa_), the accompanying figure of which is drawn from
two specimens found in the act of union (Fig. 53).

  [Illustration: FIG. 53.--Genitalia of _Helix aspersa_ Müller,
  drawn from two individuals in the act of union, from a dissection
  by F. B. Stead.

  =A.G=, albumen gland.
  =C=, coecum.
  =Cr=, crop.
  =D.S=, dart sac.
  =E=, eye (retracted).
  =Fl=, flagellum.
  =H.D=, hermaphrodite duct.
  =H.DF=, ditto, female portion.
  =H.DM=, ditto, male portion.
  =H.G=, hermaphrodite gland.
  =L=, liver.
  =M.G=, =M.G=, mucous glands.
  =Ov=, oviduct.
  =P.S=, penis sac.
  =R.M=, retractor muscle of penis.
  =Sp=, spermatheca.
  =V=, vagina.
  =V.D=, vas deferens.]

Beginning from the inside and proceeding outwards we have firstly the
_hermaphrodite gland_ or _ovo-testis_ (H.G.), a yellowish white mass
of irregular shape, embedded in the _liver_ (L.) and forming part
of its spiral but not reaching quite to the apex. Within this gland
are developed the ova and spermatozoa. The former are rather large
round cells, produced within the outer wall of the gland, while
the spermatozoa, which are produced in the more central part, are
thread-like bodies, generally aggregated in small bundles. From the
hermaphrodite gland the ova and spermatozoa pass through the upper
part of the _hermaphrodite duct_ (H.D.), which is always more or less
convoluted. Below the convoluted portion, the duct opens into the
_albumen gland_ (A.G.), a large linguiform mass of tissue which becomes
dilated at the time of pairing, and secretes a thick viscid fluid which
probably serves to envelop the ova. Up to this point both the male and
female elements follow the same course, but on their exit from the
albumen gland they diverge. The hermaphrodite duct becomes greatly
enlarged, and is partially divided by a kind of septum into a male and
female portion. These run parallel to one another, the larger or female
portion (H.DF.), through which the ova pass (and which is sometimes
termed the _uterus_) being dilated into a number of puckered folds,
while the smaller or male portion (H.DM.) is comparatively narrow,
and not dilated. At their anterior end, the two portions of the duct
separate completely from one another, the female portion being then
termed the _oviduct_ (OV.) and the male portion the _vas deferens_
(V.D.).

Following first the oviduct, we find that it soon widens into the
_vagina_ (V.), which is furnished with a pair of _mucous glands_
(M.G.), one on each side. These are much branched, and resemble little
bunches of whitish seaweed. A little above the mucous glands a long
tube diverges from the vagina, which is furnished with a produced
_coecum_ (C.) and a pouch, the _spermatheca_ (SP.) at the extreme
end. In this pouch, and in the duct leading to it, is stored the
spermatophore received in union with another snail. Just below the
mucous glands the vagina is joined by the _dart sac_ (D.S.), which is
more fully described below. Finally, at its lower end the vagina unites
with the penis sac at a point just posterior to the common orifice.

Returning now to the male organs, we find that the _vas deferens_
is the continuation of the male portion of the hermaphrodite duct,
after its final separation from the female portion. It passes under
the retractor muscle of the upper right tentacle, which has been cut
away in the specimen figured, to dissect it out. Just before the vas
deferens widens into the penis sac, it branches off into a long and
tapering tube, the _flagellum_, in which the spermatozoa are stored and
become massed together in the long packet known as the _spermatophore_.
The _penis sac_ (P.S.) is the continuation of the vas deferens beyond
the point at which the flagellum diverges. It joins the vagina at
its extreme anterior end, uniting with it to form the common genital
aperture, which cannot be exactly represented in the figure. The
_penis_ itself lies in the interior of the penis sac, and is a rather
long muscular tube which is protruded during union, but at other times
remains retracted within the sac.

In the Helicidae generally, the form of the generative organs varies
with each separate species, sometimes merely as regards the size of
the different parts, at others in the direction of greater simplicity
or complication. The mucous glands may be absent, and the flagellum
greatly reduced in size, or absent altogether.

=The Dart Sac.=--A remarkable part of the reproductive system in many
of the true Helicidae is the so-called _dart_, _Liebespfeil_, or _telum
veneris_. It consists of ‘a straight, or curved, sometimes slightly
twisted tubular shaft of carbonate of lime, tapering to a fine point
above, and enlarging gradually, more often somewhat abruptly, to
the base.’ The sides of the shaft are sometimes furnished with two
or more blades; these are apparently not for cutting purposes, but
simply to brace the stem. The dart is contained in a dart sac, which
is attached as a sort of pocket to the vagina, at no great distance
from its orifice. There are four different forms of sac. It may be
single or double, and each of these divisions may be bilobed, each lobe
containing one dart at a time. In _Helix aspersa_ the dart is about
5/16 in. in length, and ⅛ in. in breadth at its base (see Fig. 54).

It appears most probable that the dart is employed as an adjunct to
the sexual act. Besides the fact of the position of the dart sac
anatomically, we find that the darts are extruded and become embedded
in the flesh just before or during the act of copulation. It may
be regarded, then, as an organ whose punctures induce excitement
preparatory to sexual union. It only occurs in well-grown specimens.
When once it begins to form, it grows very rapidly, perhaps not more
than a week being required for its entire formation.

  [Illustration: FIG. 54.--Darts of British land snails: =A=,
  _Hyalinia excavata_ Bean; =B=, _Helix hortensis_ Müll.; =C=,
  _Helix aspersa_ Müll. (After Ashford.)]

The dart is almost confined to _Helicidae_, a certain number of
exceptions being known which border on _Helix_. _Hyalinia nitida_ and
_excavata_ are the only British species, not _Helices_, which are known
to possess it. It has not been noticed to occur in the slugs, except in
the N. American genus _Tebennophorus_. About one-third of the British
Helices are destitute of the dart.[260] _H. rufescens_ possesses a
double bilobed sac, but only two darts, which lie in the lower lobes.
It does not use the darts, and could not do so, from the relative sizes
of dart and sac; it has often been watched when uniting, but the use of
the darts has never been observed. From this it has been inferred that
the darts are degenerate weapons of defence, and that they were in fact
at one time much stronger organs and more often used.[261] This theory,
however, does not seem consistent with the whole circumstances of the
occurrence, position, and present use of the darts.

=Hermaphrodite Mollusca.=--(_b_) _Digonopora._--As an example of
the _Digonopora_ or hermaphrodite Mollusca with separate generative
apertures for the male and female organs, we may take the common
_Limnaea stagnalis_ (Fig. 55). It will be seen from the figure that
the relative positions of the hermaphrodite gland and duct, and of
the albumen gland, are the same as in _Helix_. When the oviduct parts
company from the vas deferens, it becomes furnished with several
accessory glands, one of which (GL.E.) probably serves as a reservoir
for the ova, and answers more or less to a uterus. The tube leading to
the spermatheca is short, and there is no divergent caecum. The female
orifice lies near to the external opening of the branchial cavity. The
vas deferens, which is very long, is furnished with a large prostate
gland. The penis sac is greatly dilated, and there is no flagellum. The
male orifice is behind the right tentacle, slightly in advance of the
female orifice (compare Fig. 102).

  [Illustration: FIG. 55.--Genitalia of _Limnaea stagnalis_ L.
  (from a dissection by F. B. Stead), × 2.

    =A.G=, albumen gland.
    =Ac.G=, accessory gland.
    =F.O=, female orifice.
    =Gl.E=, glandular enlargement.
    =H.D=, hermaphrodite duct.
    =H.G=, hermaphrodite gland.
    =Li=, liver.
    =M.O=, male orifice.
    =P=, penis sac.
    =Pr=, prostate.
    =R.M=, retractor muscle of penis.
    =Sp=, spermatheca.
    =V.D=, vas deferens.]

Most of the _Opisthobranchiata_, but not all, have separate sexual
orifices. Numerous variations from the type just described will be
found to occur, particularly in the direction of the development of
accessory glands, which are sometimes very large, and whose precise
purpose has in many cases not been satisfactorily determined.

_Pelecypoda._--In the dioecious Pelecypoda, which form the great
majority, the reproductive system is simple, and closely parallel in
both sexes. It consists of a pair of gonads, which are either ovaries
or testes, and a pair of oviducts or sperm-ducts which lead to a
genital aperture. The gonads are usually placed symmetrically at the
sides or base of the visceral mass. The oviduct is short, and the
genital aperture is usually within the branchial chamber, thus securing
the fertilisation of the ova by the spermatozoa, which are carried into
the branchial chamber with the water which passes through the afferent
siphon.

Hermaphrodite Pelecypoda are rare, the sexes being usually separate.
The following are assured instances: _Pecten glaber_, _P. jacobaeus_,
_P. maximus_, _Ostrea edulis_, _Cardium norvegicum_, _Pisidium
pusillum_, _Cyclas cornea_, _Pandora rostrata_, _Aspergillum
dichotomum_, and perhaps _Clavagella_. The greater number of these
have only a single genital gland (gonad) on each side, with a single
efferent duct from each, but part of the gland is male and part female,
_e.g._ in the Pectens above mentioned. _Pandora_ and _Aspergillum_ have
two distinct glands, respectively male and female, on each side, each
of the two glands possessing its separate duct, and the two ducts from
each side eventually opening near one another. It appears probable that
the _Septibranchiata_ (_Cuspidaria_, _Poromya_, _Lyonsiella_, etc.)
must also be added to the number of hermaphrodite Pelecypoda which have
separate male and female glands.

It is worthy of remark that all the hermaphrodite Pelecypoda belong to
forms decidedly specialised, while forms distinctly primitive, such as
_Nucula_, _Solenomya_, _Arca_, and _Trigonia_ are all dioecious. In
Gasteropoda similarly, the least specialised forms (the _Amphineura_,
with the exception of the _Neomeniidae_, and the _Rhipidoglossa_)
are dioecious. It is possible therefore that in the ancestors of the
Mollusca the separation of the sexes had already become the normal
type of things, and that hermaphroditism in the group is, to a certain
extent, a sign or accompaniment of specialisation.[262]

=Development of Fresh-water Bivalves.=--The vast majority of
fresh-water bivalves either pass the larval stage entirely within
the mother, and do not quit her except in a perfectly developed form
(_Cyclas_, _Pisidium_), or assume a mode of development in which free
larvae indeed occur, but are specially modified for adaptation to
special circumstances (_Unio_). _Cyclas_ and _Pisidium_, and no doubt
all the kindred genera, preserve their ova in a sort of brood-pouch
within the gills, in which the ova pass the earlier stages of their
development. But, even so, the larva of these genera retains some
traces of its original free-swimming habits, for a rudimentary velum,
which is quite useless for its present form of development, has been
detected in _Cyclas_.

The larva of _Dreissensia_ (see Fig. 47, A), so far as is at present
known, stands alone among fresh-water bivalves in being free-swimming,
and to this property has been attributed, no doubt with perfect
justice, the fact of the extraordinarily rapid spread of _Dreissensia_
over the continent of Europe (chap. xvi.). In expelling the ova,
the parent slightly opens the shells and then quickly closes them,
shooting out a small point of white slime, which is in fact a little
ball of eggs. The general course of development is precisely parallel
to that of marine _Pelecypoda_, greatly resembling, so far as form is
concerned, certain stages in the growth of the larvae of _Modiolaria_
and _Cardium_, as figured by Lovén.[263]

In June and July the larvae appear in large numbers on the surface of
the water, when in spite of their exceedingly small size, they can
be captured with a fine hand-net. They pass about eight days on the
surface, feeding apparently on minute floating algae. During this time,
the principal change they undergo is in the formation of the foot,
which first appears as a small prominence midway between the mouth
and anus, and gradually increases in length and flexibility. When the
larva sinks to the bottom, the velum soon disappears entirely, the
foot becomes exceedingly long and narrow, while the shell is circular,
strongly resembling a very young _Cyclas_.

=Larvae of Unionidae.=--The early stages of the development of _Unio_
and _Anodonta_ (so far as the species of North America, Europe, and
Asia are concerned) is of extreme interest, from the remarkable fact
that the young live for some time parasitically attached to certain
species of fresh-water fishes. In order to secure this attachment,
the larva, which is generally known as _Glochidium_, develops a long
filament which perhaps renders it aware of the neighbourhood of a fish,
and also a larval shell furnished with strong hooks by which it fastens
itself to the body of its unconscious host (Fig. 56). According to some
interesting observations made by Mr. O. H. Latter,[264] the ova pass
into the external gill of the mother, in which is secreted a nutritive
mucus on which they are sustained until they arrive at maturity and a
suitable opportunity occurs for their ‘being born.’ If this opportunity
is deferred, and the _Glochidia_ mature, their so-called ‘byssus’
becomes developed, and by being entangled in the gill filaments of the
parent, prevents their escaping. It is interesting to notice that, when
the nutritive mucus of the parent is used up, it becomes, as it were,
the turn of the children to provide for themselves a secondary mode of
attachment.

  [Illustration: FIG. 56.--=A=, _Glochidium_ immediately after it
  is hatched: _ad_, adductor muscle; _by_, ‘byssus’ cord; _s_,
  sense organs; _sh_, shell. =B=, _Glochidium_ after it has been on
  the fish for some weeks: _a.ad_, _p.ad_, anterior and posterior
  adductors; _al_, alimentary canal; _au.v_, auditory vesicle;
  _br_, branchiae; _f_, foot; _mt_, mantle. (Balfour.)]

The mother _Anodonta_ does not always retain the _Glochidium_ until
fish are in her neighbourhood. Gentle stirring of the water caused
them to emit _Glochidium_ in large masses, if the movement was not so
violent as to cause alarm. The long slimy masses of _Glochidium_ were
observed to be drawn back again within the shell of the mother, even
after they had been ejected to a distance of 2 or 3 inches.

It is a mistake to assert that the young _Glochidium_ can swim. When
they finally quit the mother, they sink to the bottom, and there
remain resting on their dorsal side, with the valves gaping upwards and
the so-called byssus streaming up into the water above them. There they
remain, until a convenient ‘host’ comes within reach, and if no ‘host’
comes within a certain time, they perish. They are evidently peculiarly
sensitive to the presence of fish, but whether they perceive them by
smell or some other sense is unknown. “The tail of a recently killed
stickleback thrust into a watch-glass containing _Glochidium_ throws
them all into the wildest agitation for a few seconds; the valves are
violently closed and again opened with astonishing rapidity for 15–25
seconds, and then the animals appear exhausted and lie placid with
widely gaping shells--unless they chance to have closed upon any object
in the water (_e.g._ another _Glochidium_), in which case the valves
remain firmly closed.”

In about four weeks after the _Glochidium_ has quitted its host, and
the permanent shell has made its appearance _within_ the two valves
of the _Glochidium_, the projecting teeth of the latter press upon
the ventral edge of the permanent shell, at a point about half way in
its lengthward measurement, retarding the growth of the shell at that
particular point, and indenting its otherwise uninterrupted curve with
an irregular notch or dent. As growth proceeds, this dent becomes less
and less perceptible on the ventral margin of the shell itself, but its
effects may be detected, in well-preserved specimens, by the wavy turn
in the lines of growth, especially near the umbones of the young shell.

Mr. Latter found that all species of fish with which he experimented
had a strong dislike to _Glochidium_ as an article of food. Sometimes
a fish would taste it “just to try,” but invariably spit it out again
in a very decided manner. The cause of unpleasantness seemed not to be
the irritation produced in the mouth of the fish by the attempt of the
_Glochidium_ to attach itself, but was more probably due to what the
fish considered a nasty taste or odour in the object of his attentions.

       *       *       *       *       *

The following works will be found useful for further study of this
portion of the subject:--

   =F. M. Balfour=, Comparative Embryology, vol. i. pp. 186–241.

   =F. Blochmann=, Ueber die Entwickelung von Neritina fluviatilis
   Müll.: Zeit. wiss. Zool. xxxvi. (1881), pp. 125–174.

   =L. Boutan=, Recherches sur l’anatomie et le développement de la
   Fissurelle: Arch. Zool. exp. gén. (2) iii. suppl. (1885), 173
   pp.

   =W. K. Brooks=, The development of the Squid (Loligo Pealii
   Les.): Anniv. Mem. Bost. Soc. Nat. Hist. 1880.

      „     „ The development of the oyster: Studies Biol. Lab. Johns
   Hopk. Univ. i. (1880), 80 pp.

   =R. von Erlanger=, Zur Entwickelung von Paludina vivipara:
   Morph. Jahrb. xvii. (1891), pp. 337–379, 636–680.

     „      „ Zur Entwickelung von Bythinia tentaculata: Mitth. Zool.
   Stat. Neap, x. (1892), pp. 376–406.

   =H. Fol=, Sur le développement des Ptéropodes: Arch. Zool. exp.
   gén. iv. (1875), pp. 1–214.

     „ Etudes sur le développement des Mollusques. Hétéropodes: ibid
   v. (1876), pp. 105–158.

     „ Etudes sur le développement des Gastéropodes pulmonés: ibid.
   viii. (1880), pp. 103–232.

   =H. Grenacher=, Zur Entwickelungsgeschichte der Cephalopoden:
   Zeit. wiss. Zool. xxiv. (1874), pp. 419–498.

   =B. Hatschek=, Ueber Entwickelungsgeschichte von Teredo: Arb.
   Zool. Inst. Univ. Wien, iii. (1881), pp. 1–44.

   =R. Horst=, On the development of the European oyster: Quart.
   Journ. Micr. Sc. xxii. (1882), pp. 339–346.

   =E. Korschelt= and =K. Heider=, Lehrbuch der vergleichenden
   Entwickelungsgeschichte der wirbellosen Thiere, Heft iii.
   (1893), pp. 909–1177 (the work is in process of translation into
   English).

   =A. Kowalewsky=, Embryogénie du Chiton polii avec quelques
   remarques sur le développement des autres Chitons: Ann. Mus.
   Hist. Nat. Mars. Zool. i. (1883), v.

   =E. Ray Lankester=, Contributions to the developmental history
   of the Mollusca: Phil. Trans. Roy. Soc. vol. 165 (1875), pp.
   1–31.

     „     „ Observations on the development of the pond-snail
   (Lymnaeus stagnalis), and on the early stages of other Mollusca:
   Quart. Journ. Micr. Sc. xiv. (1874), pp. 365–391.

     „    „ Observations on the development of the Cephalopoda: ibid.
   xv. (1875), pp. 37–47.

   =W. Patten=, The embryology of Patella: Arb. Zool. Inst. Univ.
   Wien, vi. (1886), pp. 149–174.

   =M. Salensky=, Études sur le développement du Vermet: Arch.
   Biol. vi. (1885), pp. 655–759.

   =L. Vialleton=, Recherches sur les premières phases du
   développement de la Seiche (Sepia officinalis): Ann. Sc. Nat.
   Zool. (7) vi. (1888), pp. 165–280.

   =S. Watase=, Observations on the development of Cephalopods:
   Stud. Biol. Lab. Johns Hopk. Univ. iv. (1888), pp. 163–183.

    „   „ Studies on Cephalopods: Journ. Morph. iv. (1891), pp.
   247–294.

   =E. Ziegler=, Die Entwickelung von Cyclas cornea Lam.: Zeit.
   wiss. Zool. xli. (1885), pp. 525–569.




                              CHAPTER VI

                RESPIRATION AND CIRCULATION--THE MANTLE


The principle of respiration is the same in the Mollusca as in all
other animals. The blood is purified by being brought, in successive
instalments, into contact with pure air or pure water, the effect of
which is to expel the carbonic acid produced by animal combustion, and
to take up fresh supplies of oxygen. Whether the medium in which a
mollusc lives be water or air, the effect of the respiratory action is
practically the same.

Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’
water, while those whose usual habitat is the land ‘breathe’ air. But
this rule has its exceptions on both sides. The great majority of the
fresh-water Mollusca which are not provided with an operculum (_e.g._
_Limnaea_, _Physa_, _Planorbis_), breathe air, in spite of living in
the water. They make periodic visits to the surface, and take down
a bubble of air, returning again for another when it is exhausted.
On the other hand many marine Mollusca which live between tide-marks
(_e.g._ _Patella_, _Littorina_, _Purpura_, many species of _Cerithium_,
_Planaxis_, and _Nerita_) are left out of the water, through the
bi-diurnal recess of the tide, for many hours together. Such species
invariably retain several drops of water in their branchiae, and,
aided by the moisture of the air, contrive to support life until the
water returns to them. Some species of _Littorina_ (_e.g._ our own _L.
rudis_ and many tropical species) live so near high-water mark that
at neap-tides it must frequently happen that they are untouched by
the sea for several weeks together, while they are frequently exposed
to a burning sun, which beats upon the rocks to which they cling. In
this case it appears that the respiratory organs will perform their
functions if they can manage to retain an extremely small amount of
moisture.[265]

The important part which the respiratory organs play in the economy of
the Mollusca may be judged from the fact that the primary subdivision
of the Cephalopoda into Dibranchiata and Tetrabranchiata is based
upon the number of branchiae they possess. Further, the three great
divisions of the Gasteropoda have been named from the position or
character of the breathing apparatus, _viz_. Prosobranchiata,
Opisthobranchiata and Pulmonata, while the name Pelecypoda has hardly
yet dispossessed Lamellibranchiata, the more familiar name of the
bivalves.

Respiration may be conducted by means of--(_a_) Branchiae or Gills,
(_b_) a Lung or Lung-cavity, (_c_) the outer skin.

In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of
the Gasteropoda, respiration is by means of branchiae, also known as
_ctenidia_[266], when they represent the primitive Molluscan gill and
are not ‘secondary’ branchiae (pp. 156, 159).

In all non-operculate land and fresh-water Mollusca, in the
Auriculidae, and in one aberrant operculate (_Amphibola_), respiration
is conducted by means of a lung-cavity, or rarely by a true lung,
whence the name _Pulmonata_. The land operculates (Cyclophoridae,
Cyclostomatidae, Aciculidae, and Helicinidae) also breathe air, but are
not classified as Pulmonata, since other points in their organisation
relate them more closely to the marine Prosobranchiata. Both methods of
respiration are united in _Ampullaria_, which breathes indifferently
air through a long siphon which it can elevate above the surface of the
water, and water through a branchia (see p. 158). _Siphonaria_ (Fig.
57) is also furnished with a lung-cavity as well as a branchia. Both
these genera may be regarded as in process of change from an aqueous
to a terrestrial life, and in _Siphonaria_ the branchia is to a great
extent atrophied, since the animal is out of the water, on the average,
twenty-two hours out of the twenty-four. In the allied genus _Gadinia_,
where there is no trace of a branchia, but only a lung-cavity, and
in _Cerithidea obtusa_, which has a pulmonary organisation exactly
analogous to that of _Cyclophorus_,[267] this process may be regarded
as practically completed.

  [Illustration: FIG. 57.--=A=, _Siphonaria gigas_ Sowb., Panama,
  the animal contracted in spirit: _gr_, siphonal groove on right
  side. =B=, _Gadinia peruviana_, Sowb., Chili, shell only: _gr_,
  mark of siphonal groove to right of head.]

Respiration by means of the skin, without the development of any
special organ, is the simplest method of breathing which occurs in the
Mollusca. In certain cases, _e.g._ _Elysia_, _Limapontia_, and _Cenia_
among the Nudibranchs, and the parasitic _Entoconcha_ and _Entocolax_,
none of which possess breathing organs of any kind, the whole outer
surface of the body appears to perform respiratory functions. In
others, the dorsal surface is covered with papillae of varied size and
number, which communicate with the heart by an elaborate system of
veins. This is the case with the greater number of the _Aeolididae_
(Fig. 58, compare Fig. 5, C), but it is curious that when the animal is
entirely deprived of these papillae, respiration appears to be carried
on without interruption through the skin.

  [Illustration: FIG. 58.--_Aeolis despecta_ Johnst., British
  coasts. (After Alder and Hancock.)]

In the development of a distinct breathing organ, it would seem as if
progress had been made along two definite lines, each resulting in
the exposure of a larger length of veins, _i.e._ of a larger amount
of blood, to the simultaneous operation of fresh air or fresh water.
Either (_a_) the skin itself may have developed, at more or less
regular intervals, elevations, or folds, which gradually took the form
of papillae, or else (_b_) an inward folding, or ‘invagination,’ of
the skin, or such a modification of the mantle-fold as is described
below (p. 172) may have taken place, resulting in the formation of a
cavity more or less surrounded by walls, within which the breathing
organs were ultimately developed. Sometimes a combination of both
processes seems to have occurred, and after a papilliform organ has
been produced, an extension or prolongation of the skin has taken
place, in order to afford a protection to it. Respiration by means of a
lung-cavity is certainly subsequent, in point of time, to respiration
by means of branchiae.

  [Illustration: FIG. 59.--_Chiton squamosus_ L., Bermuda: =A=,
  anus; =Br=, branchiae; =M=, mouth.]

  [Illustration: FIG. 60.--_Fissurella virescens_ Sowb., Panama,
  showing position of the branchiae: =Br=, branchiae: =E=, =E=,
  eyes; =F=, foot; =M=, mantle; =T=, =T=, tentacles.]

The branchiae seem to have been originally paired, and arranged
symmetrically on opposite sides of the body. It is not easy to decide
whether the multiple form of branchia which occurs in _Chiton_ (Fig.
59), or the simple form as in _Fissurella_ (Fig. 60), is the more
primitive. Some authorities hold that the multiple branchia has
gradually coalesced into the simple, others that the simple form has
grown, by serial repetition, into the multiple. There appears to be no
trace of any intermediate forms, and, as a matter of fact, the multiple
branchia is found only in the _Amphineura_, while one or rarely
two (never more) pairs of branchiae, occur, with various important
modifications, in the vast majority of the Mollusca.

_Amphineura._--In _Chiton_ the branchiae are external, forming a long
row of short plumes, placed symmetrically along each side of the foot.
The number of plumes, at the base of each of which lies an osphradial
patch, varies from about 70 to as few as 6 or 7. When the plumes are
few, they are confined to the posterior end, and thus approximate
to the form and position of the branchiae in the other Amphineura.
In _Chaetoderma_, the branchiae consist of two small feather-shaped
bodies, placed symmetrically on either side of the anus, which opens
into a sort of cloaca within which the branchiae are situated. In
_Neomenia_ the branchiae are still further degraded, consisting
of a single bunch of filaments lying within the cloaca, while in
_Proneomenia_ there is no more than a few irregular folds on the
cloaca-wall (Fig. 61).

  [Illustration: FIG. 61.--Terminal portions of the Amphineura,
  illustrating the gradual degradation of the branchiae, and
  their grouping round the anus in that class. =A=, _Chiton_
  (_Hemiarthrum_) _setulosus_ Carp., Torres Str.; =B=, _Chiton_
  (_Leptochiton_) _benthus_ Hadd., Torres Str.; =C=, _Chaetoderma_;
  =D=, _Neomenia_; _a_, anus; _br_, _br_, branchiae; _k_, _k_,
  kidneys; _p_, pericardium. (=A= and =B= after Haddon, =C= and =D=
  after Hubrecht.)]

In the _Prosobranchiata_, symmetrically paired branchiae occur only
in the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former
of which two perfectly equal branchiae are situated on either side
of the back of the neck. These three families taken together form
the group known as _Zygobranchiata_.[268] In all other families the
asymmetry of the body has probably caused one of the branchiae,
the right (originally left), to become aborted, and consequently
there is only one branchia, the left, in the vast majority of
marine Prosobranchiata, which have been accordingly grouped as
_Azygobranchiata_. Even in _Haliotis_ the right branchia is rather
smaller than the left, while the great size of the attachment muscle
causes the whole branchial cavity to become pushed over towards the
left side. In those forms which in other respects most nearly approach
the Zygobranchiata, namely, the Trochidae, Neritidae, and Turbinidae,
the branchia has two rows of filaments, one on each side of the long
axis, while in all other Prosobranchiata there is but one row (see Fig.
79, p. 169).

  [Illustration: FIG. 62.--_Bullia laevissima_ Gmel., showing
  branchial siphon =S=; =F=, =F=, =F=, foot; =OP=, operculum; =P=,
  penis; =Pr=, proboscis; =T=, =T=, tentacles. (After Quoy and
  Gaimard.)]

In the great majority of marine Prosobranchiata the branchia is
securely concealed within a chamber or pouch (the respiratory cavity),
which is placed on the left dorsal side of the animal, generally near
the back of the neck. For breathing purposes, water has to be conveyed
into this chamber, and again expelled after it has passed over the
branchia. In the majority of the vegetable-feeding molluscs (_e.g._
_Littorina_, _Cerithium_, _Trochus_) water is carried into the chamber
by a simple prolongation of one of the lobes or lappets of the mantle,
and makes its exit by the same way, the incoming and outgoing currents
being separated by a valve-like fringe depending from the lobe. In the
carnivorous molluscs, on the other hand, a regular tube, the _branchial
siphon_, which is more or less closed, has been developed from a fold
of the mantle surface, for the special purpose of conducting water to
the branchia. After performing its purpose there, the spent water does
not return through the siphon, but is conducted towards the anus by
vibratile cilia situated on the branchiae themselves. In a large number
of cases, this siphon is protected throughout its entire length by a
special prolongation of the shell called the canal. Sometimes, as in
_Buccinum_ and _Purpura_, this canal is little more than a mere notch
in the ‘mouth’ of the shell, but in many of the Muricidae (_e.g._ _M.
haustellum_, _tenuispina_, _tribulus_) the canal becomes several inches
long, and is set with formidable spines (see Fig. 164, p. 256). In
_Dolium_ and _Cassis_ the canal is very short, but the siphon is very
long, and is reflected back over the shell.

The presence or absence of this siphonal notch or canal forms a fairly
accurate indication of the carnivorous or vegetarian tendencies of most
marine Prosobranchiata, which have been, on this basis, subdivided
into _Siphonostomata_ and _Holostomata_. But this classification is
of no particular value, and is seriously weakened by the fact that
_Natica_, which is markedly ‘holostomatous,’ is very carnivorous, while
_Cerithium_, which has a distinct siphonal notch, is of vegetarian
tendencies.

In the Zygobranchiata the water, after having aerated the blood in the
branchiae, usually escapes by a special hole or holes in the shell,
situated either at the apex (_Fissurella_) or along the side of the
last whorl (_Haliotis_). In _Pleurotomaria_ the slit answers a similar
purpose, serving as a sluice for the ejection of the spent water, and
thus preventing the inward current from becoming polluted before it
reaches the branchiae (see Fig. 179, p. 266).

In _Patella_ the breathing arrangements are very remarkable. In spite
of their apparent external similarity, this genus possesses no such
symmetrically paired plume-shaped branchiae as _Fissurella_, but we
notice a circlet of gill-lamellae, which extends completely round the
edge of the mantle. It has been shown by various authorities that
these lamellae are in no sense morphologically related to the paired
branchiae in other Mollusca, but only correspond to them functionally.
The typical paired branchiae, as has been shown by Spengel, exist
in _Patella_ in a most rudimentary form, being reduced to a pair
of minute yellow bodies on the right and left sides of the back of
the ‘neck.’ A precisely similar abortion of the true branchiae, and
special development of a new organ to perform their work, is shown in
_Phyllidia_ and _Pleurophyllidia_ (see below under Opisthobranchiata).
This circlet of functional gills in _Patella_ has therefore little
systematic value, being only developed in an unusual position, like
the eyes on the mantle in certain _Pelecypoda_, to supply the place of
the true organs which have fallen into disuse. Accordingly Cuvier’s
class of _Cyclobranchiata_, which included _Patella_ and _Chiton_, has
no value, and has indeed long been discarded. In _Chiton_ the gills
never extend completely round the animal, but are always more or less
interrupted at the head and anus. They are the true gills, the plumes
being serially repeated in the same way as the shell plates.

  [Illustration: FIG. 63.--_Patella vulgata_ L., seen from the
  ventral side: _f_, foot; _g.l_, circlet of gill lamellae; _m.e_,
  edge of the mantle; _mu_, attachment muscle; _sl_, slits in the
  same; _sh_, shell; _v_, vessel carrying aerated blood to the
  heart; _v´_, vessel carrying blood from the heart; _ve_, small
  accessory vessels.]

  [Illustration: FIG. 64.--_Patella vulgata_ L., seen from the
  dorsal side after the removal of the shell and the black pigment
  covering the integument; the anterior portion of the mantle is
  cut away or turned back: _a_, anus; _br_, _br_, remains of the
  true branchiae (ctenidia); _i_, intestine; _k_, _k´_, kidneys;
  _k.ap_, their apertures on each side of the anus; _l_, liver;
  _m_, _m_, mantle; _mu_, attachment muscles, severed in removal of
  shell; _t_, _t_, tentacles.]

In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae,
Aciculidae, Helicinidae) which, having exchanged a marine for an
aerial life, breathe air instead of water, the branchia has completely
disappeared, and breathing is conducted, as in the Pulmonata, by a
lung-cavity. In certain genera of land operculates, _e.g._ _Pupina_,
_Cataulus_, _Pterocyclus_, a slight fissure or tube in the last whorl
(see Fig. 180, p. 266) serves to introduce air into the shell, which
is perhaps otherwise closed to air by the operculum. In _Aulopoma_,
which has no tube, the operculum admits free circulation of air. In
certain other Cyclostomatidae the apex is truncated, and air can enter
there. De Folin closed with wax the aperture of _Cycl. elegans_, and
found that on placing it in a pneumatic machine, the shell gave off
air through its whole surface. On the other hand, _Cylindrella_ and
_Stenogyra decollata_, on being submitted to the same test, showed that
the truncated part alone was permeable by air.

Fischer and Bouvier have made some interesting observations on the
breathing of a species of _Ampullaria_ (_insularum_ Orb.). The species
has, in common with all _Ampullaria_, two siphons, but while the
right siphon is but slightly developed, the left is very long, almost
twice as long as the shell (see Fig. 65). The animal, when under the
water, lengthens its siphon, brings the orifice to the surface, and by
alternately raising and depressing its head produces in the pulmonary
sac movements of ex- and inspiration; these are repeated about ten or
fifteen times at regular intervals of from six to eight seconds, a
method of respiration strongly resembling that of the Cetacea. At the
same time, branchial respiration takes place. If powdered carmine is
added to water, the particles are seen to enter the branchial cavity
by the siphon and pass out by the short right siphon. Sometimes the
animal remains under water for hours without rising to the surface to
inspire air. In _Valvata_ (Fig. 66) the branchia is very large, and
projects like a leaf or fan above the shell on the left side; on the
corresponding position on the right side is a long filiform appendage,
which some have regarded as representing the other branchia.

  [Illustration: FIG. 65.--_Ampullaria insularum_ Orb.: =A=,
  breathing water; =B=, breathing air; =Si=, siphon; =T=, upper;
  _t_, lower tentacles; =X=, pallial expansion, performing the part
  of excurrent siphon. (After Fischer and Bouvier, x ⅓.)]

_Opisthobranchiata._--A true branchia occurs only in the
Tectibranchiata and the Ascoglossa. It lies on the right side, and is
usually more or less external, being partly covered sometimes by the
shell (as in _Umbrella_, Fig. 5), sometimes by a fold of the mantle. In
the Pteropoda (which are probably derived from the Tectibranchiata),
all the Thecosomata, with the exception of _Cavolinia_, have no
specialised branchia, but probably respire through portions or the
whole of the integument. In the Gymnosomata an accessory branchia
has in many cases been developed at the posterior end of the body.
_Pneumodermon_ alone has both lateral and posterior branchiae well
developed, _Clione_ and _Halopsyche_ are destitute of either, while the
four remaining families have one branchia, sometimes lateral, sometimes
posterior.[269]

  [Illustration: FIG. 66.--_Valvata piscinalis_ Müll.: _br_,
  branchia; _fi_, filament; _f.l_, foot lobes. (After Boutan.)]

  [Illustration: FIG. 67.--_Doris_ (_Archidoris_) _tuberculata_ L.,
  Britain: _a_, anus; _br_, branchiae, surrounding the anus; _m_,
  male organ; _rh_, _rh_, rhinophores. × ⅔.]

  [Illustration: FIG. 68.--_Pleurophyllidia lineata_ Otto,
  Mediterranean: _a_, anus; _br_, secondary branchiae; _m_, mouth;
  _s.o_, sexual orifice.]

Certain of the Nudibranchiata possess no special breathing organs, and
probably respire through the skin (_Elysia_, _Limapontia_, _Cenia_,
_Phyllirrhoë_). The majority, however, have developed secondary
branchiae, in the form of prominent lobes or leaf-like processes (the
_cerata_), which are carried upon the back, without any means of
protection. These cerata are, as a rule, of extreme beauty and variety
of form, consisting sometimes of long whip-like tentaculae, in other
cases of arborescent plumes of fern-like leafage, in others of curious
bead-like appendages of every imaginable shape and colour. In _Doris_
they lie at the posterior end of the body, in a sort of rosette, which
is generally capable of retraction into a chamber. In _Phyllidia_ and
_Pleurophyllidia_ these secondary branchiae lie, as in _Patella_, on
the lateral portions of the mantle.

The Scaphopoda in all probability possess neither true nor secondary
branchiae.

_Pulmonata._--When we use the term ‘lung,’ it must be remembered that
this organ in the Mollusca does not correspond, morphologically, with
the spongy, cellular lung of vertebrates; it simply performs the same
functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined
with blood-vessels which are disposed over its vaulted surface in
various patterns of network. The pulmonary sac or cavity is therefore a
better name by which to denote this organ.

  [Illustration: FIG. 69.--_Geomalacus maculosus_ Allm., S.
  Ireland: =P.O=, pulmonary orifice.]

It seems probable, as has been already shown (pp. 18–22), that all
Pulmonata are ultimately derived from marine forms which breathed
water by means of branchiae. Thus we find intermediate forms, such as
_Siphonaria_, possessed of both a branchia and a pulmonary sac, the
former being evanescent, while in _Gadinia_ and _Amphibola_ it has
quite disappeared. In the vast majority of _Pulmonata_ no trace of
a branchia remains; its function is performed by a chamber, always
situated at the right side of the animal, and generally more or less
anterior, admitting air by a narrow aperture which is rhythmically
opened and closed. In _Arion_ and _Geomalacus_ (Fig. 69) this aperture
is in the front of the right side of the ‘shield,’ in _Limax_ (Fig. 71)
in the hinder part, in _Testacella_ (Fig. 20) it is near the extremity
of the tail, under the spire of the shell; in _Janella_ it is on
the middle of the right edge of the shield (Fig. 70). If a specimen
of _Helix aspersa_, or better, of _H. pomatia_, is held up to the
light, the beautiful arborescent vessels, with which the upper part
of the pulmonary chamber is furnished, can be clearly seen by looking
through the aperture as it dilates. It is only in the Auriculidae
that an actual spongy mass of lung material appears to exist. When
in motion, a _Helix_ inspires air much more frequently than when at
rest. Temperature, too, seems to affect the number of inspirations; it
appears doubtful whether, during hibernation, a snail breathes at all.
In any case, the amount of air required to sustain life must be small.

  [Illustration: FIG. 70.--_Janella hirudo_ Fisch., N. Caledonia:
  =G=, generative orifice; =P=, pulmonary orifice; =T=, =T=,
  tentacles. (After Fischer.)]

  [Illustration: FIG. 71.--_Limax maximus_ L.: =PO=, pulmonary
  orifice. × ⅔.]

With regard to the respiration of fresh-water Pulmonata there appears
to be some difference of opinion. It is held, on the one hand, that
the Limnaeidae only respire air, making periodic visits to the surface
to procure it, and that they perish, if prevented from doing so, by
asphyxiation. If, we are told,[270] as a _Limnaea_ is floating on
the surface of the water in a glass jar, a morsel of common salt be
dropped upon its outstretched foot, it will sink heavily to the bottom,
emitting a stream of air from its pulmonary orifice. On recovering
from the shock, it will anxiously endeavour to regain the surface, but
will have some difficulty in doing so, owing to its now much greater
specific gravity. When it succeeds, it creeps almost out of the
water, and exposes its respiratory orifice freely to the air. If the
experiment is repeated several times on the same individual, it becomes
so much weakened that it has to be taken out of the water to save its
life. Moquin-Tandon, on the other hand, is strongly of opinion[271]
that there is no absolute necessity for _Limnaea_ to obtain air by
rising to the surface, and that, if prevented from emerging, it can
obtain air from the water. When covered in by a roof of ice, _Limnaea_
has not been observed to suffer any inconvenience. Moquin-Tandon kept
_L. glabra_ and _Planorbis rotundatus_ in good health under 20 mm.
of water for eighteen and nineteen days, and relates a case in which
_Physa_ was kept alive under water for four days, and _Planorbis_ for
twelve. Young specimens, both of _Limnaea_ and _Planorbis_, do not rise
to the surface for a supply of air; they are hatched with the pulmonary
cavity full of water.

It is probable, therefore, that Limnaeidae are capable, on occasion,
of respiration through the skin. Some authorities are of opinion that
certain long and narrow lamellae, situated within the pulmonary sac,
are employed for the purpose of aqueous respiration. _Ancylus_, which
never makes periodic excursions to the surface, perhaps respires by
receiving into its pulmonary chamber the minute quantities of oxygen
given off by the vegetation on which it feeds.

Limnaeidae taken from a great depth of water, _e.g._ from 130 fathoms
in the lake of Geneva, have been examined by Forel.[272] The pulmonary
sac is full of water, but there is no transformation of organs, no
appearance of a branchia, to meet the changed circumstances of their
environment. Doubtless a good deal of respiration is done by the skin;
being soft and vascular, it respires the air dissolved in the water.
Forel cites cases of _Limnaea_ living at much shallower depths, which
come to the surface once, and then remain below for months. The oxygen
of this supply must soon have become exhausted, and the animals,
discontinuing for a time the use of the pulmonary chamber, must have
respired through the skin. Shallow-water _Limnaea_, according to the
same authority, remain beneath the surface during cold weather; when
warm weather returns they rise to the surface to take in a supply of
air. Since the water at great depths is always very cold, there is no
need for the _Limnaea_ living there to rise to the surface at all.

It is a curious fact that _Limnaea_, which have been respiring by
the skin for the whole winter, should suddenly, on the first warm
days of summer, take to rising to the surface and breathing air. But
exactly the same phenomenon is shown in the case of _Limnaea_ from
great depths. Placed in an aquarium, they _immediately_ begin rising
to the surface and inspiring air; in other words, they experience
instantaneously a complete transformation of their respiratory system.

In _Onchidium_, a land pulmonate which has retrogressed to an
amphibious or quasi-marine mode of life, there is no organ which
represents the pulmonary or branchial cavity, the so-called lung being
only a cavity of the kidney. Respiration is, however, conducted by the
skin as well, and by the dorsal papillae.[273]

Land Mollusca can sustain, for a considerable time, complete
deprivation of atmospheric air. Helices placed in an exhausted receiver
show no signs of being inconvenienced for about 20 hours, and are able
to survive for about two or three days. If detained under water, they
are very active for about 6 hours, then become motionless, the body
swells, owing to the water absorbed, and death ensues in about 36
hours. Immersion for only 24 hours is generally followed by recovery.
In the latter case, the cause of death is not so much deprivation
of air as compulsory absorption of water by the skin. The amount of
water thus taken up is surprising. Spallanzani found that a _Helix_
which weighed 18 grammes increased in weight by 13½ grammes after a
prolonged immersion. Even slugs enclosed in moist paper gained more
than 2 grammes in the course of half an hour. Experiment has shown
that the amount of carbonic acid gas produced by respiration stands in
direct relation to the amount of food consumed. Four pairs of snails
were taken which had recently awakened from their winter sleep and had
eaten heartily, and an equal number, under the same circumstances,
which had been prevented from eating. It was found that the first
four pairs produced, in consuming a given amount of oxygen, 11, 9,
10, and 13 parts respectively of carbonic acid, while the second
set produced, in consuming the same amount of oxygen, only 4, 8, 7,
and 9 parts of carbonic acid.[274] Hibernating Helices, if weighed
in December and again in April, will be found to have lost weight,
due to the expiration of carbonic acid. Owing to the difficulty of
experiment, opinions vary as to the absolute temperature of snails. It
appears to be established that several snails, if placed together in
a tube, raise the temperature one or two degrees C., but as a rule,
the temperature of a solitary _Helix_ differs very slightly from that
of the surrounding air. Increased activity, whether in respiration or
feeding, is found to raise the temperature.

  [Illustration: FIG. 72.--_Cardium edule_ L.: =A=, anal; =B=,
  branchial siphon; =F=, foot. (After Möbius.)]

W. H. Dall, writing of the branchia in _Pelecypoda_, remarks[275]
that there can be no doubt that its original form was a simple
pinched-up lamella or fold of the skin or mantle. This, elongated,
becomes a filament. Filaments united by suitable tissue, trussed,
propped, and stayed by a chitinous skeleton, result in the forms,
wonderful in number and complexity, which puzzle the student to
describe, much more to classify.

  [Illustration: FIG. 73.--_Scrobicularia piperata_ Gmel., in its
  natural position in the sand: =A=, efferent or anal siphon; =B=,
  afferent or branchial siphon. (After Möbius.)]

In Pelecypoda the branchiae are placed on each side of the body,
between the mantle and the visceral mass. They lie in a chamber known
as the _branchial cavity_. Leading into this cavity, and behind it,
are, as a rule, two tubes or siphons, one of which conducts water to
the branchiae, while the other carries it away after it has passed over
them. The lower is known as the _branchial_ or _afferent_ siphon, the
upper as the _anal_ or _efferent_ siphon (see Figs. 72 and 73). The
action of these siphons can readily be observed by placing a little
carmine in water, near to the siphonal apertures of an _Anodonta_ or
_Unio_. In many cases (_e.g._ _Psammobia_, _Tellina_, _Mya_, genera
which burrow deeply in sand) both the siphons are exceedingly long,
sometimes considerably longer than the whole shell. In some cases the
two tubes are free throughout their entire length, in others they
become fused together before their entrance within the shell (Fig.
74). In other genera, which do not burrow (_e.g._ _Ostrea_, _Pecten_,
_Arca_, _Mytilus_), the siphons are rudimentary or altogether absent
(Fig. 75).

  [Illustration: FIG. 74.--_Solecurtus strigillatus_ L., Naples:
  _s.af_, afferent siphon; _s.ef_, efferent siphon, the two uniting
  in _SS_ externally to the shell, × ½.]

  [Illustration: FIG. 75.--_Mytilus edulis_ L., attached by its
  byssus (=By=) to a piece of wood: =F=, foot; =S=, anal siphon,
  the branchial siphon being below it and not closed. (After
  Möbius.)]

The number and arrangement of the branchiae varies considerably. It
appears probable that the different degrees of complication of the
gill indicate degrees of specialisation in the different groups of
Pelecypoda, in other words, assuming that a simpler form of gill
precedes, in point of development, a more complicated form, the nature
of the gill may be taken as indicating different degrees of removal
from the primitive form of bivalve.

1. The simplest form of gill (_Nucula_, _Leda_, _Solenomya_, etc.)
is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of two
rows of very short, broad, not reflected filaments, the rows being
placed in such a way that they incline at right angles to one another
from a common longitudinal axis. The filaments are not connected with
one another, nor are the two leaves of each gill united at any point.
(_Protobranchiata._)

  [Illustration: FIG. 76.--Morphology of the branchiae
  of Pelecypoda, seen diagrammatically in section:
  =A=, _Protobranchiata_; =B=, _Filibranchiata_; =C=,
  _Eulamellibranchiata_; =D=, _Septibranchiata_; _e_, _e_, external
  row of filaments; _i_, _i_, internal row of filaments; _e´_,
  external row or plate folded back; _i´_, internal row folded
  back; _f_, foot; _m_, mantle; _s_, septum; _v_, visceral mass.
  (From A. Lang.)]

  [Illustration: FIG. 77.--Four gill filaments of _Mytilus_, highly
  magnified; _cj_, ciliary junctions; _f_, filament. (After Peck.)]

2. In the _Anomiidae_, _Arcadae_, _Trigoniidae_, and _Mytilidae_ each
gill consists of two plates or rows of much longer filaments, which
consequently occupy a much larger space in the mantle cavity (Fig.
76, B). Unable to extend beyond the limits of the mantle, filaments
are reflected or doubled back upon one another, those of the external
plate being reflected towards the outside, those of the internal plate
towards the inside. Each separate filament is not connected with the
filament next adjacent, except by surface cilia situated on small
projections on the sides of the filaments, and interlocking with the
cilia of the adjacent filament. The two superposed plates or leaves
of the gill may or may not be united by cords running between the two
parts of a filament. (_Filibranchiata._)

3. In the _Pectinidae_, _Aviculidae_, and _Ostreidae_ a further
development takes place. The filaments of each gill are reflected in
the same way as in the _Filibranchiata_, but the part thus reflected
may become completely united or ‘concresce’ with the mantle on the
exterior and with the base of the foot on the interior side. The leaves
of each gill plate, which have thus become doubled (the gills being
apparently two instead of one on each side), are folded or crumpled,
and the filaments are modified at the re-entrant angles of the fold.
(_Pseudolamellibranchiata._)

4. In all the remaining _Pelecypoda_, except class 5, in other words,
in the very large majority of families, the filaments are either
reflected, as in (3), or simple; but the process of concrescence is
so far advanced that the adjacent filaments are always intimately
connected with one another in such a way as to admit the passage of the
blood; and the leaves of each gill-plate (Fig. 76, C) are united by
cross channels in a similar way. (_Eulamellibranchiata._)

5. In certain of the _Anatinacea_ alone (_Cuspidaria_, _Lyonsiella_,
_Poromya_, _Silenia_) the gills are transformed into a more or less
muscular partition, extending from one adductor muscle to the other
(Fig. 76, D), and separating off the pallial chamber into two distinct
divisions, which communicate by means of narrow slits in the partition.
(_Septibranchiata._)

  [Illustration: FIG. 78.--Transverse section of portion of an
  outer gill plate of _Anodonta_, highly magnified: _il_, inner
  lamella; _il´_, outer lamella; _ilj_, interlamellar junctions;
  _v_, large vertical vessels. (After Peck.)]

Thus the process of gill development in the Pelecypoda appears to lead
up from a simple to a very complex type. In its original form, at all
events in the most primitive form known to us, the gill is a series of
short filaments, quite independent of one another, strung in two rows;
then the filaments become longer and double back, while at the same
time they begin to show signs of adhesion, as yet only superficial, to
one another. In a further stage, the reflected portions become fused to
the adjacent surfaces of the foot and mantle, while the interlamellar
junctions serve to lock the two gill-plates together; finally, the
mere ciliary junction of adjacent filaments is exchanged for intimate
vascular connection, while the gill-plates as a whole become closely
fused together in a similar manner.

This theory of origin is strengthened by closer observation of the
phenomena of a single group. Taking the Septibranchiata as an instance,
we find that in _Lyonsiella_ the branchiae unite with the mantle
in such a way as to form two large pallial chambers, the structure
of the branchiae being preserved, and their lamellae covering the
partition. A further stage is observed in _Poromya_. There, a similar
partition exists, but it has become muscular, preserving, however, on
each side two groups of branchial lamellae, separated one from the
other by a series of slits, which form a communication between the
two pallial chambers. A further stage still is seen in _Silenia_.
There the same muscular partition exists, but the branchial lamellae
on either side have disappeared, the slits between the two chambers,
which occur in _Poromya_, still persisting, but separated into three
groups. _Cuspidaria_ represents the last stage in the development. In
the ventral chamber there appears nothing at all corresponding to a
branchia; the surface of the partition appears perfectly uniform, but
on careful examination three little separate orifices, remains of the
three groups of orifices in _Silenia_, are observed.[276]

_Relation between Branchiae and Heart._--The object of the branchiae
being, as has been already stated, to aerate the blood on its way to
the heart, we find that the heart and the branchiae stand in very
important structural relations to one another. When the branchiae
are in pairs, we find that the auricles of the heart are also
paired, the auricle on the right and left sides being supplied by
the right and left branchiae respectively. This is the case with the
Dibranchiate Cephalopods (_Argonauta_, _Octopus_, _Loligo_, etc.),
the Zygobranchiate Prosobranchs (_Fissurella_, _Haliotis_), and all
_Pelecypoda_. In the _Amphineura_ (_Chiton_, etc.) there are two
auricles corresponding to the two sets of multiple branchiae. In the
case of the Tetrabranchiate Cephalopods (_Nautilus_) there are four
auricles corresponding to each of the four branchiae. Compare Fig. 79,
A, B, C, D, E.

On the other hand, when the branchia is single, or when both branchiae
are on the same side, and one is aborted and functionless, the auricle
is single too, and on the same side as the branchia. This is the case
with the Tectibranchiate Opisthobranchs (_Philine_, _Scaphander_,
etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa,
Taenioglossa, and Ptenoglossa), and the other Azygobranchiate
Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right
auricle exists, as well as the left, but is simply a closed sac, the
coalescing of the two gills on the left side having thrown all the work
upon the left auricle. Compare Fig. 79, F, G, H.

  [Illustration: FIG. 79.--Diagram illustrating the relations
  between branchiae, heart, and aorta in the Mollusca: =A=, In
  Chiton; =B=, Pelecypoda; =C=, Dibranchiate Cephalopoda; =D=,
  Tetrabranchiate Cephalopoda; =E=, Prosobranchiata Zygobranchiata;
  =F=, Prosobranchiata Azygobranchiata; =G=, Prosobranchiata
  Monotocardia; =H=, Opisthobranchiata Tectibranchiata: 1,
  Ventricle; 2, Auricle; 3, Aorta; 3_a_, Cephalic aorta; 3_b_,
  Visceral aorta; 3_c_, Posterior aorta. (From A. Lang.)]


                         =Circulatory System=

All Mollusca, without exception, possess a circulatory system of more
or less complexity. The centre of the system is the heart, which
receives the aerated blood from the breathing organs, and propels it to
every part of the body. In the Scaphopoda alone there appears to be no
distinct heart.

The heart may consist simply of a single auricle and ventricle, and
an aorta opening out of the ventricle. From the aorta the blood is
conveyed to the various parts of the body by arteries. Veins convey the
blood back to the breathing organs, after passing over which it returns
by the branchial or pulmonary vein to the heart, thus completing the
circuit.

As regards position, the heart is situated within the pericardium,
a separate chamber which in the Pelecypoda, Cephalopoda, and the
bilaterally symmetrical Gasteropoda lies on the median line, while in
the asymmetrical Gasteropoda it is on one or other of the sides of the
body, usually the right. The veins connected with the branchiae, and
consequently the auricle into which they open, are situated _behind_
the ventricle in the Opisthobranchiata (whence their name), while in
the Prosobranchiata they are situated _in front of_ the ventricle.

The number of auricles corresponds to the number of branchiae. Thus
there is only one auricle in the great majority of Prosobranchiata
(which are accordingly classified as _Monotocardia_), and also in
the Opisthobranchiata, while the Pulmonata have a single auricle
corresponding to the pulmonary chamber. There are two auricles in
the Amphineura, in a small group of Gasteropoda, hence known as
_Diotocardia_, in all Pelecypoda, and in the Dibranchiate Cephalopoda.
In the Tetrabranchiate Cephalopoda alone there are four auricles
corresponding to the four branchiae.

A single aorta occurs only in the Amphineura and in the Tetrabranchiate
Cephalopoda. In all the other groups there are two aortae, leading
out of the anterior and posterior ends of the ventricle in Pelecypoda
and Dibranchiate Cephalopoda, while a single aorta leads out of the
posterior end alone, and subsequently bifurcates, in most of the
Gasteropoda. One aorta, the cephalic, supplies the front part of the
body, the oesophagus, stomach, mantle, etc.; the other, the visceral
aorta, supplies the posterior part, the liver and sexual organs.

The general circulatory system in the Mollusca has not yet been
thoroughly investigated. As a general rule, the blood driven from the
ventricle through the aorta into the arteries, passes, on reaching
the alimentary canal and other adjacent organs, into a number of
irregular spaces called _lacunae_. These in their turn branch into
_sinuses_, or narrow tubes covered with muscular tissue, which
penetrate the body in every direction. In the Dibranchiate Cephalopoda
true capillaries are said to occur, which in some cases form a direct
communication between the arteries and veins. According to some
authorities[277] capillaries and veins exist in certain Pelecypoda in
connexion with the intestinal lacunae, but this again is regarded by
others as not established. A similar difference of opinion occurs with
regard to the precise function of the foot-pore which occurs in many
Mollusca, some holding that it serves as a means for the introduction
of water into the blood-vascular system, while others regard it as a
form of secretion gland, the original purpose of which has perhaps
become lost.

=Blood.=--As a rule, the blood of the Mollusca--_i.e._ not the
corpuscles but the liquor sanguinis--is colourless, or slightly tinged
with blue on exposure to the air. This is due to the presence of a
pigment termed _haemocyanin_, in which are found traces of copper and
iron, the former predominating. _Haemoglobin_, the colouring matter
of the blood in Vertebrates, is, according to Lankester,[278] of very
restricted occurrence. It is found--(1) in special corpuscles in the
blood of _Solen legumen_ (and _Arca Noae_); (2) in the general blood
system of _Planorbis_; (3) in the muscles of the pharynx and jaws
of certain Gasteropoda, _e.g._ _Limnaea_, _Paludina_, _Littorina_,
_Chiton_, _Aplysia_. This distribution of haemoglobin is explained by
Lankester in reference to its chemical activity; whenever increased
facilities for oxidisation are required, then it may be present to
do the work. The Mollusca, being as a rule otiose, do not possess it
generally diffused in the blood, as do the Vertebrata. The actively
burrowing _Solen_ possesses it, and perhaps its presence in _Planorbis_
is to be explained from its respiring the air of stagnant marshes.
Its occurrence in the pharyngeal muscles and jaws of other genera may
be due to the constant state of activity in which these organs are
kept.[279]

According to Tenison-Woods[280] a species of _Arca_ (_trapezia_ Desh.)
and two species of _Solen_, all Australian, have red blood. It is
suggested that in these cases the habits of the animal (the _Solen_
burrowing deeply in sand, the _Arca_ in mud) require some highly
oxidising element, surrounded as the creature is by ooze. In _Arca
pexata_ (N. America) the blood is red, the animal being familiarly
known as the ‘bloody clam.’ Burrowing species, however, are not all
distinguished by this peculiarity. Tenison-Woods finds red fluids in
the buccal mass of many Gasteropoda, _e.g._ in species of _Patella_,
_Acmaea_, _Littorina_, _Trochus_, _Turbo_, giving the parts the
appearance of raw meat.


                              =The Mantle=

On the dorsal side of the typical molluscan body, between the visceral
sac and the shell, lies a duplicature of the integument, generally
known as the _mantle_. The depending sides of the mantle, which are
usually somewhat thickened, enclose between themselves and the body
mass a chamber of varying size and shape, called the _mantle cavity_,
which communicates freely with the external air or water, and encloses
and furnishes a protection for the organ or organs of respiration. On
its upper or dorsal surface the mantle is closely applied to the shell
throughout its whole extent, the cells with which it is furnished
secreting the materials from which the shell is formed (see p. 255).
The whole mantle is capable, to some degree, of secreting shelly
matter, but the most active agent in its production is the mantle edge
or margin.

In the Prosobranchiata the mantle cavity, for reasons which have
already been explained, is found on the left side of the animal, its
front portion being in many cases produced into a tubular siphon.
Within the mantle cavity are found, besides the branchia, the anus,
the apertures of the kidneys, and the osphradium. In the pulmonata the
mantle fold encloses a so-called lung-cavity. The front edge of the
mantle coalesces with the integument of the neck in such a way as to
enclose the cavity very completely, the only communication with the
outer air being by means of the contractile breathing or pulmonary
aperture on the right side. In the Tectibranchiate Opisthobranchs the
mantle fold is inconsiderable, and is usually not of sufficient extent
to cover the branchia, while in the Nudibranchs, which have no true
branchiae, it disappears altogether.

In the Pelecypoda the mantle cavity is equally developed on each side,
enclosing the two sets of branchiae. The mantle may thus be regarded as
consisting of two equal portions, which form a sort of lining to the
two valves. The lower or ventral portion of the mantle edges may be
simple, or provided with ocelli (_Pecten_, _Arca_), tentacles, cilia
(_Lima_, _Lepton_), or doubled folds. The two portions of the mantle
touch one another along the whole line of the edge of the two valves,
and, although thus in contact, may remain completely separate from one
another, or else become permanently united at one or more points. This
fusion of the mantle edges corresponds to important changes in the
organisation of the animal as a whole. The anal and branchial siphons
are no more than prolongations of the mantle edges on the posterior
side into a tubular form. These ‘siphons’ exhibit the siphonal form
more distinctly according as the adjacent portions of the mantle become
more definitely fused together.

  [Illustration: FIG. 80.--Diagram illustrating the various
  stages in the closing of the mantle in Pelecypoda: =A=, mantle
  completely open; =B=, rudiments of siphons, mantle still
  completely open; =C=, mantle closed at one point; =D=, mantle
  closed at two points, with complete formation of siphonal
  apertures; =E=, development of siphons, ventral closure more
  extended; =F=, mantle closed at three points, with fourth
  orifice: _f_, foot; _s.a_, _s.b_, anal and branchial siphons;
  1, 2, 3, first, second, and third points of closure of mantle.
  (After A. Lang.)]

This progressive fusion of the mantle edges may be taken as indicating
definite stages in the development of the Pelecypoda. A perfectly free
mantle edge, joined at no point with the edge of the adjacent mantle,
occurs in _Nucula_, _Arca_, _Anomia_, and _Trigonia_ (see Fig. 80,
A, B). Here there is nothing in the nature of a siphon, either anal
or branchial; in other words, no contrivance exists to prevent the
spent water which has passed over the branchiae from becoming mixed
with the fresh water which is to reach them. When the mantle edges
are fused at _one point only_, this is invariably on the middle part
of the posterior side, thus separating off an anal opening which may
become prolonged into a tube-like form. At the same time the adjacent
underlying portions of the mantle edges draw together, without actually
coalescing, to form an opening for the incurrent stream of water,
the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the
case with most _Mytilidae_ (see Fig. 75) with _Cardita_, _Astarte_,
and _Pisidium_. In the next stage the branchial opening is separated
off by the concrescence of the mantle edges beneath it, and we have
the mantle united _in two places_, thus forming three openings, the
ventral of which is the opening for the protrusion of the foot (Fig.
80, D). This is the case in _Yoldia_, _Leda_, the majority of the
Eulamellibranchiata (_e.g._ _Lucina_, _Cyrena_, _Donax_, _Psammobia_,
_Tellina_, _Venus_, _Cardium_, _Mactra_), and all Septibranchiata.
In _Chama_ and _Tridacna_ the fused portions of the mantle become
more extended, and in _Pholas_, _Xylophaga_, _Teredo_, _Pandora_, and
_Lyonsia_ this concrescence takes place over the greater length of the
whole mantle edge, so that the mantle may be regarded as closed, with
the exception of the three apertures for the foot and the two siphons
(Fig. 80, E).

In certain genera there occurs, besides these three apertures, a
fourth, in the line of junction between the pedal and branchial
orifices. It appears probable that this fourth orifice (which has been
regarded by some as an inlet for water when the siphons are retracted),
stands in relation to the byssal apparatus (Fig. 80, F). In _Lyonsia_,
for instance, a thick byssus protrudes through the orifice, which is
large and open. In _Solen_, _Lutraria_, _Glycimeris_, _Cochlodesma_,
_Thracia_, _Aspergillum_, and a few more genera, which have no byssus,
the orifice is very small and narrow. It is possible that in these
latter cases, the byssal apparatus having become atrophied, the orifice
has been correspondingly reduced in size.[281]

=Mantle Reflected over the Shell.=--It is sometimes the case that the
mantle edges tend to double back over the external surface of the
shell, and to enclose it to a greater or less extent. When this process
is carried to an extreme, the edges of the reflected mantle unite, and
the shell becomes completely internal. We see an incipient stage of
this process in _Cypraea_ and _Marginella_, where the bright polish
on the surface of the shell is due to the protection afforded by the
lobes of the mantle. A considerable portion of the shell of _Scutus_
is concealed in a similar way, while in _Cryptochiton_, _Lamellaria_,
and _Aplysia_ the shell is more or less completely enclosed. Among
Pulmonata, it is possible that in forms like _Vitrina_, _Parmacella_,
_Limax_, and _Arion_, we have successive stages in a process which
starts with a shell completely external, as in _Helix_, and ends, not
merely by enveloping the shell in the mantle, but by effecting its
disappearance altogether. In _Vitrina_ and some allied genera we
have a type in which the mantle lobes are partly reflected over the
shell, which at the same time exhibits rather less of a spiral form
than in _Helix_. In the stage represented by _Parmacella_, the mantle
edges have coalesced over the whole of the shell, except for a small
aperture immediately over the spire; the nucleus alone of the shell
is spiral, the rest considerably flattened. In _Limax_ the shell has
become completely internal, and is simply a flat and very thin plate,
the spiral form being entirely lost, and the nucleus represented by a
simple thickening at one end of the plate. In _Arion_, the final stage,
we find that the shell, being no longer needed as a protection to the
vital organs, has either become resolved into a number of independent
granules, or else has entirely disappeared.

Some indications of a similar series of changes occur in the
Pelecypoda. The mantle edge of _Lepton_ is prolonged beyond the area
of the valves, terminating in some cases in a number of filaments. In
_Galeomma_ and _Scintilla_ the valves are partially concealed by the
reflected mantle lobes, and in a remarkable form recently discovered
by Dall[282] (_Chlamydoconcha_) the shell is completely imbedded in
the mantle, which is perforated at the anterior end by an orifice for
the mouth, and at the posterior end by a similar orifice for the anus.
In all these cases, except _Lepton_, it is interesting to notice that
the hinge teeth have completely disappeared, the additional closing
power gained by the external mantle rendering the work done by a hinge
unnecessary. It is quite possible, on the analogy of the Gasteropoda
mentioned above, and also, it may be added, of the Cephalopoda and
other groups, that we have here indicated the eventual occurrence of a
type of Pelecypoda altogether deprived of valves, a greatly thickened
mantle performing the part of a shell.[283]

       *       *       *       *       *

The following works will be found useful for further study of this
portion of the subject:--

   =F. Bernard=, Recherches sur les organes palléaux des
   Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7) ix. (1890),
   pp. 89–404.

   =G. Cuvier=, Le Régne animal (ed. V. Masson); Mollusca, Text and
   Atlas.

   =C. Grobben=, Beiträge zur Kenntniss des Baues von Cuspidaria
   (Neaera) cuspidata Olivi, nebst Betrachtungen über das System
   der Lamellibranchiaten: Arb. Zool. Inst. Wien, x. (1893), pp.
   101–146.

   =E. Ray Lankester=, Encyclopaedia Britannica, 9th ed., vol. xvi.
   (1883), Art. ‘Mollusca.’

   =A. Ménégaux=, Recherches sur la circulation des Lamellibranches
   marins: Besançon, 1890.

   =K. Mitsukuri=, On the structure and significance of some
   aberrant forms of Lamellibranchiate gills: Q. Journ. Micr. Sc.,
   N.S. xxi. (1881), pp. 595–608.

   =H. L. Osborn=, On the gill in some forms of Prosobranchiate
   Mollusca: Stud. Biol. Lab. Johns Hopk. Univ. iii. (1884), pp.
   37–48.

   =R. Holman Peck=, The structure of the Lamellibranchiate gill:
   Q. Journ. Micr. Sc., N.S. xvii. (1877), pp. 43–66.

   =P. Pelseneer=, Contributions à l’étude des lamellibranches:
   Arch. Biol. xi. (1891), pp. 147–312.




                              CHAPTER VII

          ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE
                       FOOT--THE NERVOUS SYSTEM


                     _Organs of Sense_: I. _Touch_

Tactile organs, although occurring in some of the Mollusca, do not
appear to attain special or marked development, except in a few cases.
The whole surface of the skin, and particularly of the foot, is very
sensitive to the slightest impression. Nearly all Gasteropoda are
furnished with at least two cephalic tentacles, projecting like horns
from each side of the fore part of the head. At or near the base of
these are generally situated the eyes. In the Helicidae the eyes are
situated, not at the base, but at the apex of the tentacles, and in
that case--except in _Vertigo_--a second pair of shorter tentacles
appears beneath the longer pair. It frequently happens that several
senses are centred in a single organ; thus the upper tentacles of
snails not only carry the eyes and serve to a certain extent as tactile
organs, but they also carry the organs of smell.

The edges of the mantle, which are sometimes specialised into lobes,
appear to be keenly sensitive to touch in all Gasteropoda.

In _Cypraea_ (Fig. 81) these lobes, or tentaculae, are a prominent
feature of the animal, and also in certain genera of the Trochidae
(Fig. 82). In most of the carnivorous land Pulmonata--_e.g._
_Testacella_, _Rhytida_, _Ennea_--there are developed, under the lower
pair of tentacles, and close to the mouth, large labial palps or
feelers. These are connected with the cerebral ganglion by a very large
nerve, and may therefore be supposed to be of extreme sensitiveness.
In some of the large carnivorous forms (_Glandina_, _Aerope_, compare
Fig. 21, p. 54) these palpae are of great size, and curl upwards like
an enormous pair of moustaches. When a _Glandina_ seizes its prey, the
palpae (see Fig. 83) appear to enfold it and draw it in towards the
mouth.

  [Illustration: FIG. 81.--_Cypraea moneta_ L., showing tentaculae
  at edge of mantle, which partly envelops the shell: =Si=, siphon;
  =M=, =M=, mantle; =F=, foot; =T´=, tentaculae at edge of mantle.
  (After Quoy and Gaimard.) × 3/2.

  [Illustration: FIG. 82.--_Monodonta canalifera_ Lam., New
  Ireland, showing mantle lobes. (After Quoy and Gaimard.)]

  [Illustration: FIG. 83.--_Glandina_ seizing its prey, with buccal
  papillae turned back. (Strebel.)]

It is in the Opisthobranchiata that the organs of touch attain their
maximum development. Many of this group are shell-less or possess a
small internal shell, and accordingly, in the absence of this special
form of defence, a multiplied sense of touch is probably of great
service. Thus we find, besides the ordinary cephalic tentacles,
clusters or crowns of the same above the head of many Nudibranchiata,
with lobe-like prolongations of the integument, and tentacular
processes in the neighbourhood of, or surrounding the branchiae (see
Figs. 58 and 84), or even projecting from the whole upper surface of
the body (Fig. 5, C).

In the Pelecypoda, the chief organs of touch are the foot, which is
always remarkably sensitive, especially towards its point, the labial
palps on each side of the mouth, and the siphons. In certain cases the
mantle border is prolonged into a series of threads or filaments. These
are particularly noticeable in _Pecten_, _Lepton_, and _Lima_ (Fig.
85), the mantle lobes of the common _L. hians_ of our own coasts being
very numerous, and of a bright orange colour. In many genera--_e.g._
_Unio_, _Mactra_--this sensibility to touch appears to be shared
by the whole mantle border, although it is not furnished with any
special fringing. The ‘arms’ of the Cephalopoda appear to be keenly
sensitive to touch, and this is particularly the case with the front
or tentacular pair of arms, which seem to be employed in an especial
degree for exploration and investigation of strange objects.

  [Illustration: FIG. 84.--_Idalia Leachii_ A. and H., British
  seas; _br_, branchiae. (After Alder and Hancock.)]

  [Illustration: FIG. 85.--_Lima squamosa_ Lam., Naples, showing
  tentacular lobes of mantle (_t_, _t_); _a_, anus; _ad.m_,
  adductor muscle; _br_, _br_, branchiae; _f_, foot; _sh_, shell.]

=Taste.=--The sense of taste is no doubt present, to a greater or less
extent, in all the head-bearing Mollusca. In many of these a special
nerve or nerves has been discovered in the pharynx, connecting with the
cerebral ganglion; this no doubt indicates the seat of the faculty of
taste. The Mollusca vary greatly in their likings for different kinds
of food. Some seem to prefer decaying and highly odoriferous animal
matter (_Buccinum_, _Nassa_), others apparently confine themselves to
fresh meat (_Purpura_, _Natica_, _Testacella_), others again, although
naturally vegetarian, will not refuse flesh on occasion (_Limax_,
_Helix_).

Mr. W. A. Gain[284] has made some interesting experiments on the taste
of British land Mollusca, as evidenced by the acceptance or rejection
of various kinds of food. He kept twelve species of _Arion_ and
_Limax_, and eight species of _Helix_ in captivity for many months, and
tried them with no less than 197 different kinds of food, cannibalism
included. Some curious points came out in his table of results.
_Amalia gagates_ appears to be surprisingly omnivorous, for out of 197
kinds of food it ate all but 25; _Arion ater_ came next, eating all but
40. _Limax arborum_, on the other hand, was dainty to a fault, eating
only seven kinds of food, and actually refusing Swedes, which every
other species took with some avidity. Certain food was rejected by all
alike, _e.g._ London Pride, Dog Rose, Beech and Chestnut leaves, Spruce
Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate
greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries,
_Boletus edulis_, and common grasses. Few of our common weeds or
hedgerow flowers were altogether rejected. _Arion_ and _Limax_ were
decidedly less particular in their food than _Helix_, nearly all of
them eating earthworms and puff-balls, which no _Helix_ would touch.
_Arion ater_ and _Limax maximus_ ate the slime off one another, and
portions of skin. _Cyclostoma elegans_ and _Hyalinia nitida_ preferred
moist dead leaves to anything else.


                              II. _Sight_

=Position of Eyes.=--In the majority of the head-bearing Mollusca
the eyes are two in number, and are placed on, or in the immediate
neighbourhood of the head. Sometimes they are carried on projecting
tentacles or ‘ommatophores,’ which are either simple (as in
Prosobranchiata) or capable of retraction like the fingers of a
glove (_Helix_, etc.). Sometimes, as in a large number of the marine
Gasteropoda, the eyes are at the outer base of the cephalic tentacles,
or are mounted on the tentacles themselves, but never at the tip
(compare Fig. 60, p. 153 and Fig. 98, p. 199). In other cases they are
placed somewhat farther back, at the sides of the neck. The Pulmonata
are usually subdivided into two great groups, _Stylommatophora_ and
_Basommatophora_ (Fig. 86), according as the eyes are carried on the
tip of the large tentacles (_Helix_, and all non-operculate land
shells), or placed at the inner side of their base (_Limnaea_, _Physa_,
etc.). In land and fresh-water operculates, the eyes are situated at
the outer base of the tentacles.

  [Illustration: FIG. 86.--=A=, _Limnaea peregra_ Müll.; _e_, _e_,
  eyes; _t_, _t_, tentacles; =B=, _Helix nemoralis_ Müll.; _e_,
  _e_, eyes; _t_, _t_, tentacles; _p.o_, pulmonary orifice.]

In the Helicidae, careful observation will show that the eyes are not
placed exactly in the centre of the end of the tentacle, but on its
upper side, inclining slightly outwards. The eye is probably pushed on
one side, as it were, by the development of the neighbouring olfactory
bulb. The sense of smell being far more important to these animals than
the sense of sight, the former sense develops at the expense of the
latter.

=Organisation of the Molluscan Eye.=--The eye in Mollusca exhibits
almost every imaginable form, from the extremely simple to the
elaborately complex. It may be, as in certain bivalves, no more than
a pigmented spot on the mantle, or it may consist, as in some of the
Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an
aqueous and vitreous humour, a retina, and an optic nerve, or of some
of these parts only.

In most land and fresh-water Mollusca the eye may be regarded, roughly
speaking, as a ball connected by an exceedingly fine thread (the optic
nerve) with a nerve centre (the cerebral ganglion). In _Paludina_
this ball is elliptic, in _Planorbis_ and _Neritina_ it is drawn out
at the back into a conical or pear shape. In _Helix_ (Fig. 87) there
is a structureless membrane, surrounding the whole eye, a lens, and a
retina, the latter consisting of a nervous layer, a cellular layer, and
a layer of rods containing pigment, this innermost layer (that nearest
the lens) being of the thickness of half the whole retina.

  [Illustration: FIG. 87.--Eye of _Helix pomatia_ L., retracted
  within the tentacle; _c_, cornea; _ep_, epithelial layer; _l_,
  lens; _op.n_, optic nerve; _r_, retina. (After Simroth.)]

Comparing the eyes of different Gasteropoda together, we find that
they represent stages in a general course of development. Thus in
_Patella_ the eye is scarcely more than an invagination or depression
in the integument, which is lined with pigmented and retinal cells.
The next upward stage occurs in _Trochus_, where the depression
becomes deeper and bladder-shaped, and is filled with a gelatinous or
crystalline mass, but still is open at the top, and therefore permits
the eye to be bathed in water. Then, as in _Turbo_, the bladder becomes
closed by a thin epithelial layer, which finally, as in some _Murex_,
becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. 88),
which probably corresponds with the ‘vitreous humour’ of other types.

  [Illustration: FIG. 88.--Eyes of Gasteropoda, showing arrest of
  development at successive stages: =A=, _Patella_; =B=, _Trochus_;
  =C=, _Turbo_; =D=, _Murex_; _ep_, epidermis; _l_, lens; _op.n_,
  optic nerve; _r_, retina; _v.h_, vitreous humour. (After Hilger.)]

In _Nautilus_ the eye is of a very simple type. It consists of a
cup-shaped depression, with a small opening which is not quite closed
by the integument. The retina consists of cells which line the
interior of the depression, and which communicate directly with the
branches of the optic nerve, there being no iris or lens. This type of
eye, it will be observed, corresponds exactly with that which occurs in
_Patella_. It appears also to correspond to a stage in the development
of eyes in the Dibranchiata (_e.g._ _Octopus_, _Sepia_, _Loligo_).
Lankester has shown[285] that in _Loligo_ the eye first appears as a
ridge, enclosing an oval area in the integument. By degrees the walls
of this area close in, and eventually join, enclosing the retinal cells
within the chamber in which the lens is afterwards developed (Fig.
89). It thus appears that in some cases the development of the eye is
arrested at a point which in other cases only forms a temporary stage
towards a higher type of organisation.

  [Illustration: FIG. 89.--Three stages in the development of the
  eye of _Loligo_; _r_, _r_, ridge, enclosing _p.o.c_, primitive
  optic chamber; _or_, orifice between the closing ridges;
  _s.o.c_, secondary optic chamber; _ci_, _ci_, ciliary body; _l_,
  rudimentary lens; _R_, retina. (After Lankester.)]

  [Illustration: FIG. 90.--Eye in =A=, _Loligo_; =B=, _Helix_ or
  _Limax_; =C=, _Nautilus_: _a.o.c_, anterior optic chamber; _c_,
  cornea; _int_, integument; _ir_, iris; _l_, lens; _l´_, external
  portion of lens; _op.n_, optic nerve; _op.g_, optic ganglion;
  _p.o.c_, posterior optic chamber; _r_, retina. (After Grenacher.)]

The developed eye in the dibranchiate Cephalopods consists of a
transparent cornea, which may or may not be closed over the front of
the lens. Behind the cornea is a narrow chamber (the anterior optic
chamber) which is continued for three parts round the whole circle
of the eye, and into which project the front portion of the lens and
the folds of the iris. Throughout its whole extent, the anterior optic
chamber is lined by the integument, the portion of which on the inner
side is the choroid. The lens is divided into an outer and inner
segment by a thin membrane, and is supported by the ciliary body which
forms a continuation of the retina. The main portion of the lens lies
within the posterior optic chamber, at the back and sides of which is
found the retina (Grenacher).

There can be no doubt that the Cephalopoda use their eyes to observe,
but there is nothing to show that any other Mollusca use their eyes
for this purpose, the sense of smell in their case largely taking the
place of visual observation. Madame Jeannette Power once saw[286]
the _Octopus_ in her aquarium holding a fragment of rock in one of
its arms, and watching a _Pinna_ which was opening its valves. As
soon as they were perfectly open, the Poulpe, with incredible address
and promptitude, placed the stone between the valves, preventing
the _Pinna_ from closing again, upon which it set about devouring
its victim. The next day the Poulpe was seen, after crushing some
_Tellina_, to stretch himself down close by a _Triton nodiferus_, and
watch it attentively. After four hours the _Triton_ emerged from its
shell, when the _Octopus_ sprang upon it, and surrounded it with its
arms.

=Powers of Vision in Land Mollusca.=--The Helicidae are undoubtedly
very short-sighted. Seldom emerging from their retreats except in
twilight and darkness, they are naturally myopic, and see better in a
subdued than in a bright light. Experiment has shown that a _Helix_ can
perceive an object better at 6 centimetres distance in a weak light
than at 4 or 5 millimetres in a strong one. _Cyclostoma elegans_ and
_Paludina vivipara_ are comparatively long-sighted, perceiving objects
at a distance of 20 to 30 centimetres.[287] The increased power of
vision is due, in these two cases, to increased elaboration in the
construction of the eye, _Paludina_ possessing a large and almost
spherical lens, to which the vitreous humour closely adheres, while in
_Cyclostoma_ the lens is remarkably hard, and the aqueous humour very
abundant. According to V. Willem,[288] the Pulmonata are very sensitive
to the slightest movement of the air or jarring of the surface on which
they crawl, but are so short-sighted as only to perceive a confused
image of a large object at about 1 cm., and to distinguish the form of
objects at not more than 1 or 2 mm. The senses of touch and smell are
far more active than that of sight. A bean-pod enclosed in a narrow
glass case and placed before a hungry snail was not noticed, but when
taken out of the case and placed 8 cm. behind the snail, the latter at
once turned towards it to devour it.

Some interesting experiments were conducted by the same author with the
view of ascertaining whether snails avoid or court the light. He placed
a number of species in different wooden boxes, which were divided into
a light and a dark compartment, having previously well soaked the boxes
in water to secure a humid atmosphere and surface, and so induce the
snails to move about. The result showed that nearly all species have a
marked predilection one way or the other, but not all in the same way.
_Helix aspersa_, _Arion empiricorum_, six species of _Limax_, and three
of _Planorbis_, are lovers of darkness, while _H. nemoralis_, _Succinea
putris_, and two species of _Limnaea_ are lovers of light. _Physa
fontinalis_ stands alone in being quite indifferent.

M. Willem endeavoured further to discover whether any of the Mollusca
possessed ‘dermatoptic perception,’ or the faculty of perceiving
variation of light by means of the skin alone. He accordingly repeated
the above-mentioned experiments, having previously extirpated the
eyes in all cases. The result was remarkable. In a few instances the
experiment was not conclusive, but _H. aspersa_, _A. empiricorum_,
several species of _Limax_, and one _Limnaea_ shunned or sought the
light just as they had done when their eyes were present. A few marine
Mollusca (_Littorina littorea_, _Trochus cinerarius_, _T. umbilicatus_,
_Patella vulgata_) were also shown to be exceedingly sensitive to the
impact of a shadow, whether with or without their eyes.

=Blind and Eyeless Mollusca.=--In a large number of marine Mollusca
which habitually creep about half buried in wet sand (_Bullia_,
_Sigaretus_, _Scaphander_, _Philine_), eyes are altogether absent. In
some species of _Natica_ and _Sigaretus_, and in _Doris_, eyes are
developed, but are enclosed in a thick layer of skin, through which
they can probably do little more than faintly appreciate different
degrees of light and darkness. _Chiton_ has cephalic eyes in the
embryo, but loses them in the adult stage. The two great _Auricula_,
_A. auris Judae_ and _A. auris Midae_, which habitually creep about
in the liquid mud of mangrove swamps, have entirely lost their eyes.
Certain pelagic Mollusca seem to have a tendency, which is not easily
explained, to lose their eyes or the power of seeing with them. Thus
_Ianthina_ has no eyes at all. Pteropoda as a rule have no eyes,
and the few that have (_Creseis_, _Cavolinia_) possess only certain
pigmented spots placed near to the nervous centres. In the Heteropoda,
however, and the Cephalopoda, many of which are pelagic, the eyes are
unusually large.

  [Illustration: FIG. 91.--_Sigaretus laevigatus_ Lam., a species
  frequenting wet sand, and destitute of external eyes: =F=,
  anterior portion of foot. (After Souleyet.)]

=Eyes in Deep-sea and Underground Mollusca.=--Deep-sea Mollusca, as a
rule, possess no visual organs, or possess them only in a rudimentary
state, but this rule has its exceptions. Dr. Pelseneer found[289] no
trace of eyes in two species of _Pleurotoma_ from 1850 and 1950 fath.,
none in a _Fossarus_ from 1400 f., none in a _Puncturella_ from 1340 f.
A remarkable form of _Voluta_ (_Guivillea_) from 1600 f. possessed eyes
which could hardly be functional, as they were destitute of pigment,
and exhibited other changes of structure. On the other hand, it is
remarkable to notice that in three different species of _Trochus_ from
450 f., 565 f., and 1375 f., the eyes were pigmented and well developed.

In land Mollusca which live beneath the surface of the ground or in
absolute darkness, the eyes are generally more or less modified.
Thus in _Testacella_, which usually burrows deeply in the soil, but
occasionally emerges into the open air, the eyes are very small, but
distinct and pigmented. Our little _Caecilianella acicula_, which
is never seen above the surface, is altogether destitute of eyes. A
species of _Zospeum_, a _Helix_, and a _Bithynella_ from dark caves
in Carniola have suffered a similar loss. On the other hand, a small
_Hyalinia_ from a dark cave in Utah (probably a recent addition to the
cave fauna) has the eyes normally developed.

=Eyes of Onchidium.=--Many species of _Onchidium_, a naked land
pulmonate which creeps on rocks near high-water mark, are provided
with dorsal eyes of various degrees of organisation, and in numbers
varying up to nearly one hundred. The tropical _Onchidium_ are the
prey of a fish (_Periophthalmus_) which skips along the beach by the
aid of its large ventral fins, and feeds principally on insects and
_Onchidium_. Karl Semper suggests[290] that the eyes are of service to
_Onchidium_ as enabling it to apprehend the shadow of the approaching
_Periophthalmus_, and defend itself by suddenly contracting certain
glands on the skin and expressing a liquid secretion which flies
into the air like shot and frightens the _Periophthalmus_ away. This
theory for it is no more than theory--may or may not be true, but it
is remarkable that _Onchidium_ with dorsal eyes have precisely the
same geographical distribution as _Periophthalmus_, and that where no
_Periophthalmus_ exists, _e.g._ on our own S.W. coasts, the _Onchidium_
are entirely destitute of dorsal eyes. In those species of _Onchidium_
which have no dorsal eyes, the latter are on the tips of the tentacles,
as in _Helix_. The eyes are developed _on_ the head, and afterwards
ascend with the growth of the ommatophores, while in _Helix_ the
ommatophores are formed first, and the eyes developed upon them.[291]

=Dorsal Eyes in the Chitonidae.=--The remarkable discoveries of Moseley
with regard to the dorsal eyes of _Chiton_ were first published in
1884.[292] He happened to notice, while examining a specimen of
_Schizochiton incisus_, a number of minute black dots on the outer
surface of the shell, which appeared to refract light as if composed
of glass or crystal. These ‘eyes,’ in all the species of _Chiton_ yet
examined, are restricted to the outer surface of the exposed area of
the shell, never being on the laminae of insertion or on the girdle.
In certain sub-genera of _Chiton_ the eyes are scattered irregularly
over the surface, in others they are arranged symmetrically in rows
diverging from the apex of each plate, but in old specimens the eyes
towards the apices are generally rubbed off by erosion or abrasion.
Moseley regarded the occurrence of scattered eyes as indicating an
original stage of development, when the eyes were at first disposed
irregularly all over the surface of the shell; the gathering into
regular rows showing a later stage.

  [Illustration: FIG. 92.--Dorsal eyes of _Chitonidae_, showing the
  various forms of arrangement in the first and fourth valves of 1,
  1_a_, _Acanthopleura spinigera_ Sowb., E. Indies, × 2; 2, 2_a_,
  _Tonicia suezensis_ Reeve, Suez, × 3; 3, 3_a_, _Acanthopleura
  granulata_ Gmel., W. Indies, × 2; 4, 4_a_, _Tonicia lineolata_
  Fremb., Chili, × 2. From specimens in the Museum of Zoology,
  Cambridge.]

The eyes appear to be invariably more numerous on the anterior plate.
Thus in _Corephium aculeatum_ there are about 12,000 in all, of which
more than 3000 are on the anterior plate. In _Schizochiton_ they are
arranged in very symmetrical rows, six of which are situated on the
anterior, and only two, sometimes only one, on the central plates. In
_Tonicia marmorata_ the eyes are sunk in little cup-shaped depressions
of the shell, possibly to escape abrasion. As regards shape and size,
in _Ch. incisus_ they are circular, and about 1/35 inch in diameter,
this being the largest size known; in _Ch. spiniger_ and _Ch.
aculeatus_ they are oval, measuring about 1/400 x 1/600 inch. There are
no eyes in _Chiton_ proper, nor in _Mopalia_, _Maugeria_, _Lorica_,
and _Ischnochiton_.[293] None of our English species appear to possess
them.[294]

=Eyes in Bivalve Mollusca.=--Some, possibly most, of the Pelecypoda
possess, in the larval state, true paired eyes at the oral end of the
body. These become aborted as the animal develops, since that part of
the body becomes entirely screened from the light by the growth of the
shell. To compensate for their loss, numerous _ocelli_, or pigmented
spots sensitive to the action of light, are in many cases developed
on different parts of the mantle, functionally corresponding to the
‘eyes’ of _Chiton_ described above. As in _Chiton_, too, we have here
an interesting series of instances in which true eyes have suffered
total obliteration, through disuse, and, as if to restore to the animal
in some measure its lost sense, visual organs of a low power have
subsequently been developed and are now observed in various stages of
specialisation.

=Concentration of Eyes in Special Parts of the Mantle.=--Sharp has
shown[295] that in several species of _Ostrea_, _Cardium_, _Anomia_,
_Lima_, _Avicula_, _Arca_, and _Tellina_ pigmented cells, with a
highly refractive cuticle, are scattered over a considerable portion
of the mantle. Experiment has proved the powers of ‘vision,’ _i.e._
of sensitiveness to different degrees of light, possessed by these
organs. In _Dreissena polymorpha_, _Tapes decussatus_, and two species
of _Venus_ these cells are concentrated on that particular part of the
mantle which is not always covered by the shell, _i.e._ the siphon,
but since the siphon can be completely retracted within the shell,
there is no special provision for their protection. A further step
is shown in the case of _Mya arenaria_, where the siphon is scarcely
capable of complete retraction. Here, while some of the pigment cells
are scattered about over the surface of the siphon, the majority
are placed in grooves at the base of the siphonal tentacles, forming
an intensely black band round them. A higher stage still is shown in
_Solen vagina_, _S. ensis_, and _Mactra solidissima_, where the cells
are situated only in the siphonal grooves, which are more or less
specialised in numbers and complexity.

_Arca Noae_, according to Patten, is very sensitive to any sudden
change in the amount of light falling upon its mantle-edge. A faint
shadow cast upon it by the hand is sufficient to cause it to close
its valves quickly, but always one or two seconds afterwards, the
promptitude in all cases depending upon the depth of the shadow.
Sensitiveness in this direction was found to depend greatly upon
the vitality of the animals themselves, since it always became less
in those specimens which had been kept for long in confinement. A
shadow was not always necessary to make them close. An ordinary
black pencil, if approached within two or three inches with extreme
caution, produced the same result, while a glass rod brought within
the same distance, and even moved rapidly to and fro, appeared to
cause no alarm. Sensitiveness to change in intensity of light was
experimentally noticed by the same author in the case of _Ostrea_,
_Mactra_, _Avicula_ (to a special extent), and _Cardium_. It is very
remarkable to find that increased elaboration in the structure of the
eyes does not necessarily carry with it increased sensitiveness, _i.e._
higher visual powers. _Avicula_, which is only provided with a few
scattered ommatidia, which would entirely escape the notice of any one
who had not seen them better developed elsewhere, was considerably more
sensitive to light and shade than _Arca_, with its eyes of conspicuous
size and much more perfect organisation, instantly contracting the
mantle upon the impact of a shadow so faint as to be invisible to the
experimenter.[296]

=Visual Faculties of Solen and Ostrea.=--The visual power of _Solen_
may be exemplified by any one who is walking along almost any of our
sandy bays at extreme low-water mark. If the day be warm and sunny,
numbers of _Solen_ will be seen raising themselves an inch or two out
of their holes; but if you wish to catch them you must approach very
cautiously, and on no account allow your shadow to fall upon them, or
they will pop down into their burrows in an instant, and it is vain
to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson
Smith, with reference to oysters,[297] ‘the creatures are to the
light above them; the shadow [of the boat] as it passes overhead is
instantaneously noted, and, snap! the lips are firmly closed.’

=Ocelli of Pecten.=--In _Pecten_ and _Spondylus_ the _ocelli_ are
remarkably large and prominent, shining like precious stones, and are
placed along the two edges of the mantle so as to receive the light
when the shell gapes (Fig. 93). In _Pecten opercularis_, _jacobaeus_,
and _maximus_ their number varies from 80 to 120. In _Spondylus
gaederopus_, a very inequivalve shell, 60 have been counted on the
right or fixed valve, and 90 on the left or upper valve. Each ocellus
is connected, by means of its optic nerve, with the large circumpalleal
nerve, and so with the branchial ganglion. They possess a cornea,
lens, choroidea, and optic nerve, and, according to Hickson,[298] bear
a considerable resemblance to the vertebrate type of eye. In spite
of this, the power of vision in these genera does not appear at all
superior to that of other Pelecypoda.

  [Illustration: FIG. 93.--_Pecten opercularis_ L., showing the
  _ocelli_ on the two edges of the mantle.]

  [Illustration: FIG. 94.--Compound eyes (_c.e_) of _Arca barbata_
  L.; _m.l_, mantle fold; _omm_, ommatidia. (After Patten.)]

According to the elaborate investigations of Patten, the ‘eyes’ in
_Arca_ occur upon the middle or ‘ophthalmic’ fold of the mantle-edge,
which is thickened at the end to admit of their reception. Along this
is ranged a row of dark brown spots of various sizes, which are larger
at the anterior and posterior ends of the mantle-edge, but smaller and
more numerous towards the middle. These brown spots, or ‘eyes,’ are
many of them compound, being made up of the fusion of a number of
ommatidia (from 10 to 80) into one large round eye, which is generally
elevated above the surface of the surrounding epithelium. Sometimes
these eyes themselves tend to fuse together. In one specimen of _Arca
Noae_, 133 of these faceted eyes were counted in one mantle border, and
102 in the other.

There can be little doubt that the development of these functional
eyes, or sensitive spots, in bivalve Mollusca, is due to special
needs. They appear to be entirely absent in fresh-water bivalves (with
the exception of _Dreissensia_, which is obviously a marine genus
recently become fresh-water), while they are most abundant in genera
living between tide marks (_Solen_, _Mya_, _Mactra_), and most highly
specialised in a genus that is, for a bivalve, of singularly active
habits (_Pecten_). Now genera living in sand between tide marks, as
the three above-mentioned genera are in the habit of doing, and also
protruding their siphons, and occasionally a considerable portion
of their shells, out of their burrow, are manifestly very much at
the mercy of their watchful enemies the gulls, and anything which
would enable them to apprehend the approach of their enemies would be
greatly to their advantage. Here, perhaps, lies the explanation of the
greater elaboration of these pigmented spots in littoral genera, as
compared with those inhabiting deeper water. _Pecten_, again, a genus
distinguished by great activity, which can ‘fly’ for considerable
distances in the water by flapping its valves together and expelling
the water from the apertures at either side of the hinge, may be
greatly assisted by its ocelli in directing its flight so as to escape
its enemies.


                             III. _Smell_

The sense of smell--touch at a distance, as Moquin-Tandon has called
it--is probably the most important sense which the Mollusca possess,
and is unquestionably far more valuable to them than that of sight.
Any one who has ever enjoyed the fun of hauling up lobster pots
will recollect that part of the contents was generally a plentiful
sprinkling of _Buccinum_, _Nassa_, and _Natica_, attracted by the smell
of the stinking piece of fish with which the trap was baited. According
to Mr. J. S. Gibbons,[299] _Bullia rhodostoma_ congregates in hundreds
on gigantic medusae which are stranded on the sandy bays near the
Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the
common _Neptunea antiqua_ “are procured on the Cheshire coast by the
fishermen placing a dead dog on the sands at low-water mark during
spring tides. The bait is then completely covered with stones, which
are piled up like a cairn. On the next turn of the tide the heap of
stones is visited, and the whelks are found on the surface in great
numbers, having been apparently attracted by the smell of the bait,
but unable to get at it.” Mr. W. A. Lloyd kept specimens of _Nassa
reticulata_ in a tank in the sand, at the bottom of which they usually
remained buried. If a piece of meat of any kind were drawn over the
sand, the _Nassa_ would appear above the surface in a few minutes.
Half-picked beef or mutton bones, if placed in the tank, were covered
in a few minutes. In fact, no animal matter, whether living or dead,
could be introduced without the _Nassa_ smelling it, and coming up to
see what they could get.[301]

Any one can experiment for themselves on the olfactory powers of our
common snails or slugs. Moquin-Tandon records[302] two interesting
cases, one communicated to him by letter, the other occurring to
himself. His correspondent, a M. Parenteau, was one day walking along a
dusty high-road, when he noticed, near the middle of the road, an empty
bean-pod and two Arions eating it. Attributing the meeting of feeders
and food to mere chance, he was walking on, when he noticed a second
bean-pod, and, about two yards away from it, a third _Arion_, hurrying
straight towards it. When the _Arion_ had yet more than a yard to
traverse, M. Parenteau picked up the bean and put it in his pocket. The
_Arion_ stopped, raised its head, and turned in every direction, waving
its tentacles, but without advancing. M. Parenteau then carried the
bean to the other side of the road, and put it in a small hole behind a
piece of stone. The _Arion_, after a moment’s indecision, started off
straight for the bean. Again the position of the precious morsel was
changed, and again the _Arion_ made for it, this time without being
further tantalised. M. Moquin-Tandon noticed, one rainy day in the
botanical gardens at Toulouse, two _Limax maximus_ approaching a rotten
apple from different directions. He changed the position of the apple
several times, placing it at a sufficient distance, to be sure they
could not see it, but they always hit it off correctly, after raising
their heads and moving their long tentacles in every direction. It then
occurred to him to hold the apple in the air, some centimetres above
the head of the _Limax_. They perceived where it was, raised their
heads and lengthened their necks, endeavouring to find some solid body
on which to climb to their food.

Several of the land Mollusca have the power of exhaling a disagreeable
smell, _Hyalinia alliaria_ smelling strongly of garlic, and _Stenogyra
decollata_ of laudanum; but this need not be any argument for the sense
of smell in the creatures themselves.

=Position of Olfactory Organs in Pulmonata.=--Most authorities are
of opinion that the olfactory organs are situated in the tentacles.
Moquin-Tandon considered that in the Helicidae and Limacidae the sense
of smell is confined to the little knob or elevation at the end of
the longer tentacles, close to the eye. He found that when he cut off
these tentacles both in _Limax_ and _Arion_, the creatures were quite
unable to discover the whereabouts even of strongly-scented food. The
same author believed that in the Basommatophora the sense of smell
was present in the whole of the tentacle, which is covered with an
exceedingly sensitive ciliated epithelium. Lacaze-Duthiers, however,
places the olfactory sense in this group at the outer side of the base
of the tentacles, near to the eyes. Some authorities[303] deny that the
Helicidae have the olfactory organ at the tip of the tentacles, and
locate it in a pedal gland near the mouth, which contains conspicuous
sensitive cells. A _Helix_ whose tentacles had been removed manifested
its repulsion to the smell of spirits of turpentine, while another
_Helix_, which was unmutilated, did not object to the turpentine being
held between its tentacles. Altogether, then, the exact position of the
smell-organ in the Helicidae must be considered as not yet thoroughly
determined. Simroth holds that the sense of smell is distributed over
the whole soft integument, and is especially concentrated in the
feelers, and in the neighbourhood of the respiratory orifice.[304]

In nearly all marine Mollusca yet examined, the organ of smell or
_osphradium_ is in situation intimately connected with the breathing
organs, being generally placed near their base, with the object,
apparently, of testing the quality of the water before it passes over
the branchiae. It consists of a patch of the epithelium, modified in a
special manner, and connected by its own nerve with one of the visceral
ganglia.

An osphradium does not necessarily occur in all genera; for instance,
it has not been detected in _Fissurella_. It is most highly specialised
in the Conidae, and in the carnivorous Gasteropoda generally. In
_Buccinum undatum_, for instance, it is very large indeed, and, from
its plumed form, has sometimes been mistaken for an accessory branchia
(Fig. 95). In _Haliotis_ it is paired, one lying in close proximity to
each of the two branchiae, but in _Turbo_ it is single, corresponding
to the single branchia. In _Chiton_ there is an osphradium at the
base of each separate gill filament, making a total of twenty or more
on each side. Its position in _Physa_ and in _Cyclostoma_ will be
seen by reference to Figs. 103 and 104 (p. 205). In the Pelecypoda
the osphradia are paired, and lie adjacent to the posterior adductor
muscle, close to the hinder end of the axis of the branchiae. In the
Tetrabranchiate Cephalopoda there are two osphradia, placed between
the bases of the two pairs of gills. In the Dibranchiates on the other
hand, a groove above the eyes has been regarded as the seat of the
organ of smell. This groove contains sensory and ciliated cells, and
appears to be connected with a special nerve centre of its own, which
ultimately is derived from the cerebral ganglion.

  [Illustration: FIG. 95.--_Buccinum undatum_ L., deprived of its
  shell, showing the relative position of branchia (_br_) and
  osphradium (_os_); _m_, mucous glands; _s_, siphon. The portion
  of the mantle covering the osphradium has been removed.]

Scarcely any instances of the exercise of the sense of smell on the
part of bivalve Mollusca have been recorded. Something of the sort,
however, seems to have been present in a case related by Mr. R. L.
King.[305] A skull of a fox had been placed in a small ditch in order
to soak, and after a few days, when taken out, was found to be covered
with _Pisidium pusillum_ to the number of at least two hundred,
which had been probably attracted from the water in the immediate
neighbourhood by the smell of the decaying flesh.


                             IV. _Hearing_

Experiments made with a view to ascertain whether the Mollusca are
sensitive to noises have usually led to the conclusion that they
are deaf to very loud sounds. This is the more curious, because an
undoubted auditory apparatus has been discovered in a large number
of genera. In the case of an experiment, it is not easy to be sure
that the animal is not affected, at least in part, by the shock or
jar, rather than by the actual sound. In some experiments, however,
conducted at the Plymouth Marine Biological Laboratory, Mr. Bateson
found[306] that _Anomia_ could be made to shut its shell by smearing
the glass of the tank with the finger in such a way as to make a
creaking sound. It was evident that the cause of alarm was not the
jarring of the solid framework of the tank, for the same result
occurred when the object on which the _Anomia_ were fixed was suspended
in the water by a thread. It was found that the sound had to be of a
particular pitch to excite the attention of the mollusc.

As a rule the organ of hearing is nothing more than a small vesicle
or sac (the _otocyst_), filled with a fluid secretion, in which
are suspended one or usually more calcareous concretions known as
_otoliths_. By means of cilia, which connect with sense-cells, these
otoliths are given a peculiar movement or oscillation in the medium
in which they are suspended. The number of the otoliths varies in
different genera and species; there are several hundreds in _Arion_ and
_Limax_, about a hundred in _Helix pomatia_, _nemoralis_, _hispida_,
_arbustorum_, _rotundata_, _Succinea putris_, and _Limnaea stagnalis_;
about fifty in _Planorbis contortus_ and _Physa fontinalis_, only
one in _Cyclostoma elegans_. The number increases with age. In young
specimens of _Limn. stagnalis_ as few as ten, nine, and seven have been
noticed.[307]

The otocysts are always paired, and, in Gasteropoda, are placed close
to the pedal ganglia. The acoustic nerve, however, has been shown by
Lacaze-Duthiers to connect with the cerebral ganglia in certain cases.
The otocysts are never on the surface of the body and are rarely
connected with it by any passage or tube; it is probable therefore that
sound reaches them simply through the medium of the tissues.

In the _Pelecypoda_ the otocyst is similarly situated near the pedal
ganglion, and is probably (though this has not yet been proved)
similarly connected with the cerebral. There is only a single otolith.
Pelseneer finds[308] in Nuculidae alone a free communication between
the otocyst and the exterior. _Anodonta_ has been observed[309] to
withdraw its foot into the shell at the noise of an opening door, a
loud voice, or a shrill whistle, whether in a basin of water or lying
on a study table.

  [Illustration: FIG. 96.--Illustrating the otocyst in =A=,
  _Anodonta_, =B=, _Cyclas_; _ot_, otolith; _a_, _b_, _c_, _c´_,
  cellular layers surrounding the chamber; _ci_, cilia on interior
  walls of chamber: =C=, an otolith crushed. (After Simroth.)]

Delage extirpated the otocysts in certain Octopoda, and obtained some
unexpected results. He found that remarkable effects were produced upon
the animal’s powers of locomotion, so that it was unable to preserve
its proper balance in the water when in rapid motion, but its body
was forced to undergo a form of rotation more or less pronounced.
He concluded that the otocysts must possess, besides their auditory
functions, a power which stands in some relation to the proper
orientation of the body in locomotion, a power which is not wholly
supplied by sight and touch alone. The otocysts may thus regulate
locomotion by stimulating muscular acts which tend to keep the body in
the straight line during the process of movement.[310]


                               =The Foot=

One of the most characteristic organs of the Mollusca is the foot,
which, under one form or another, occurs throughout the whole phylum.
The foot is a thickening, on the ventral side, of a portion of the
integument of the animal, modified to serve different forms of
motion. It attains its maximum relative area in the Chitonidae, many
Nudibranchs, and the slugs generally, in nearly all of which there is
no portion of the body which is not subtended by the foot. Here too it
presents the form of a regular disc or ellipse, which is more or less
produced. In many cases, however, the foot becomes modified in such a
way that we are enabled to recognise well-marked anterior and posterior
portions, which have received the name of _propodium_ and _metapodium_
respectively, while the intervening central portion is termed the
_mesopodium_.

  [Illustration: FIG. 97.--_Sigaretus laevigatus_ Lam., showing
  excessive development of the propodium (_pr_) and metapodium
  (_met_) in a mollusc living in sand (the shell, which covers only
  the liver and adjacent parts, has been removed); _l_, liver;
  _s.ap_, aperture of proboscis, here deflected from the median
  line; _t_, _t_, tentacles. (After Quoy and Gaimard.)]

The propodium is most strongly developed in genera which crawl about in
wet sand, _e.g._ _Natica_, _Sigaretus_, _Oliva_, _Harpa_, _Scaphander_
(Figs. 97 and 98, and compare Fig. 91). In such cases it seems to serve
as a sort of fender or snow-plough, to push the sand away on both sides
of the path the animal is traversing. In some species of _Sigaretus_
the propodium becomes as it were banked up against the head and
proboscis, which are thus unnaturally elevated, or tend to disappear
altogether. _Bullia_ (Fig. 62), which crawls about rapidly on wet sand,
appears to attain its object by a wide extension of the foot on all
sides, and so slides over the sand instead of ploughing through it;
the little lappets at the end of the ‘tail’ probably serve as rudders.

In _Melampus_ and _Pedipes_ the propodium is marked off by a groove
across the ventral surface. When the animal is in motion it first
advances the propodium and then pulls the rest of the foot after it
with the looping gait of certain caterpillars. In many Cyclostomatidae
this groove, instead of being transverse, is longitudinal, and the
animal advances first the right and then the left segment of the foot,
which gives it a swaying motion from side to side.

Upon the metapodium lies the operculum, when it occurs. As a rule the
metapodium is not sharply marked off from the rest of the foot. In
_Strombus_ (Fig. 99) it becomes erected into a sort of hump or column,
on the top of which the operculum is situated.

  [Illustration: FIG. 98.--_Oliva textilina_ Lam., showing how the
  front part of the foot (_f_) is developed into a sort of fender,
  the propodium (_pr_); _e_, _e_, eyes; _m.ap_, front appendage
  of mantle; _m.ap´_, hinder appendage of mantle, folded into
  the suture when the animal is at rest; _si_, siphon; _t_, _t_,
  tentacles. (After Quoy and Gaimard.)]

The _epipodium_ is a prominent fold or border, which occurs upon the
upper edge of the foot in most Diotocardia. In _Haliotis_ it is of
considerable breadth, and is covered by a number of lobes which spring
from a moss-like prolongation of the skin. From the epipodium are
developed the lateral tentaculae of _Monodonta_ (Fig. 82, p. 178), and
of other sub-genera of the Trochidae.[311]

In the Opisthobranchiata the lateral edges of the foot (the
_parapodia_) are frequently produced into broad folds or wing-like
extensions, which in many cases tend to fold over the shell, and, in
conjunction with the mantle, eventually imbed it altogether. By the
wavy motion of the parapodia the animal is enabled to progress through
the water. The paired natatory lobes of the Pteropoda are simply the
parapodia of the Tectibranchs modified for swimming purposes.

  [Illustration: FIG. 99.--_Strombus lentiginosus_ Lam., showing
  the modified form of the foot (_f_): _e_, _e_, eyes on their
  pedicels; _mp_, metapodium; _op_, operculum; _p_, penis; _pr_,
  proboscis; _t_, _t_, tentacles. (After Quoy and Gaimard.)]

It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda,
groups which have, for the most part, exchanged a crawling for
a swimming life, that the modifications of the foot are most
considerable. In _Oxygyrus_ and _Atlanta_, for instance, the propodium
and metapodium are sharply distinguished from the mesopodium, and
no doubt have acquired, as a means of propulsion, the power of
separate movement, the animal swimming with these portions of the
foot uppermost. In _Carinaria_ and _Pterotrachea_ the metapodium has
probably become continuous with the long axis of the body, while
the so-called ‘foot’ with its sucker represents only the original
propodium. In the Cephalopoda the arms and funnel represent the
modified foot, the sides of which are prolonged into a number of very
long specialised tentaculae. In the adult Cephalopod some of the arms
have assumed a position in advance of the mouth, the latter being in
fact surrounded by a circle of arms. But in the Cephalopod embryo the
mouth opens as in the Gasteropoda, _i.e._ in advance of the arms, and
it is only gradually that it becomes encircled by them. Arms and funnel
alike are found to be innerved from the pedal ganglion.[312]

The pointed axe-shaped foot, which is characteristic of the majority of
Pelecypoda, is doubtless derived from a form more akin to the flattened
‘sole’ of the Gasteropoda. A foot with something of this disc-shaped
base actually occurs in some of the Nuculidae, the parapodia being
furnished with pleats which recall similar formations in other Orders
(Fig. 100). The principal modifications of the foot are due to its
employment as a burrowing organ. In genera which burrow but slightly
it is small and feebly developed, while in genera which habitually
excavate, it becomes the largest and strongest organ of the body. At
the same time it has a tendency to shift its position from the ventral
to the anterior margin, accompanied by a corresponding narrowing of
the shell, until it arrives at the position seen in Mollusca of the
shape of _Mya_, _Pholas_, and _Solen_. In sedentary or attached genera,
_e.g._ _Pecten_, _Chama_, _Ostrea_, the foot tends to become aborted.

  [Illustration: FIG. 100.--_Yoldia limatula_ Say, Greenland,
  showing the short plumed branchiae (_br_, _br_), the
  gasteropodous foot (_f_), and the large labial palps (_l.p_,
  _l.p_): =A=, as seen from the ventral margin; =B=, from the left
  side, with the mantle turned back; _a.m_, position of anterior
  adductor muscle; _i_, intestine; _l_, liver; _m_, _m_, mantle.]

The _byssus gland_, secreting a byssus of horny threads, is
characteristic of many Pelecypoda, and may be observed by any one
in the common mussel. It occurs in the larvae of many species which
do not possess a byssus in the adult stage. The pedal gland of many
Prosobranchiates, which secretes a tough and almost thread-like slime,
is possibly homologous with the byssus gland of bivalves.


                            =Nervous System=

The Mollusca possess a nervous system, which usually consists of a
number of nerve centres or _ganglia_, linked together by bands (the
_commissures_) and sending out thread-like nerves which ramify into the
various organs. The character of the nervous system varies greatly
in different groups, ranging as it does from a condition of extreme
complexity, in which the ganglia are numerous and the commissures
equally so, to that of considerable simplicity, in which ganglia are
almost entirely absent.

The most important ganglia are (1) the _cerebral_,[313] which are
always placed above or on either side of the mouth, and from which
proceed the nerves of the eyes and tentacles; (2) the _pedal_, which
in Gasteropoda are situated below the oesophagus, in Pelecypoda at the
base of the foot, and from which the nerves of the foot and sometimes
the acoustic nerve arise; (3) the _pleural_,[314] whose position varies
considerably, but is always below the oesophagus and slightly above
the pedal ganglia; these innervate the mantle, branchiae, heart, and
viscera generally.

=Gasteropoda.=--The simplest form of nerve system as thus understood
occurs in the Amphineura, and more particularly in the Chitons. Here
we find four longitudinal nerve-cords, parallel to one another for
nearly the whole length of the mollusc. The two exterior cords probably
represent the pleural, the two interior the pedal nervous system.
There being no head or tentacles, but simply a mouth at the anterior
end, the cerebral ganglia do not exist, but they are represented by
the curved ring formed by the massing together of the two nerve-cords
on each side. The only distinct ganglia are a pair of buccal ganglia
(which are developed on a pair of commissures which pass forward from
the cerebral mass and innervate the lips and buccal region), and a much
smaller group, the sublingual. The two pedal cords are united by a
number of transverse parallel connectives, which recall similar modes
of connexion in the Chaetopod worms and in Arthropoda.

This quadruple set of nerve-cords is characteristic of all the
Amphineura, but the absence of ganglia is most marked in the Chitons.
In _Proneomenia_ and _Neomenia_ there is a distinct cerebral
ganglion, formed by the massing of the two ganglia into one, while
in _Proneomenia_ the lateral cords are joined to the pedal, as well
as the pedal to one another, by connectives. In _Chaetoderma_ the
cerebral ganglia, though adjacent, are distinct, and both the pedal and
lateral cords connect directly with them, while there are no transverse
connectives.

The remaining three great divisions of Gasteropoda, namely, the
Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as
comprising two distinct types of nervous condition, according as the
loop formed by the two visceral nerve-cords is twisted over itself,
forming a figure of 8, or continues straight and uncrossed. In the
former case, we get the condition known as _streptoneurous_, in the
latter that as _euthyneurous_.[315] The _Euthyneura_ include the whole
of the Opisthobranchiata[316] and Pulmonata, the _Streptoneura_ all the
Prosobranchiata.

  [Illustration: FIG. 101.--Nervous system of the Amphineura:
  =A=, _Proneomenia_; =B=, _Neomenia_; =C=, _Chaetoderma_; =D=,
  _Chiton_; _c_, cerebral ganglia; _l_, _l_, lateral cords; _pc_,
  posterior commissure; _s_, sublingual commissure or ring, with
  ganglia; _v_, _v_, pedal cords. (Alter Hubrecht.)]

The simplest form of nervous system in the euthyneurous Gasteropoda
occurs in the Opisthobranchiata. The cerebral, pleural, and pedal
ganglia tend to become concentrated in a ring-like form, united by
short commissures at the posterior end of the pharynx. The visceral
loop is in some cases long, and the two or three visceral ganglia are
then situated at its posterior extremity. The nervous system of the
Pulmonata is of a similar type, the visceral loop being often much
shorter, and tending to draw in towards the central group of ganglia.
The tentacular and optic nerves are, as usual, derived from the
cerebral ganglion, with which also the octocysts are probably connected
by rather long nerves. A pair of buccal ganglia innervate the buccal
mass, and are united by commissures with the right and left cerebral
ganglia. The osphradial nerve springs from one of the ganglia on the
visceral loop, the osphradium itself being situated (in _Limnaea_)
immediately above the pulmonary orifice and adjacent to the anus (Fig.
102). This massing of the ganglia is still better illustrated by the
accompanying figure of _Physa_ (Fig. 103), in which the animal is
represented as if transparent, so that the ganglia and nerves are seen
through the tissues.

  [Illustration: FIG. 102.--I. Nervous system of _Limnaea
  stagnalis_ L. The oesophagus has been cut and pulled forwards
  through the nerve-collar, so as to expose the lower surface of
  the buccal mass(dissected by F. B. Stead).

  =B.M=, buccal mass.

  =B.G=, buccal, =C.G=, cerebral, =Os.G=, osphradial, =Pe.G=, pedal
  ganglia.

  =Pl.G=, pleural ganglia.

  =Op.N=, optic, =Os.N=, osphradial, =Te.N=, tentacular nerve.

  =Ot=, otocyst; =V.L=, visceral loop.

  =R=, rectum, dotted in to show its position relative to the
  osphiadium.

  II. Right side of the head of _Limnaea stagnalis_. The
  overhanging flap of the mantle has been cut in the middle line,
  and the right half twisted back, so as to expose the pulmonary
  orifice, etc. The points =A= =A= on the mantle edge were
  continuous before the mantle was cut; the line =BA= is part of
  the free edge of the mantle.

  =An=, anus; =F=, female generative orifice; =J=, portion of
  jaw; =M=, male generative orifice under right tentacle; =Os=,
  osphradium; =P.O=, pulmonary orifice.]

Of the streptoneurous Gasteropoda, the nervous system of _Fissurella_
and _Haliotis_ shows distinct points of similarity to that of the
Amphineura. The pedal nerves are united by transverse commissures
throughout their entire length, while a double commissure unites the
cerebral ganglia to the mass from which the pedal nerves proceed.
In the great majority of the Streptoneura the ganglia (except the
visceral) are more concentrated and the commissures are consequently
much shorter. The accompanying figure of _Cyclostoma_, in which
the animal is represented as in that of _Physa_ just described,
illustrates this grouping of the ganglia, the twist of the visceral
loop, and the position of the visceral ganglia at its posterior end.

  [Illustration: FIG. 103.--Nervous system of _Physa acuta_
  Drap., showing the massing of the ganglia at the hinder end of
  the pharynx: _e_, _e_, eyes; _m_, mouth; _m.l_, _m.l_, mantle
  lappets; _o.f_, female generative orifice; _o.m_, male generative
  orifice; _os_, osphradium. (After Lacaze-Duthiers.)]

  [Illustration: FIG. 104.--Example of a streptoneurous Gasteropod
  (_Cyclostoma elegans_ Drap.): _c.g_, _c.g_, cerebral ganglia;
  _e_, _e_, eyes; _os_, osphradium; _ot_, _ot_, otocysts; _p.g_,
  _p.g_, pedal ganglia; _pl.g_, _pl.g_, pleural ganglia; _sp.g_,
  supra-intestinal ganglion; _sb.g_, sub-intestinal ganglion;
  _t.n_, tentacle nerve; _v.g_, visceral ganglion. (After
  Lacaze-Duthiers.)]

=Scaphopoda.=--In the Scaphopoda the nervous system resembles that of
the Pelecypoda. The cerebral and pleural ganglia lie close together,
while the pedal ganglia are placed in the anterior part of the foot,
connected with the cerebral ganglia by long commissures; the visceral
loop is rather long, and the two visceral ganglia are adjacent to the
anus.

=Pelecypoda.=--The nervous system in the Pelecypoda is the simplest
type in which well-marked ganglionic centres occur. The ganglia are
few, symmetrically placed, and are usually at a considerable distance
apart. There are, as a rule, three distinct pairs of ganglia, the
cerebral (cerebro-pleural), pedal, and visceral. The cerebral are
formed by the fusion of the cerebral and pleural ganglia, which,
however, in some cases (Protobranchiata) continue distinct.[317] They
lie above or on each side of the mouth, united by a commissure of
varying length. Another pair of commissures unites them with the pedal
ganglia, which are placed at the base of the foot, and are usually
very close together, sometimes (as in _Anodonta_) becoming partially
fused. The length of these commissures depends upon the distance
between mouth and foot; thus they are very long in _Mya_ and _Modiola_,
and very short in _Pecten_. In cases where the foot is rudimentary or
becomes aborted through disuse (_e.g._ _Ostrea_), the pedal ganglia
may dwindle or disappear altogether. The visceral ganglia are on the
ventral side of the posterior adductor muscle, beneath the rectum,
and innervate the branchiae, osphradia, and the whole of the visceral
sac. A pair of cerebro-visceral commissures traverses the base of the
foot, surrounding it with a comparatively short loop (compare Fig. 106,
_c.v.c´_), while a long commissure, which runs round the entire edge
of the mantle, and supplies branching nerves to the mantle border and
siphons (Fig. 106, _c.v.c_), may also connect the visceral and cerebral
ganglia.

  [Illustration: FIG. 105.--Nervous system of Pelecypoda: _A_,
  _Teredo_; _B_, _Anodonta_; _C_, _Pecten_; _a_, _a_, cerebral
  ganglia; _b_, pedal ganglia; _c_, visceral ganglia. (After
  Gegenbaur.)]

=Cephalopoda.=--In the Cephalopoda the concentration of ganglia
attains its maximum, and may perhaps be regarded as approaching the
point at which a definite brain may be said to exist. Another point
of distinction is the formation of special small ganglia upon the
nerve-cords in different parts of the body. In the Tetrabranchiata
(_Nautilus_) the cerebral and pedal ganglia form a broad ring which
surrounds the oesophagus, the former giving out the optic nerves,
with their special optic ganglion, and a pair each of buccal and
pharyngeal ganglia, the latter the nerves of the arms and funnel. The
visceral loop is still present in the form of a separate band, which
innervates the branchiae, osphradia, and viscera generally, forming
a special genital ganglion in connexion with the reproductive organs.
The principal ganglia of the Dibranchiata are still more concentrated,
even the visceral loop being possibly united with the rest in forming
an unbroken mass in which scarcely any trace of commissures can be
detected. The pedal ganglion becomes separated into two portions,
one of which innervates the arms, the other the funnel. Two peculiar
ganglia (the stellate ganglia) supply a number of branching nerves to
the mantle.

  [Illustration: FIG. 106.--Nervous system of _Cardium edule_
  L.: _a.m_, anterior adductor muscle; _br_, branchiae; _br.n_,
  branchial nerve; _c.g_, _c.g_, cerebral ganglia; _c.p.c_,
  cerebro-pedal commissure; _c.v.c’_, cerebro-visceral commissure;
  _c.v.c_, cerebro-visceral commissure of mantle; _l.p_, labial
  palps: _m_, mouth; _p.g_, pedal ganglion; _p.m_, posterior
  adductor muscle; _v.g_, visceral ganglion. (After Drost, × 3.)]

       *       *       *       *       *

   =E. L. Bouvier=, Système nerveux, morphologie générale et
   classification des Gastéropodes prosobranches: Ann. Sc. Nat.
   Zool. (7), iii. 1887, pp. 1–510.

   =J. Brock=, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool.
   xlviii. 1889, pp. 67–83.

   =O. Bütschli=, Bemerkungen über die wahrscheinliche Herleitung
   der Asymmetrie der Gasteropoda, etc.: Morph. Jahrb. xii. 1886,
   pp. 202–222.

   =B. Haller=, Zur Kenntniss der Muriciden. I. Anatomie des
   Nervensystems: Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp.
   87–106.

   „ Untersuchungen über marine Rhipidoglossen. II. Textur des„
   Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885,
   pp. 319–436.

   =H. Grenadier=, Abhandlungen zur vergleichenden Anatomie des
   Auges: Abh. Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii.
   1886, pp. 1–64.

   =A. P. Henchman=, The Origin and Development of the Central
   Nervous System in _Limax maximus_: Bull. Mus. C. Z. Harv. xx.
   1890, pp. 169–208.

   =V. Hensen=, Ueber das Auge einiger Cephalophoren: Zeit. wiss.
   Zool. xv. 1865, pp. 157–242.

   =C. Hilger=, Beiträge zur Kenntniss des Gasteropodenauges:
   Morph. Jahrb. x. 1885, pp. 352–371.

   =Lacaze-Duthiers=, Otocystes ou Capsules auditives des
   Mollusques (Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp.
   97–166.

   „ „ Du système nerveux des Mollusques gastéropodes pulmonés
   aquatiques: _ibid._ pp. 437–500.

   =P. Pelseneer=, Recherches sur le système nerveux des
   Ptéropodes: Arch. Biol. vii. 1887, pp. 93–130.

   „ Sur la valeur morphologique des bras et la composition du
   système nerveux central des Cephalopodes: Arch. Biol. viii.
   1888, pp. 723–756.

   =H. Simroth=, Ueber die Sinneswerkzeuge unserer einheimische
   Weichthiere: Zeit. wiss. Zool. xxvi. 1876, pp. 227–348.

   =J. W. Spengel=, Die Geruchsorgane und das Nervensystem der
   Mollusken: Zeit. wiss. Zool. xxxv. 1881, pp. 333–383.




                             CHAPTER VIII

        THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS


The digestive tract, or, as it is often termed, the alimentary canal or
gut, is a very important feature of the Mollusca. It may be regarded as
consisting of the following parts: (1) a _mouth_ or _oral aperture_:
(2) a _throat_ or _pharynx_; (3) an _oesophagus_, leading into (4) a
_stomach_, (5) an _intestine_ and _rectum_, ending in (6) an _anus_.

The primitive positions of mouth and anus were presumably at the
anterior and posterior ends of the animal, as in the Amphineura and
symmetrical Mollusca generally. But the modifications of original
molluscan symmetry, which have already been referred to (p. 154,
compare pp. 245, 246), have resulted in the anus becoming, in the great
majority of Gasteropoda, twisted forward, and occupying a position on
some point in the right side in dextral, and in the left in sinistral
species.

The process of digestion, as the food passes from one end of the tract
to the other, is performed by the aid of the secretions of various
glands, which open into the alimentary canal at different points in
its course. The principal of these are the _salivary glands_, situated
on the pharynx and oesophagus, and the _liver_, _biliary_ or _hepatic
gland_, connecting with the stomach. With these may be considered the
_anal_ and _ink-glands_, which, in certain genera, connect with the
terminal portion of the rectum.

1. The _mouth_ is generally, as in the common snail and periwinkle,
placed on the lower part of the head, and may be either a mere
aperture, circular or semicircular, in the head-mass, or, as is more
usual, may be carried on a blunt snout (compare Fig. 6, p. 10, and Fig.
68, p. 159), which is capable of varying degrees of protrusion. From
the retractile snout has doubtless been derived the long proboscis
which is so prominent a feature of many genera (compare Figs. 1, B, and
99), and in some (_e.g._ _Mitra_, _Dolium_) attains a length exceeding
that of the whole body. As a rule, Mollusca provided with a proboscis
are carnivorous, while those whose mouth is on the surface of the head
are Vegetable feeders, but this rule is by no means invariable. The
mouth is thickened round the aperture into ‘lips,’ which are often
extensile, and appear capable of closing upon and grasping the food.
In the Pelecypoda the mouth is furnished, on each side, with a pair
of special external lobes, the ‘labial palps,’ which appear to be of
a highly sensitive nature, and whose object it is to collect, and
possibly to taste, the food before it passes into the mouth.

2. The _Pharynx, Jaws, and Radula_.--Immediately behind the lips the
mouth opens into the muscular throat, pharynx, or buccal mass. The
pharynx of the Glossophora, _i.e._ of the Gasteropoda, Scaphopoda,
and Cephalopoda, is distinguished from that of the Pelecypoda,[318]
by the possession of two very characteristic organs for the rasping
or trituration of food before it reaches the oesophagus and stomach.
These are (_a_) the _jaw_ or _jaws_, and (_b_) the _radula_,[319]
_odontophore_, or _lingual ribbon_. The jaws bite the food, the
radula tears it up small before it passes into the stomach to undergo
digestion. The jaws are not set with teeth like our own; roughly
speaking, the best idea of the relations of the molluscan jaw and
radula may be obtained by imagining our own teeth removed from our jaws
and set in parallel rows along a greatly prolonged tongue.[320]

In nearly all land Pulmonata the jaw is single, and is placed behind
the upper lip. If a common _Helix aspersa_ be observed crawling up
the inside of a glass jar, or feeding on some succulent leaf, the
position and action of the jaw can be readily discerned. It shows
very black when the creature opens its mouth, and under its operation
the edge of a lettuce leaf shows a regular series of little curved
indentations, in shape not unlike the semicircular bites inflicted by
a schoolboy upon his bread and butter. The jaw of _Helix_ (Fig. 107,
B) is arched in shape, and is strengthened by a number of projecting
vertical ribs. That of _Limax_ (A) is straighter, and is slightly
striated, without vertical ribs. In _Bulimulus_ (C) the arch of the
jaw is very conspicuous, and the upper edges are always denticulated;
in _Orthalicus_ there is a central triangular plate with a number of
overlapping plates on either side; in _Succinea_ (E) there is a large
square accessory plate above the jaw proper. The form of the jaw
is peculiar not only to the genus but to the species as well. Thus
the jaw of _H. aspersa_ is specifically distinct from that of _H.
pomatia_, and that of _H. nemoralis_ is distinct from both. Wiegmann
has observed[321] that in young _Arion_, _Limax_, and _Helix_, the jaw
consists of two pieces, which coalesce by fusion in the adult, thus
indicating a stage of development in advance of the double jaw which
is found in most of the non-pulmonate Mollusca. In all fresh-water
Pulmonata there are two small accessory side plates besides the jaw
proper (Fig. 107, F).

  [Illustration: FIG. 107.--Jaws of various Pulmonata: =A=, _Limax_
  (_gagates_ Drap., Lancashire, × 15); =B=, _Helix_ (_acutissima_
  Lam., Jamaica, × 15); =C=, _Bulimulus_ (_depictus_ Reeve,
  Venezuela, × 20); =D=, _Achatina_ (_fulica_ Fér., Mauritius, ×
  7); =E=, _Succinea_ (_elegans_ Riss., Aral District, × 30); =F=,
  _Limnaea_ (_stagnalis_ L., Cambridge, × 30).]

Nearly all the non-carnivorous Prosobranchiata, land, fresh-water,
and marine alike, are provided with two large lateral jaws. Many of
these are sculptured with the most elaborate patterns, and appear to
be furnished with raised teeth, like a file. In the Nudibranchiata the
jaws are of great size and beauty of ornamentation (Fig. 109).

  [Illustration: FIG. 108.--Jaws of =A=, _Triton australis_
  Lam., Sydney; =B=, _Ampullaria fasciata_ Reeve, Demerara; =C=,
  _Calliostoma punctulatum_ Mart., New Zealand; =D=, _Cyclophorus
  atramentarius_ Sowb., Sanghir; all × 15.]

   [Illustration: FIG. 109.--Jaws of =A=, _Chromodoris
  gracilis_ Iher., × 15; =B=, _Scyllaea pelagica_ L., × 7; =C=,
  _Pleurobranchus plumula_ Mont., × 10; =D=, _Pleurobranchaea
  Meckelii_ Lam., × 5/2.]

The carnivorous genera, whether marine (_e.g._ _Conus_, _Murex_,
_Buccinum_, _Nassa_) or land (_e.g._ _Testacella_, _Glandina_,
_Streptaxis_, _Ennea_), are entirely destitute of jaws, the reason
probably being that in all these cases the teeth of the radula are
sufficiently powerful to do the work of tearing up the food without
the aid of a masticatory organ as well. Jaws are also wanting in the
Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.

In the Cephalopoda the jaws, or ‘beaks,’ as they are called, are most
formidable weapons of attack. In shape they closely resemble the beaks
of a parrot, but the hook on the dorsal side of the mouth does not,
as in birds, close over the lower hook, but fits under it. Powerful
muscles govern these mandibles, which must operate with immense effect
upon their prey (Fig. 110).

  [Illustration: FIG. 110.--Jaws of _Sepia_: =A=, _in situ_ within
  the buccal mass, several of the arms having been cut away; =B=,
  removed from the mouth and slightly enlarged.]

  [Illustration: FIG. 111.--_Patella vulgata_ L., showing the
  normal position of the radula, which is doubled back in a bow;
  the shell has been removed, and the whole visceral mass is turned
  forward, exposing the dorsal surface of the muscular foot:
  _gr_, longitudinal groove on this surface; _i_, _i_, intestine;
  _l_, liver; _m_, _m_, mantle edge; _mu_, muscles (cut through)
  fastening the visceral mass to the upper sides of the foot; _ov_,
  ovary; _r_, radula; _u.f_, upper or dorsal surface of the foot.]

_The Radula._[322]--When the food has passed beyond the operation
of the jaw, it comes within the province of the radula, the front
part of which perhaps co-operates to a certain extent with the jaw in
performing the biting process. The function of the radula as a whole
is to tear or scratch, not to bite; the food passes over it and is
carded small, the effect being very much the same as if, instead of
dragging a harrow over the surface of a field, we were to turn the
harrow points upwards, and then drag the field over the harrow.

The radula itself is a band or ribbon of varying length and breadth,
formed of chitin, generally almost transparent, sometimes beautifully
coloured, especially at the front end, with red or yellow.[323] It lies
enveloped in a kind of membrane, in the floor of the mouth and throat,
being quite flat in the forward part, but usually curving up so as to
line the sides of the throat farther back, and in some cases eventually
forming almost a tube. The upper surface, _i.e._ the surface over
which the food passes, is covered with teeth of the most varied shape,
size, number, and disposition, which are almost invariably arranged in
symmetrical rows. These teeth are attached to the cartilage on which
they work by muscles which serve to erect or depress them; probably
also the radula as a whole can be given a forward or backward motion,
so as to rasp or card the substances which pass over it.

The teeth on the front part of the radula are often much worn (Fig.
112), and probably fall away by degrees, their place being taken by
others successively pushed up from behind. At the extreme hinder end of
the radula the teeth are in a nascent condition, and there are often
as many as a dozen or more scarcely developed rows. Here, too, lie the
cells from which the teeth are originally formed.

The length and breadth of the radula vary greatly in different genera.
In _Littorina_ it is very narrow, and several times the length of the
whole animal. It is kept coiled away like a watch-spring at the back
of the throat, only a small proportion of the whole being in use. I
have counted as many as 480 rows in the common _Littorina littorea_.
In _Patella_ it is often longer than the shell itself, and if the
radula of a large specimen be freshly extracted and drawn across the
hand, the action of the hooks can be plainly felt. In _Aerope_, the
Turbinidae generally, and _Haliotis_ it is very large. In _Turritella_,
_Aporrhais_, _Cylichna_, _Struthiolaria_, and the Cephalopoda it
is small in proportion to the size of the animal. In the Pulmonata
generally it is very broad, the length not exceeding, as a rule, thrice
the breadth; in most other groups the breadth is inconsiderable, as
compared to the length.

The Radula is wanting in two families of Prosobranchiata, the Eulimidae
and Pyramidellidae, which are consequently grouped together as the
section Gymnoglossa. It is probable that in these cases the radula
has aborted through disuse, the animals having taken to a food
which does not require trituration. Thus several genera contained
in both these families are known to live parasitically upon various
animals--Holothurians, Echinoderms, etc.--nourishing themselves on
the juices of their host. In some cases, the development of a special
suctorial proboscis compensates for the loss of radula (see pp. 76–77).
In _Harpa_ there is no radula in the adult, though it is present in the
young form. No explanation of this fact has yet been given. It is also
absent in the Coralliophilidae, a family closely akin to _Purpura_,
but invariably parasitic on corals, and probably nourished by their
exudations. There is no radula in _Entoconcha_, an obscure form
parasitic on the blood-vessels of _Synapta_, or in _Neomenia_, a genus
of very low organisation, or in the Tethyidae, or sea-hares, or in one
or two other genera of Nudibranchiata.

  [Illustration: FIG. 112.--Example of a front portion of a radula
  (_Cantharus ringens_ Reeve, Panama), much worn by use. × 70.]

The number of teeth in the radula varies greatly. When the teeth are
very large, they are usually few in number, when small, they are
very numerous. In the carnivorous forms, as a rule, the teeth are
comparatively few and powerful, while in the phytophagous genera they
are many and small. Large hooked and sickle-shaped teeth, sometimes
furnished with barbs like an arrow-head, and poison-glands, are
characteristic of genera which feed on flesh; vegetable feeders, on
the contrary, have the teeth rounded, and blunter at the apex, or, if
long and narrow, so slender as to be of comparatively little effect.
Genera which are normally vegetarian, but which will, upon occasion,
eat flesh, _e.g._ _Limax_ and _Hyalinia_, exhibit a form of teeth
intermediate between these two extremes (see Fig. 140, A).

In _Chaetoderma_ there is but one tooth. In _Aeolis coronata_ there are
about 17, in _A. papillosa_ and _Elysia viridis_ about 19, in _Glaucus
atlanticus_ about 21, in _Fiona nobilis_ about 28. In the common
whelk (_Buccinum undatum_) there are from 220 to 250, in the common
periwinkle about 3500. As many as 8343 have been counted in _Limnaea
stagnalis_, about 15,000 in _Helix aspersa_ (that is, about 400,000
to the square inch), about 30,000 in _Limax maximus_, and as many as
40,000 in _Helix Ghiesbreghti_, a large species from Mexico; they are
very numerous also in _Nanina_, _Vitrina_, _Gadinia_, and _Actaeon_.
But _Umbrella_ stands far and away the first, as far as number of teeth
is concerned. In both _U. mediterranea_ and _U. indica_ they entirely
baffle calculation, possibly 750,000 may be somewhere near the truth.

The teeth on the radula are almost invariably disposed in a kind of
pattern, exactly like the longitudinal rows of colour in a piece of
ribbon, down the centre of which runs a narrow stripe, and every band
of colour on one side is repeated in the same relative position on
the other side. The middle tooth of each row--the rows being counted
across the radula, not longitudinally--is called the _central_ or
_rachidian_ tooth; the teeth next adjacent on each side are known as
the _laterals_, while the outermost are styled _uncini_ or _marginals_.
As a rule, the distinction between the laterals and marginals is fairly
well indicated, but in the Helicidae and some of the Nudibranchiata it
is not easy to perceive, and in these cases there is a very gradual
passage from one set to the other.

The central tooth is nearly always present. It is wanting in certain
groups of Opisthobranchiata, some of the carnivorous Pulmonata, and
in the Conidae and Terebridae, which have lost the laterals as well.
_Voluta_ has lost both laterals and marginals in most of the species,
and the same is the case with _Harpa_. In _Aeolis_, _Elysia_, and some
other Nudibranchiata the radula consists of a single central row. Other
peculiarities will be described below in their proper order.

The extreme importance of a study of the radula depends upon the fact,
that in each species, and _a fortiori_ in each genus and family, the
radula is characteristic. In closely allied species the differences
exhibited are naturally but slight, but in well-marked species the
differences are considerable. The radula, therefore, serves as a test
for the distinction of genera and species. For instance, in the four
known recent genera of the family Strombidae, _viz._ _Strombus_,
_Pteroceras_, _Rostellaria_, and _Terebellum_, the radula is of the
same general type throughout, but with distinct modifications for each
genus; and the same is true, though to a lesser extent, for all the
species hitherto examined in each of the genera. These facts are true
for all known genera, differences of the radula corresponding to and
emphasising those other differences which have caused genera to be
constituted. The radula therefore forms a _basis of classification_,
and it is found especially useful in this respect in dealing with the
largest class of all, the Gasteropoda, and particularly with the chief
section of this order, the Prosobranchiata. Thus we have--

                                       { (_a_) _Toxoglossa_
                                       { (_b_) _Rachiglossa_
                      { _Monotocardia_ { (_c_) _Taenioglossa_
                      {                { (_d_) _Ptenoglossa_
    =Prosobranchiata= {                { (_e_) _Gymnoglossa_
                      {
                      { _Diotocardia_  { (_f_) _Rhipidonlossa_
                                       { (_g_) _Docoglossa_[324]

(_a_) _Toxoglossa._--Only three families, Terebridae, Conidae, and
Cancellariidae, belong to this section. There is no central tooth,
and no laterals, the radula consisting simply of large marginals on
each side. In _Conus_ these are of great size, with a blunt base which
contains a poison-gland (see p. 66), the contents of which are carried
to the point by a duct. The point is always singly and sometimes doubly
barbed (Fig. 116). When extracted, the teeth resemble a small sheaf
of arrows (Figs. 113, 115). A remarkable form of radula, belonging to
_Spirotropis_ (a sub-genus of _Drillia_, one of the Conidae), enables
us to explain the true history of the radula in the Toxoglossa. Here
there are five teeth in a row, a central tooth, and one lateral and one
marginal on each side, the marginals being very similar in shape to the
characteristic shafts of the Conidae (Fig. 114). It is evident, then,
that the great mass of the Toxoglossa have lost both their central
and lateral teeth, and that those which remain are true uncini or
marginals. _Spirotropis_ appears to be the solitary survival of a group
retaining the primitive form of radula.

  [Illustration: FIG. 113.--Radula of _Bela turricula_ Mont. × 70.]

  [Illustration: FIG. 114.--Portion of radula of _Spirotropis
  carinata_ Phil., Norway. × 70.]

  [Illustration: FIG. 115.--Eight teeth from the radula of _Terebra
  caerulescens_ Lam. × 60.]

The arrangement of teeth in all these sections is expressed by a
formula applicable to each transverse row of the series. The central
tooth, if present, is represented by 1, and the laterals and marginals,
according to their number, on each side of the central figure. Thus the
typical formula of the Toxoglossa is 1.0.0.0.1, the middle 0 standing
for the central tooth which is absent, and the 0 on each side of it for
the absent laterals; the 1 on each extreme represents the one uncinus
in each row. Thus the formula for _Spirotropis_, which has also one
lateral on each side and a rachidian or central tooth, is 1.1.1.1.1.
Often the formula is given thus: 1.0.0.0.1 / 30, 1.1.1.1.1 / 42, where
30 and 42 stand for the average number of _rows_ of teeth in _Conus_
and _Spirotropis_ respectively; the same is sometimes expressed thus:
1.0.0.0.1 × 30; 1.1.1.1.1 × 42.

  [Illustration: FIG. 116.--A tooth from the radula of _Conus
  imperialis_ L., S. Pacific, × 50, showing barb and poison duct.]

  [Illustration: FIG. 117.--Portion of the radula of _Melongena
  vespertilio_ Lam., Ceylon. × 30.]

  [Illustration: FIG. 118.--Portion of the radula of _Eburna
  japonica_ Sowb., China. × 30.]

  [Illustration: FIG. 119.--Portion of the radula of _Murex regius_
  Lam., Panama. × 60.]

(_b_) The _Rachiglossa_ comprise the 12 families Olividae, Harpidae,
Marginellidae, Volutidae, Mitridae, Fasciolariidae, Turbinellidae,
Buccinidae, Nassidae, Columbellidae, Muricidae, and Coralliophilidae.
Certainly most and probably all of these families are or have been
carnivorous, the Coralliophilidae being a degraded group which have
become parasitic on corals, and have lost their teeth in consequence.
The characteristics of the group are the possession of a central tooth
with from one cusp (_Boreofusus_) to about fourteen (_Bullia_), and
a single lateral more or less cuspidate, the outer cusp of all being
generally much the largest. Thus in _Melongena respertilio_ (Fig. 117)
the central tooth is tricuspid, the central cusp being the smallest,
while the laterals are bicuspid; in _Eburna japonica_ (Fig. 118) the
central tooth is 5-cusped, the two outer cusps being much the smallest.
The teeth, on the whole, are sharp and hooked, with a broad base and
formidable cutting edge. In the Olividae, _Turricula_, _Buccinopsis_,
and the Muricidae the laterals are unicuspid and somewhat degraded
(Fig. 119). In _Mitra_ and the Fasciolariidae they are very broad and
finely equally toothed like a comb (Figs. 120, 121). The whole group is
destitute of marginals.

  [Illustration: FIG. 120.--Portion of the radula of _Imbricaria
  marmorata_ Swains. × 80.]

  [Illustration: FIG. 121.--Three rows of teeth from the radula of
  _Fasciolaria trapezium_ Lam. × 40.]

  [Illustration: FIG. 122.--Six teeth from the radula of _Cymbium
  diadema_ Lam., Torres Strait. × 25.]

  [Illustration: FIG. 123.--Examples of degraded forms of radula:
  =A=, _Cantharus pagodus_ Reeve, Panama (nascent end), × 40; =A´=,
  same radula, central and front portion; =B=, _Columbella varia_
  Sowb., Panama, × 50.]

  [Illustration: FIG. 124.--Three rows of the radula of _Sistrum
  spectrum_ Reeve, Tonga, × 80. The laterals to the right are not
  drawn in.]

Several remarkable peculiarities occur. _Harpa_ loses the radula
altogether in the adult. In the young it has lost only the laterals,
and consists of nothing but the central tooth. _Marginella_ has no
laterals; the central tooth is small and comb-shaped, with blunt cusps.
In _Voluta_ the laterals are generally lost, but in _Volutomitra_ and
one species of _Voluta_[325] they are retained. The central tooth
usually has three strong cusps, and is very thick and coloured a deep
red or orange (Fig. 122); in the sub-genus _Amoria_ it is unicuspid, in
shape rather like a spear-head with broadened wings; in _Volutolyria_
it is of a different type, with numerous unequal denticulations,
something like the laterals of _Mitra_ or _Fasciolaria_. Of the
Mitridae, _Cylindromitra_ has lost the laterals. Among the Buccinidae,
_Buccinopsis_ possesses a curiously degraded radula, the central
tooth having no cusps, but being reduced to a thin basal plate, while
the laterals are also weakened. This degradation from the type is a
remarkable feature among radulae, and appears to be characteristic,
sometimes of a whole family, _e.g._ the Columbellidae (Fig. 123, B),
sometimes of a genus, sometimes again of a single species. Thus in
_Cantharus_ (a sub-genus of _Buccinum_) the radula is typical in the
great majority of species, but in _C. pagodus_ Reeve, a large and
well-grown species, it is most remarkably degraded, both in the central
and lateral teeth (Fig. 123, A). This circumstance is the more singular
since _C. pagodus_ lives at Panama side by side with _C. ringeus_
and _C. insignis_, both of which have perfectly typical radulae. It
is probable that the nature of the food has something to do with the
phenomenon. Thus _Sistrum spectrum_ Reeve was found to possess a very
aberrant radula, not of the common muricoid type, but with very long
reed-like laterals. This singularity was a standing puzzle to the
present writer, until he was fortunate enough to discover that _S.
spectrum_, unlike all other species of _Sistrum_, lives exclusively on
a branching coral.

The dental formula for the _Rachiglossa_ is thus 1.1.1, except in those
cases where the laterals are absent, when it is 0.1.0.

  [Illustration: Fig. 125.--Portion of the radula of _Cassis
  sulcosa_ Born., × 40. The marginals to the right are not fully
  drawn.]

(_c_) The _Taenioglossa_ comprise 46 families in all, of which the
most important are Tritonidae, Cassididae, Cypraeidae, Strombidae,
Cerithiidae, Turritellidae, Melaniidae, Littorinidae, Rissoidae,
Paludinidae, Ampullariidae, Cyclophoridae, Cyclostomatidae, and
Naticidae. The radula is characterised by a central tooth of very
variable form, the prevailing type being multicuspid, the central cusp
the largest, on a rather broad base; a single lateral, which is often a
broad plate, more or less cusped, and two uncini, rather narrow, with
single hooks, or slightly cusped. The accompanying figures of _Cassis_,
_Vermetus_, and _Cypraea_, and those of _Littorina_ and _Cyclophorus_
given on pp. 20, 21, are good examples of typical taenioglossate
radulae.

  [Illustration: FIG. 126.--Four rows of teeth from the radula of
  _Vermetus grandis_ Gray, Andamans. × 40.]

In _Homalogyra_ the radula is much degraded, the central tooth is
large and triangular on a broad base, the lateral is represented only
by a thin oblong plate, and the uncini are absent. In some species
of _Jeffreysia_ the uncini are said to be absent, while present in
others. _Lamellaria_ has lost both its uncini, but the radula of the
allied _Velutina_ is quite typical. A peculiar feature in this group
is the tendency of the marginals to increase in number. A stage in
this direction is perhaps seen in _Ovula_, _Pedicularia_, and the
_Cyclostomatidae_. Here the outermost of the two marginals is by far
the larger and broader, and is strongly pectinated on its upper edge;
in the _Cyclostomatidae_ the pectinations are rather superficial;
in _Ovula_ (where both marginals are pectinated) they are decidedly
deeper; in _Pedicularia_ they are deeper still, and make long slits
in the tooth, tending to subdivide it altogether. In _Turritella_ the
number of marginals is said to vary from none (in _T. acicula_) to
three (_T. triplicata_), but the fact wants confirmation. _Solarium_
is an aberrant form, possessing simply a number of long uncini, which
recall those of _Conus_ or _Pleurotoma_, and is therefore hard to
classify; the allied _Torinia_ has a radula which appears allied
to _Ovula_ or _Pedicularia_. In _Triforis_ the teeth are identical
throughout, very small, about 27 in a row, tricuspid on a square base,
cusps short.

The normal formula of the _Taenioglossa_ is 2.1.1.1.2; in _Lamellaria_,
1.1.1; in _Triforis_, 13.1.13, or thereabouts.

  [Illustration: FIG. 127.--Two rows of the radula of _Cypraea
  tigris_ L. × 30.]

  [Illustration: FIG. 128.--Portion of the radula of _Ianthina
  communis_ Lam. × 40.]

(_d_) _Ptenoglossa._--This section consists of two families only,
which certainly appear remarkably dissimilar in general habits
and appearance, _viz._, the Ianthinidae and Scalariidae. In all
probability their approximation is only provisional. The radula,
which in _Ianthina_ is very large, and in _Scalaria_ very small,
possesses an indefinite number of long hooked teeth, of which the
outermost are the largest. The central tooth, if present (it does not
occur in _Ianthina_), is the smallest in the series, and thus recalls
the arrangement in some of the carnivorous _Pulmonata_ (p. 232). In
_Ianthina_ the radula is formed of two large divisions, with a gap
between them down the middle.

The formula is ∞.1.∞ or ∞.O.∞ according as the central tooth in
_Scalaria_ is or is not reckoned to exist.

(_e_) _Gymnoglossa._--In the absence of both jaw and radula it is
not easy to classify the two families (Eulimidae and Pyramidellidae)
which are grouped under this section. Fischer regards them as modified
Ptenoglossa; one would think it more natural to approximate them to the
Taenioglossa.

  [Illustration: FIG. 129.--Portion of the radula of _Margarita
  umbilicalis_ Brod., Labrador. × 75 and 300.]

(_f_) _Rhipidoglossa._--This section consists of seventeen families,
the most important being the Helicinidae, Neritidae, Turbinidae,
Trochidae, Haliotidae, Pleurotomariidae, and Fissurellidae. The radula
is characterised by--

(1) The extraordinary development of the uncini, of which there are so
many that they are always reckoned as indefinitely numerous. They are
long, narrow, hooked, and often cusped at the top, and crowded together
like the ribs of a fan, those at the extreme edge not being set
straight in the row, but curving away backwards as they become smaller;
in _Solariella_ alone, where there are from five to ten, can they be
counted.

  [Illustration: FIG. 130.--Portion of the radula of _Nerita
  albicilla_ L., Andaman Is., with central tooth highly magnified:
  _c_, _c_, the capituliform tooth. × 40.]

(2) The varying number of the laterals. The average number of these
is five on each side; in some cases (_Livona_) there are as many as
nine, in some (_Neritopsis_) only three. The lateral next to the uncini
(which is specially large in the Neritidae, and is then known as the
_capituliform_ tooth) is regarded by some authorities as the first
uncinus, by others as the sole representative of the laterals, the
teeth on the inner side of it being reckoned as multiplied central
teeth. According to this latter view, _Livona_ will have as many
as seventeen central teeth. Taking five as the average number of
‘laterals,’ we shall have the following different ways of constituting
the rhipidoglossate formula, the first being that to which preference
is given, _viz._:--

   (1) ∞.5.1.5.∞, _i.e._ one central, five laterals, including
   the ‘last lateral’ tooth.

   (2) (∞.1).4.1.4.(1.∞), regarding the ‘last lateral’ as first
   uncinus, but specialising it by a number.

   (3) ∞.1.(4.1.4).1.∞, regarding the ‘last lateral’ as the only
   lateral.

In the Neritidae and the derived fresh-water genera (_Neritina_,
_Navicella_) the first lateral, as well as the capituliform
tooth, is very large, and in shape rather like the blade bone of
a shoulder of mutton; the intervening laterals are very small. In
_Neritopsis_ (a degraded form) the central tooth and first lateral
are entirely wanting. In the neritiform land-shells (_Helicina_,
_Proserpina_) the first lateral is no larger than the others, while
the capituliform tooth is enormous. _Hydrocena_ is a very aberrant
and apparently degraded form; the laterals between the first and the
capituliform tooth are all wanting. In _Haliotis_, _Scissurella_,
and _Pleurotomaria_ the five laterals are of fairly equal size; in
_Fissurella_ we again meet with a large capituliform tooth, with very
small laterals.

(_g_) The _Docoglossa_ are in direct contrast with the Rhipidoglossa
in possessing few and strong teeth, instead of many and weak. There
are only three families, Acmaeidae, Patellidae, and Lepetidae. In some
of the Acmaeidae there are not more than two teeth in a row, while in
no species are there more than twelve. The radula is, however, very
long; there are as many as 180 rows in _Patella vulgata_. The teeth
are thick, generally of a very deep red horn colour, rather opaque.
The cartilage in which they are set is remarkably thick, and in some
species whose teeth are very few a considerable portion of this
cartilage is left quite bare.

  [Illustration: FIG. 131.--Portion of the radula of _Patella
  cretacea_ Reeve, seen in half profile. × 40.]

Although the teeth are so few, the arrangement is by no means simple.
The special feature of the group is the multiplication of identical
centrals. Of these there are two in _Acmaea_, and four, as a rule,
in _Patella_. Thus in these two genera there is seldom an absolutely
_central_ tooth. Either laterals or marginals are liable to be lost,
but there are never more than two of either in _Acmaea_, and never
more than two laterals and three marginals in _Patella_. Thus the
formula varies from 0.0.(1 + 0 + 1).0.0 in _Pectinodonta_, 2.2.(1 + 0
+ 1).2.2 in _Collisellina_ (both Acmaeidae), to 3.2.(1 + 0 + 1).2.3
in _Patinella_, and 3.1.(2 + 0 + 2).1.3 in _Patella_ proper. In the
Lepetidae there is an absolutely central tooth, which appears to be
made up of the coalescence of several teeth, no laterals, and about two
marginals; formula, 2.0.1.0.2.

   [Illustration: FIG. 132.--Two rows of the radula of
  _Pterotrachea mutica_ Les., Naples. × 60.]

The radula of the _Heteropoda_ is quite characteristic, and shows
no sign of affinity with any other Prosobranchiate. The central tooth
is large, broad, tricuspid, and denticulated on a broad base; the
single lateral is strong, often bicuspid; the two marginals simple,
long, falciform; formula, 2.1.1.1.2 (Fig. 132).

  [Illustration: FIG. 133.--=A=, Portion of the radula of _Chiton_
  (_Acanthopleura_) _spiniger_] Sowb., Andamans, × 30; =B=, portion
  of the radula of _Dentalium entalis_ L., Clyde, × 50.]

=Amphineura.=--(_a_) _Polyplacophora._--The radula of the _Chitonidae_
is quite unique. It resembles that of the _Docoglossa_ in being very
long, and composed of thick and dark horn-coloured teeth. The number of
teeth, however, is considerably greater, amounting almost invariably
to seventeen in each row. There are three rather small central teeth,
the two outer of these being similar; next comes a very large lateral
(the _major_ lateral), usually tricuspid, which is followed by two much
smaller laterals, which are scarcely more than accessory plates; then
a very large and arched marginal (the _major_ uncinus), at the outer
side of which are three accessory plates. Some consider there is only
one central tooth, and count the two small teeth on each side of it as
laterals.

Thus the formula is either (3 + 1).(2 + 1).3.(1 + 2).(1 + 3) or (3 +
1).(2 + 1 + 1).1.(1 + 1 + 2).(1 + 3).

(_b_) _Aplacophora._--Of this rather obscure order, _Chaetoderma_ has a
single strong central tooth, _Neomenia_ has no radula, _Proneomenia_
and _Lepidomenia_ have about twenty falciform teeth, much larger at one
end of the radula than the other; formula, 0.1.0.

=Opisthobranchiata.=--The radula of the Opisthobranchiata is
exceedingly variable in shape, size, and number and character of
teeth. Not only do allied families differ greatly from one another,
but allied genera often possess radulae widely distinct in plan.
Thus, among the Polyceridae, _Goniodoris_ has no central tooth, one
large lateral and one marginal (form. 1.1.0.1.1); _Doridunculus_ the
same, with five marginals (form. 5.1.0.1.5); _Lamellidoris_ one each
of median, laterals, and marginals (1.1.1.1.1); _Idalia_, _Ancula_,
and _Thecacera_ the same as _Goniodoris_; _Crimora_ several each of
laterals and marginals. Even species of the same genus may differ; thus
the formula for _Aeolis papillosa_ is 0.1.0, but for _Ae. Landsbergi_
1.1.1; for _Philine aperta_ 1.0.1, but for _Philine pruinosa_ 6.0.6.

  [Illustration: FIG. 134.--Two teeth from the radula of _Aeolis
  papillosa_ L. × 55.]

It must not be forgotten, however, that a simple repetition of the same
tooth, whether lateral or marginal, does not necessarily constitute
an important characteristic, nor does the presence or absence of a
central tooth. In most of the cases mentioned above, the difference in
the number of laterals and marginals is due to the multiplication of
identical forms, while the central tooth, when present, is often a mere
plate or narrow block without cusps, whose presence or absence makes
little difference to the character of the radula as a whole.

There appear to be three well-marked types of radula among the
Opisthobranchiata.

(_a_) Radula with a single strong central tooth, rows few. This form
is characteristic of the Aeolididae, Fionidae, Glaucidae, Dotoidae,
Hermaeidae, Elysiidae (Fig. 135), and Limapontiidae. In the Aeolididae
it is sometimes accompanied by a single lateral. The same type occurs
in _Oxynoe_, and in _Lobiger_ (= _Lophocercus_).

(_b_) Radula with the first lateral very strongly developed. This type
may take the form of (1) a single lateral, no central or marginals,
_e.g._ _Onchidoris_, _Scaphander_ (Fig. 137, A), _Philine_ (certain
species), _Ringicula_, or (2) first lateral strongly developed, and
repeated in succeeding laterals (2–6) on a smaller scale, _e.g._
_Philine_ (certain species). A few marginals are sometimes added,
_e.g._ in _Polycera_, _Lamellidoris_ (where there is a degraded central
tooth, Fig. 137, B), _Idalia_, and _Ancula_.

  [Illustration: FIG. 135.--Radula of _Elysia viridis_ Mont. × 40.
  Type (_a_).]

  [Illustration: FIG. 136.--Portion of the radula of _Gadinia
  peruviana_ Sowb., Chili. × 250. Type (_c_).]

(_c_) Radula with an indefinite number of marginals, laterals (if
present) merging into marginals, central tooth present or absent,
inconspicuous, teeth all very small. This type of radula, among the
Nudibranchiata, is characteristic of certain sub-genera of _Doris_
(_e.g._ _Chromodoris_, _Aphelodoris_, _Casella_, _Centrodoris_), of
_Hypobranchiaea_ and _Pleurophyllidia_; among the Tectibranchiata,
of _Actaeon_, many of the Bullidae, _Aplustrum_, the Aplysiidae,
_Pleurobranchus_, _Umbrella_ and _Gadinia_ (Figs. 136 and 137, C).

In the _Pteropoda_ there are two types of radula. The Gymnosomata,
which are in the main carnivorous, possess a radula with a varying
number (4–12) of sickle-shaped marginals, central tooth present or
absent. In the Thecosomata, which feed on a vegetable diet, there are
never more than three teeth, a central and a marginal on each side;
teeth more or less cusped on a square base.

=Pulmonata.=--The radula of the Testacellidae, or carnivorous land
Mollusca, is large, and consists of strong sickle-shaped teeth with
very sharp points, arranged in rows with or without a central tooth,
in such a way that the largest teeth are often on the outside, and the
smallest on the inside of the row (as in _Rhytida_, Fig. 139). The
number and size of the teeth vary. In _Testacella_ and _Glandina_,
they are numerous, consisting of from 30 to 70 in a row, with about
50 rows, the size throughout being fairly uniform. In _Aerope_ they
are exceedingly large, and only eight in a row, the outermost marginal
being probably the largest single tooth in the whole of the Mollusca.
The central tooth is always obscure, being, when present, simply a
weaker form of the weakest lateral; in genera with only a few teeth in
a row it is generally absent altogether.

  [Illustration: FIG. 137.--Portions of the radula of
  Opisthobranchiata, illustrating types (_b_) and (_c_); =A=,
  _Scaphander lignarius_ L.; =A´=, one of the teeth seen from the
  other side, × 40; =B=, _Lamellidoris bilamellata_ L., Torbay, ×
  60; =C=, _Hydatina physis_ L., E. Indies, × 75.]

The first family of jaw-bearing snails, the Selenitidae, is distinctly
intermediate. The possession of a jaw relates it to the main body of
Helicidae, but the jaw is not strong, while the teeth are still, with
the exception of the central, thoroughly Testacellidan. The central
tooth is quite rudimentary, but it is something more than a mere
weak reproduction of the marginals. There are no true laterals. The
Limacidae show a further stage in the transition. Here the central
tooth has a definite shape of its own, tricuspid on a broad base, which
is more or less repeated in the first laterals; these, as they approach
the marginals, gradually change in form, until the outer marginals
are again thoroughly Testacellidan.[326] This is the general form
of radula, varied more or less in different genera, which occurs in
_Nanina_, _Helicarion_, _Limax_, _Parmacella_, and all the sub-genera
of _Zonites_. It is certain that some, and probable that all of these
genera will, on occasion, eat flesh, although their usual food appears
to be vegetable. The jaw is more powerful than in the Selenitidae, but
never so large or so strongly ribbed as in _Helix_ proper.

  [Illustration: FIG. 138.--Portion of the radula of _Glandina
  truncata_ Gmel. × 40.]

  [Illustration: FIG. 139.--Portion of the radula of _Rhytida
  Kraussii_ Pfr., S. Africa. × 25.]

When we reach the Helicidae, we arrive at a type of radula
in which the aculeate form of tooth--so characteristic of the
Agnatha--disappears even in the marginals, and is replaced by teeth
with a more or less quadrate base; the laterals, which are always
present, are intermediate in form between the central and the
marginals, and insensibly pass into the latter. In size and number
of cusps the first few laterals resemble the central tooth; in the
extreme marginals the cusps often become irregular or evanescent. As
a rule, the teeth are set squarely in the rows, with the exception of
the extreme marginals, which tend to slope away on either side. In
some Helicidae there is a slight approximation to the Zonitidae in the
elongation of the first marginals.

The above is the type of radula occurring in the great family
Helicidae, which includes not only _Helix_ proper, with several
thousand species, but also _Arion_, _Bulimus_, _Ariolimax_, and other
genera. The jaw is almost always strongly transversely ribbed.

In the _Orthalicidae_ (Fig. 140, C) the teeth of the radula, instead of
being in straight rows, slope back at an angle of about 45 degrees from
the central tooth. The central and laterals are very similar, with an
obtuse cusp on rather a long stem; the marginals become bicuspid.

In the _Bulimulidae_, which include the important genera _Placostylus_,
_Amphidromus_, _Partula_, _Amphibulimus_, and all the groups of South
American _Bulimulus_, the jaw is very characteristic, being thin,
arched, and denticulated at the edges, as if formed of numerous
narrow folds overlapping one another. The radula is like that of the
Helicidae, but the inner cusp of the laterals is usually lengthened and
incurved. In _Partula_ the separation between laterals and marginals is
very strongly marked.

The remaining families of Pulmonata must be more briefly described. In
the _Cylindrellidae_ there are three distinct types of radula: (_a_)
Central tooth a narrow plate, laterals all very curiously incurved
with a blunt cusp, no marginals (Fig. 140, D); (_b_) radula long and
narrow, central tooth as in (_a_), two laterals, and about eight small
marginals; (_c_) much more helicidan in type, central and laterals
obtusely unicuspid, marginals quite helicidan. Type (_c_) is restricted
to Central America, types (_a_) and (_b_) are West Indian.

_Pupidae_: Radula long and narrow; teeth of the helicidan type,
centrals and laterals tricuspid on a quadrate base, marginals very
small, cusps irregular and evanescent. This type includes _Anostoma_,
_Odontostomus_, _Buliminus_, _Vertigo_, _Strophia_, _Holospira_,
_Clausilia_, and _Balea_.

_Stenogyridae_, including _Achatina_, _Stenogyra_, and all its
sub-genera: Central tooth small and narrow, laterals much larger,
tricuspid, central cusp long, marginals similar, but smaller.

_Achatinellidae_: Two types occur; (_a_) teeth in very oblique rows,
central, laterals, and marginals all of the same type, base narrow,
head rather broad, with numerous small denticles (_Achatinella_ proper,
with _Auriculella_ and _Tornatellina_, Fig. 140, E); (_b_) central
tooth small and narrow, laterals bicuspid, marginals as in _Helix_
(_Amastra_ and _Carelia_).

  [Illustration: FIG. 140.--Portions of the radula of =A=,
  _Hyalinia nitidula_ Drap., Yorkshire, with central tooth, first
  lateral, and a marginal very highly magnified; =B=, _Helix
  pomatia_ L., Kent, showing central tooth, laterals, and one
  extreme marginal, the two former also highly magnified; =C=,
  _Orthalicus undatus_ Brug., Trinidad, with three laterals highly
  magnified; =D=, _Cylindrella rosea_ Pfr., Jamaica, central tooth
  and laterals, the same very highly magnified; =E=, _Achatinella
  vulpina_ Fér., Oahu, central tooth (_c_) and laterals, the same
  highly magnified.]

_Succineidae_: Central and laterals helicidan, bi- or tricuspid on a
quadrate plate, marginals denticulate on a narrow base; jaw with an
accessory oblong plate.

_Janellidae_: Central tooth very small, laterals and marginals like
_Achatinellidae_ (_a_).

_Vaginulidae_: Central, laterals, and marginals unicuspid throughout,
on same plan.

_Onchidiidae_: Rows oblique at the centre, straight near the edges;
central strong, tricuspid; laterals and marginals very long, falciform,
arched, unicuspid.

_Auriculidae_: Teeth very small; central narrow, tricuspid on rather a
broad base; laterals and marginals obscurely tricuspid on a base like
_Succinea_.

_Limnaeidae_: Jaw composed of one upper and two lateral pieces; central
and lateral teeth resembling those of Helicidae; marginals much
pectinated and serriform (Fig. 141, A). In _Ancylus_ proper the teeth
are of a very different type, base narrow, head rather blunt, with
no sharp cusps, teeth similar throughout, except that the marginals
become somewhat pectinated (Fig. 141, B); another type more resembles
_Limnaea_.

  [Illustration: FIG. 141.--Portions of the radula of =A=, _Limnaea
  stagnalis_ L., with the central tooth and two first laterals,
  and two of the marginals, very highly, magnified; =B=, _Ancylus
  fluviatilis_ Müll., with two of the marginals very highly
  magnified; =C=, _Physa fontinalis_ L., with central tooth and two
  of the marginals very highly magnified.]

_Physidae_: Jaw simple, but with a fibrous growth at its upper edge,
which may represent an accessory plate; radula with very oblique
rows, central tooth denticulate, laterals and marginals serriform,
comb-like, with a wing-like appendage at the superior outer edge (Fig.
141, C).

_Chilinidae_: Central tooth small, cusped on an excavated triangular
base, marginals five-cusped, with a projection as in _Physa_, laterals
comb-like, serrations not deep.

_Amphibolidae_: Central tooth five-cusped on a broad base, central
cusp very large; two laterals only, the first very small, thorn-like,
the second like the central tooth, but three-cusped; laterals simple,
sabre-shaped.

=Scaphopoda.=--In the single family (_Dentaliidae_) the radula is
large, and quite unlike that of any other group. The central tooth is a
simple broad plate; the single lateral is strong, arched, and slightly
cusped; the marginal a very large quadrangular plate, quite simple;
formula, 1.1.1.1.1 (Fig. 133, B).

=Cephalopoda.=--The radula of the Cephalopoda presents no special
feature of interest. Perhaps the most remarkable fact about it is its
singular uniformity of structure throughout a large number of genera.
It is always very small, as compared with the size of the animal, most
of the work being done by the powerful jaws, while the digestive powers
of the stomach are very considerable.

The general type of structure is a central tooth, a very few laterals,
and an occasional marginal or two; teeth of very uniform size and shape
throughout. In the Dibranchiata, marginals are entirely absent, their
place being always taken, in the Octopoda, by an accessory plate of
varying shape and size. This plate is generally absent in the Decapoda.
The central tooth is, in the Octopoda, very strong and characteristic;
in _Eledone_ and _Octopus_ it is five-cusped, central cusp strong; in
_Argonauta_ unicuspid, in _Tremoctopus_ tricuspid. The laterals are
always three in number, the innermost lateral having a tendency to
assume the form of the central. In _Sepia_ the two inner laterals are
exact reproductions of the central tooth; in _Eledone_, _Sepiola_,
_Loligo_, and _Sepia_, the third lateral is falciform and much the
largest.

  [Illustration: FIG. 142.--Portion of the radula of _Octopus
  tetracirrhus_ D. Ch., Naples, × 20.]

In _Nautilus_, the only living representative of the Tetrabranchiata,
there are two sickle-shaped marginals on each side, each of which has
a small accessory plate at the base. The two laterals and the central
tooth are small, very similar to one another, unicuspid on a square
base.

  [Illustration: FIG. 143.--Alimentary canal of _Helix aspersa_
  L.: _a_, anus; _b.d_, _b.d´_, right and left biliary ducts;
  _b.m_, buccal mass; _c_, crop; _h.g_, hermaphrodite gland; _i_,
  intestine; _i.o_, opening of same from stomach (pyloric orifice);
  _l_, _l´_, right and left lobes of liver; _m_, mouth; _oe_,
  oesophagus; _r_, rectum; _s.d_, salivary duct; _s.g_, salivary
  gland; _st_, stomach; _t_, left tentacle. (After Howes and
  Marshall, slightly modified.)]

_Salivary glands_ are found in most Glossophora. They occur in one or
two pairs on each side of the pharynx and oesophagus, the duct usually
leading forwards and opening into the anterior part of the pharynx
(see Figs. 143, 144). They are exceptionally large in the carnivorous
Gasteropoda. In certain genera, _e.g._ _Murex_, _Dolium_, _Cassis_,
_Pleurobranchus_, the secretions of these glands are found to contain
a considerable proportion (sometimes as much as 4·25 per cent) of
free sulphuric acid. This fact was first noticed by Troschel, who,
while handling a _Dolium galea_ at Messina, saw the creature spit a
jet of saliva upon a marble slab, which immediately produced a brisk
effervescence. A number of the genera thus provided bore through the
shells of other Mollusca and of Echinoderms, to prey upon their soft
tissues, and it is possible that the acid assists in the piercing of
the shell by converting the hard carbonate of lime into sulphate of
lime, which can easily be removed by the action of the radula.[327] In
the majority of the Cephalopoda there are two pairs of salivary glands,
one lying on each side of the mouth, the other on the middle of the
oesophagus.

  [Illustration: FIG. 144.--Alimentary canal, etc., of _Sepia
  officinalis_ L.: _a_, anus; _b.d_, one of the biliary ducts;
  _b.m_, buccal mass; _c_, coecum; _i_, ink-sac; _i.d_, duct of
  same; _j_, jaws; _l.l_, lobes of the liver; _oe_, oesophagus;
  _p_, pancreatic coeca; _r_, rectum; _s.g_, salivary glands; _st_,
  stomach. (From a specimen in the British Museum.)]

  [Illustration: FIG. 145.--Gizzard of _Scaphander lignarius_
  L.: =A=, showing position with regard to oesophagus (_oe_) and
  intestine (_i_), the latter being full of comminuted fragments
  of food; _p_, left plate; _p´_, right plate; _p.ac_, accessory
  plate; =B=, the plates as seen from the front, with the
  enveloping membranes removed, lettering as in =A=. Natural size.]

  [Illustration: FIG. 146.--Section of the stomach of _Melongena_,
  showing the gastric plates (_g.p_, _g.p_,) for the trituration of
  food; _b.d_, biliary duct; _g.g_, genital gland; _i_, intestine;
  _l_, liver; _oe_, oesophagus; _st_, stomach. (After Vanstone.)]

3. _The Oesophagus._--That part of the alimentary canal which lies
between the pharynx and the stomach (in Pelecypoda between the mouth
and stomach) is known as the oesophagus. Its exact limits are not easy
to define, since in many cases the tube widens so gradually, while the
muscular structure of its walls changes so slowly that it is difficult
to say where oesophagus ends and stomach begins. As a rule, the
oesophagus is fairly simple in structure, and consists of a straight
and narrow tube. In the Pulmonata and Opisthobranchiata it often widens
out into a ‘crop,’ which appears to serve the purpose of retaining a
quantity of masticated food before it passes on to the stomach. In
_Octopus_ and _Patella_ the crop takes the form of a lobular coecum.
In the carnivorous Mollusca the oesophagus becomes complicated by
the existence of a varying number of glands, by the action of which
digestion appears to begin in some cases before the food reaches the
stomach proper.

4. _The Stomach._--At the posterior end of the oesophagus lies the
muscular pouch known as the stomach, in which the digestion of the
food is principally performed. This organ may be, as in _Limax_, no
more than a dilatation of the alimentary canal, or it may, as is
usually the case, take the form of a well-marked bag or pocket. The two
orifices of the stomach are not always situated at opposite ends; when
the stomach itself is a simple enlargement of the wall of one side of
the alimentary canal, the cardiac or entering orifice often becomes
approximated to the orifice of exit (pyloric orifice).

The walls of the stomach itself are usually thickened and strengthened
by constrictor muscles. In some Nudibranchs (_Scyllaea_, _Bornella_)
they are lined on the inside with chitinous teeth. In _Cyclostoma_, and
some _Bithynia_, _Strombus_, and _Trochus_ there is a free chitinous
stylet within the stomach.[328] In _Melongena_ (Fig. 146) the posterior
end of the oesophagus is provided with a number of hard plate-like
ridges, while the stomach is lined with a double row of cuticular
knobs, which are movable on their bases of attachment, and serve the
purpose of triturating food.[329] _Aplysia_ has several hard plates,
set with knobs and spines, and similar organs occur in the Pteropoda.
But the most formidable organ for the crushing of food is possessed by
the Bullidae, and particularly by _Scaphander_ (Fig. 145). Here there
is a strong gizzard, consisting of several plates connected by powerful
cartilages, which crush the shells, which are swallowed whole.

Into the stomach, or into the adjacent portions of the digestive tract,
open the ducts which connect with the so-called _liver_. The functions
of this important organ have not yet been thoroughly worked out. The
liver is a lobe-shaped gland of a brown-gray or light red colour, which
in the spirally-shelled families usually occupies the greater part of
the spire. In the Cephalopoda, the two ducts of the liver are covered
by appendages which are usually known as the pancreatic coeca; the
biliary duct, instead of leading directly into the stomach, passes
into a very large coecum (see Fig. 144) or expansion of the same,
which serves as a reservoir for the biliary secretions. At the point
of connexion between the coecum and stomach is found a valve, which
opens for the issue of the biliary products into the stomach, but
closes against the entry of food into the coecum. In most Gasteropoda
the liver consists of two distinct lobes, between which are embedded
the stomach and part of the intestine. In many Nudibranchiata the liver
becomes ‘diffused’ or broken up into a number of small diverticula or
glands connecting with the stomach and intestine. The so-called cerata
or dorsal lobes in the Aeolididae are in effect an external liver,
the removal of which to the outside of the body gives the creature
additional stomach-room.

=The Hyaline Stylet.=--In the great majority of bivalves the intestine
is provided with a blind sac, or coecum of varying length. Within
this is usually lodged a long cylindrical body known as the _hyaline_
or _crystalline stylet_. In a well-developed _Mytilus edulis_ it is
over an inch in length, and in _Mya arenaria_ between two and three
inches. The bladder-like skin of the stylet, as well as its gelatinoid
substance, are perfectly transparent. In the Unionidae there is no
blind sac, and the stylet, when present, is in the intestine itself. It
is said to be present or absent indifferently in certain species.

The actual function performed by the hyaline stylet is at present a
matter of conjecture. Haseloff’s experiments on _Mytilus edulis_ tend
to confirm the suggestion of Möbius, that the structure represents
a reserve of food material, not specially secreted, but a chemical
modification of surplus food. He found that under natural conditions
it was constantly present, but that specimens which were starved lost
it in a few days, the more complete the starvation the more thorough
being the loss; it reappeared when they were fed again. Schulze, on the
other hand, believes that it serves, in combination with mucus secreted
by the stomach, to protect the intestine against laceration by sharp
particles introduced with the food. W. Clark found that in _Pholas_ the
stylet is connected with a light yellow corneous plate, and imagined
therefore that it acts as a sort of spring to work the plate in order
to comminute the food, the two together performing somewhat the
function of a gizzard.[330]

5. and 6. _The Intestine, Rectum, and Anus._--The intestine, the wider
anal end of which is called the rectum, almost invariably makes a bend
forward on leaving the stomach. This is the case in the Cephalopoda,
Scaphopoda, and the great majority of Gasteropoda. The exceptions are
the bilaterally symmetrical Amphineura, in which the anus is terminal,
and many Opisthobranchiata, in which it is sometimes lateral (Fig. 68,
p. 159), sometimes dorsal (Fig. 67). The intestine is usually short in
carnivorous genera, but long and more or less convoluted in those which
are phytophagous. In all cases where a branchial or pulmonary cavity
exists, the anus is situated within it, and thus varies its position
according to the position of the breathing organ. Thus in _Helix_ it
is far forward on the right side, in _Testacella_, _Vaginula_, and
_Onchidium_ almost terminal, in _Patella_ at the back of the neck,
slightly to the right side (Fig. 64, p. 157).

In the rhipidoglossate section of the Diotocardia (_Trochus_,
_Haliotis_, etc.) the rectum passes through the ventricle of the heart,
a fact which, taken in conjunction with others, is evidence of their
relationship to the Pelecypoda.

  [Illustration: FIG. 147.--Ink-sac of _Sepia_, showing its
  relation to the rectum: _a_, anus; _d_, duct of sac; _i.g_,
  ink-gland; _i.r_, portion of the sac which serves as a reservoir
  for the ink; _o_, orifice of ink-gland; _r_, rectum; _sp_, double
  set of sphincter muscles controlling upper end of duct. (Modified
  from Girod.)]

In nearly all Pelecypoda the intestine is very long and convoluted,
being sometimes doubled forward over the mouth. Towards its terminal
part it traverses the ventricle of the heart, except in _Ostrea_,
_Anomia_, _Teredo_, and a few more. The anus is always at the posterior
end of the animal, adjacent to and slightly above the adductor muscle.

Anal glands, which open into the rectum close to the anus, are present
in some Prosobranchiata, _e.g._ _Murex_, _Purpura_. In the Cephalopoda
the anal gland becomes of considerable size and importance, and is
generally known as the ink-sac (Fig. 147); it occurs in all known
living genera, except _Nautilus_. The ink-sac consists of a large bag
generally divided into two portions, in one of which the colouring
matter is secreted, while the other acts as a reservoir for its
storage. A long tube connects the bag with the end of the rectum, the
mouth of the tube being controlled, in _Sepia_, by a double set of
sphincter muscles.


                              =The Kidneys=

The kidneys, nephridia,[331] renal or excretory organs, consist
typically of two symmetrical glands, placed on the dorsal side of
the body in close connexion with the pericardium. Each kidney opens
on the one hand into the mantle cavity, close to the anus (see Fig.
64, p. 157), and on the other, into the pericardium. The venous blood
returning from the body passes through the vascular walls of the
kidneys, which are largely formed of cells containing uric acid. The
blood thus parts with its impurities before it reaches the breathing
organs.

The kidneys are paired in all cases where the branchiae are paired,
and where the heart has two auricles, _i.e._ in the Amphineura, the
Diotocardia (with the exception of the Neritidae), the Pelecypoda,
and all Cephalopoda except _Nautilus_, which has four branchiae,
four auricles, and four kidneys. In other Gasteropoda only one
kidney survives, corresponding to the left kidney of Zygobranchiate
Gasteropods.

Besides their use as excretory organs the kidneys, in certain groups of
the Mollusca, stand in very close relation to the genital glands. In
some of the Amphineura the generative products, instead of possessing
a separate external orifice of their own, pass from the genital gland
into the pericardium and so out through the kidneys (see Fig. 61 C, D,
p. 154). In the Diotocardia it is the right kidney alone which serves,
besides its excretory functions, as a duct for the emission of the
generative products, the left kidney being at the same time greatly
reduced in size. Thus in _Patella_ the left nephridium is small, the
right being much larger; both function as excretory organs, but the
right serves as a mode of conveyance for the seminal products as well.
In certain Pelecypoda (_e.g._ _Yoldia_, _Avicula_, _Modiola_, _Pecten_,
_Spondylus_) the genital glands communicate directly, and with a
similar object, with the renal pouch on the same side of the body, but
in the majority of cases the orifices are distinct.

       *       *       *       *       *

The following memoirs will be found useful for further study of this
portion of the subject:--

   =D. Barfurth=, Ueber den Bau und die Thätigkeit der
   Gasteropodenleber: Arch. Mikr. Anat. xxii. (1883), pp. 473–524.

   =Th. Behme=, Beiträge zur Anatomie und Entwickelungsgeschichte
   des Harnapparates der Lungenschnecken: Arch. Naturges. iv.
   (1889), pp. 1–28.

   =R. Bergh=, Semper’s Reisen im Archipelago der Philippinen;
   Nudibranchiata: Theil ii. Band ii. (1870–78), Band iii.
   (1880–1892).

   =W. G. Binney=, Terrestrial Air-breathing Mollusks of the United
   States: Bull. Mus. C. Z. Harv. iv. (1878), 450 pp.

   „ On the Jaw and Lingual Membrane of North American Terrestrial
   Pulmonata: Proc. Ac. Nat. Sc. Philad. (1875), pp. 140–243.

   =J. T. Cunningham=, The renal organs (Nephridia) of Patella:
   Quart. Journ. Micr. Sc. xxiii. (1883), pp. 369–375.

     „       „ Note on the structure and relations of the kidney in
   Aplysia: Mitth. Zool. Stat. Neap. iv. (1883), pp. 420–428.

   =R. von Erlanger=, On the paired Nephridia of Prosobranchs,
   etc.: Quart. Journ. Micr. Sc. xxxiii. (1892), pp. 587–623.

   =H. Fischer=, Recherches sur la Morphologie du Foie des
   Gastéropodes: Bull. Scient. France Belg. xxiv. (1892), pp.
   260–346.

   =C. Grobben=, Morphologische Studien über den Harn- und
   Geschlechtsapparat, sowie die Leibeshöhle, der Cephalopoden:
   Arb. Zool. Inst. Wien, v. (1884), pp. 179–252.

      „ Die Pericardialdrüse der Gasteropoden: ibid. ix. (1890), pp.
   35–56.

   =B. Haller=, Beiträge zur Kenntniss der Niere der
   Prosobranchier: Morph. Jahrb. xi. (1885), pp. 1–53.

   =A. Hancock=, On the structure and homologies of the renal organ
   in the Nudibranchiate Mollusca: Trans. Linn. Soc. xxiv. (1864),
   pp. 511–530.

   =A. Köhler=, Microchemische Untersuchung der Schneckenzungen:
   Zeits. Gesamm. Naturw. viii. (1856), pp. 106–112.

   =Ad. Oswald=, Der Rüsselapparat der Prosobranchier: Jena. Zeits.
   Naturw. N.F. xxi. (1893), pp. 114–162.

   =R. Perrier=, Recherches sur l’anatomie et l’histologie du rein
   des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), viii.
   (1889), pp. 61–315.

   =C. Semper=, Reisen im Archipelago der Philippinen; Land
   Pulmonata: Theil ii. Band iii. (1870–77).

   =C. Troschel=, Das Gebiss der Schnecken: Berlin, 1856–1892.

   =W. G. Vigelius=, Ueber das Excretionssystem der Cephalopoden:
   Niederl. Arch. Zool. v. (1880), pp. 115–184.




                              CHAPTER IX

       THE SHELL, ITS FORM, COMPOSITION AND GROWTH--DESIGNATION
                         OF ITS VARIOUS PARTS


The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly
applied to the Mollusca, the intrinsic beauty and grace of the
shells themselves, resulting in the passion for their collection,
their durability and ease of preservation, as compared with the
non-testaceous portion,--all these considerations tend to unduly exalt
the value of the shell as part of the organism as a whole, and to
obscure the truth that the shell is by no means the most important of
the organs.

At the same time it must not be forgotten that the old systems of
classification, which were based almost entirely on indications drawn
from the shell alone, have been strangely little disturbed by the new
principles of arrangement, which depend mainly on structural points in
the animal. This fact only tends to emphasise the truth that the shell
and animal are in the closest possible connexion, and that the shell
is a living part of the organism, and is equally sensitive to external
influences.

A striking instance of the comparative valuelessness of the shell alone
as a primary basis of classification is furnished by the large number
of cases in which a _limpet-shaped_ shell is assumed by genera widely
removed from one another in cardinal points of organisation. This
form of shell occurs in the common limpet (Patellidae), in _Ancylus_
(Limnaeidae), _Hemitoma_ (Fissurellidae), _Cocculina_ (close to
Trochidae), _Umbrella_ and _Siphonaria_ (Opisthobranchiata), while in
many other cases the limpet form is nearly approached.

Roughly speaking, about three-quarters of the known Mollusca, recent
and fossil, possess a univalve, and about one-fifth a bivalve shell.
In _Pholas_, and in some species of _Thracia_, there is a small
accessory hinge plate; in the Polyplacophora, or Chitons, the shell
consists of eight plates (see Fig. 2, p. 8), usually overlapping. A
certain proportion of the Mollusca have no shell at all. In many of
these cases the shell has been present in the larva, but is lost in the
adult.

The shell may be

(1) _External_, as in the great majority of both univalves and
bivalves.

(2) _Partly external_, _partly internal_; _e.g._ _Homalonyx_,
_Hemphillia_, some of the _Naticidae_, _Scutum_, _Acera_, _Aplustrum_
(Figs. 148 and 149).

  [Illustration: FIG. 148.--_Aplustrum aplustre_ L. Mauritius,
  showing the partly internal shell (=S=); =F=, foot; =LL=,
  cephalic lappets; =TT=, double set of tentacles. (After Quoy and
  Gaimard.)]

  [Illustration: FIG. 149.--_Sigaretus laevigatus_ Lam., showing
  shell partially immersed in the foot; =F=, anterior prolongation
  of the foot. (After Souleyet.)]

(3) _Internal_; _e.g._ _Philine_, _Gastropteron_, _Pleurobranchus_,
_Aplysia_, _Limax_, _Arion_, _Hyalimax_, _Parmacella_, _Lamellaria_,
_Cryptochiton_, and, among bivalves, _Chlamydoconcha_.

(4) _Absent_; _e.g._ all _Nudibranchiata_ and _Aplacophora_, many
_Cephalopoda_, a few land Mollusca, _e.g._ all _Onchidiidae_,
_Philomycus_, and _Vaginula_.

=The Univalve Shell.=--In univalve Mollusca the normal form of the
shell is an elongated cone twisted into a spiral form round an axis,
the spiral ascending to the left. Probably the original form of the
shell was a simple cone, which covered the vital parts like a tent.
As these parts tended to increase in size, their position on the
dorsal side of the animal caused them gradually to fall over, drawing
the shell with them. The result of these two forces combined, the
increasing size of the visceral hump, and its tendency to pull the
shell over with it, probably resulted in the conversion of the conical
into the spiral shell, which gradually came to envelop the whole
animal. Where the visceral hump, instead of increasing in size, became
flattened, the conical shape of the shell may have been modified into a
simple elliptical plate (_e.g._ _Limax_), the nucleus representing the
apex of the cone. In extreme cases even this plate dwindles to a few
calcareous granules, or disappears altogether (_Arion_, _Vaginula_).

=Varieties of the Spiral.=--Almost every conceivable modification of
the spiral occurs, from the type represented by _Gena_, _Haliotis_,
_Sigaretus_, and _Lamellaria_, in which the spire is practically
confined to the few apical whorls, with the body-whorl inordinately
large in proportion, to a multispiral form like _Terebra_, with about
twenty whorls, very gradually increasing in size.

  [Illustration: FIG. 150.--Examples of shells with =A=, a
  flattened spire (_Polygyratia_); =B=, a globose spire (_Natica_);
  =C=, a greatly produced spire (_Terebra_).]

As a rule, the spire is more or less obliquely coiled round the axis,
each whorl being partially covered, and therefore hidden by, its
immediate successor, while the size of the whorls, and therefore the
diameter of the spire as a whole, increases somewhat rapidly. The
effect of this is to produce the elevated spire, the shell of six to
ten whorls, and the wide aperture, of the normal type of mollusc, the
whelk, snail, periwinkle, etc.

Sometimes, however, the coil of the whorls, instead of being oblique,
tends to become horizontal to the axis, and thus we have another series
of gradations of form, from the excessively produced spire of _Terebra_
to the flattened disc of _Planorbis_, _Polygyratia_, _Euomphalus_, and
_Ammonites_. The shell of many species of _Conus_ practically belongs
to the latter type, each whorl folding so closely over its predecessor
that the spiral nature of the shell is not perceived until it is looked
at at right angles to the spire.

  [Illustration: FIG. 151.--Examples of shells with disconnected
  whorls; =A=, _Cyathopoma cornu_ Mf., Philippines; =B=,
  _Cylindrella hystrix_ Wright, Cuba. (Both × 4.)]

  [Illustration: FIG. 152.--Example of a shell whose apical whorls
  alone are coiled, and the remainder produced in a regular curve.
  (_Cyclosurus Mariei_ Morel., Mayotte.)]

In some cases the regularly spiral form is kept, but the whorls are
completely disconnected; _e.g._ some _Scalaria_, _Spirula_; among
fossil Cephalopoda, _Gyroceras_, _Crioceras_, and _Ancyloceras_; and,
among recent land Mollusca, _Cylindrella hystrix_ and _Cyathopoma
cornu_ (Fig. 151). Sometimes only the last whorl becomes disconnected
from the others, as in _Rhiostoma_ (see Fig. 180, p. 266),
_Teinostoma_, and in the fossil _Ophidioceras_ and _Macroscaphites_.
Sometimes, again, not more than one or two whorls at the apex are
spirally coiled, and the rest of the shell is simply produced or
coiled in an exceedingly irregular manner, _e.g._ _Cyclosurus_,
_Lituites_, _Orygoceras_, _Siliquaria_ (Fig. 153), _Vermetus_. In
_Coecum_ (Fig. 170, p. 260) the spiral part is entirely lost, and the
shell becomes simply a cylinder. In a few cases the last whorl is
coiled irregularly backwards, and is brought up to the apex, so that
the animal in crawling must carry the shell with the spire downwards,
as in _Anostoma_ (Fig. 154), _Opisthostoma_ (Fig. 208, p. 309),
_Strophostoma_, and _Hypselostoma_ (Fig. 202 A, p. 302).

  [Illustration: FIG. 153.--_Siliquaria anguina_ Lam., showing
  scalariform coil of upper whorls and irregular extension of the
  lower.]

  [Illustration: FIG. 154.--_Anostoma globulosum_ Lam., Brazil.
  (After P. Fischer.)]

  [Illustration: FIG. 155.--Various forms of the internal plate in
  _Capulidae_: =A=, _Calyptraea_ (_Mitrularia_) _equestris_ Lam.,
  E. Indies; =B=, _Crucibulum scutellatum_ Gray, Panama; =C=,
  _Ergaea plana_ Ad., and Reeve, Japan; =D=, _Galerus chinensis_
  L., Britain; =E=, _Crepipatella dilatata_ Lam., Callao; =F=,
  _Trochita maculata_ Quoy, N. Zealand; =G=, _Crepidula fornicata_
  Lam., N. America.]

Some genera of the _Capulidae_, in which the shell is of a broadly
conical form or with scarcely any spire, develop an internal plate
or process which serves the purpose of keeping the animal within the
shell, and does the work of a strong attachment muscle. In _Mitrularia_
this process takes the form of a raised horse-shoe; in _Crucibulum_ it
is cup-shaped, with the edge free all round; in _Galerus_, _Ergaea_,
_Crepipatella_, and _Trochita_ we get a series of changes, in which
the edge of the cup adheres to the interior of the shell, and then
gradually flattens into a plate. In _Crepidula_ proper this plate
becomes a regular partition, covering a considerable portion of the
interior (Fig. 155 G). _Hipponyx_ secretes a thin calcareous plate on
the ventral surface of the foot, which intervenes like an operculum
between the animal and the substance to which it adheres.

_Sinistral, or Left-handed Shells._--The vast majority of univalve
spiral shells are normally _dextral_, _i.e._ when held spire uppermost,
with the aperture towards the observer, the aperture is to the right
of the axis of the spire. If we imagine such a shell to be a spiral
staircase, as we ascended it we should always have the axis of the
spire to our left.

Sinistral or ‘reversed’ forms are not altogether uncommon, and may be
grouped under four classes:--

(1) Cases in which the _genus is_ normally sinistral; (2) cases in
which the _genus_ is _normally dextral_, but _certain species_ are
_normally sinistral_; (3) cases in which the shell is _indifferently
dextral or sinistral_; (4) cases in which _both genus and species_ are
_normally dextral_, and a sinistral form is an _abnormal monstrosity_.

  [Illustration: FIG. 156.--_Fulgur perversum_ L., Florida. × ½.]

  [Illustration: FIG. 157.--Illustration of the gradation of forms
  in _Ampullaria_ between a dextral (=A=) and an ultra-dextral
  species (=F=).]

In all cases of sinistral monstrosity, and all in which a sinistral and
dextral form are interchangeable (sections 3 and 4 above), the position
of the apertures of the internal organs appears to be relatively
affected, _i.e._ the body is sinistral, as well as the shell. This has
been proved to be the case in all specimens hitherto examined, and may
therefore be assumed for the rest. The same uniformity, however, does
not hold good in all cases for genera and species normally sinistral
(sections 1 and 2). As a rule, the anal and genital apertures are, in
these instances also, to the left, but not always. In _Spirialis_,
_Limacina_, _Meladomus_, and _Lanistes_ the shell is sinistral, but
the animal is dextral. This apparent anomaly has been most ingeniously
explained by Simroth, Von Ihering, and Pelseneer. The shell, in all
these cases, is not really sinistral, but _ultra-dextral_. Imagine
the whorls of a dextral species capable of being flattened, as in a
_Planorbis_, and continue the process, still pushing, as it were, the
spire downwards until it occupies the place of the original umbilicus,
becoming turned completely ‘inside out,’ and we have the whole
explanation of these puzzling forms. The animal remains dextral, the
shell has become sinistral. A convincing proof of the truth of this
is furnished by the operculum. It is well known that the twist of the
operculum varies with that of the shell; when the shell is dextral, the
operculum is sinistral, with its nucleus near the columella, and _vice
versâ_. In these ultra-dextral shells, however, where it is simply the
method of the enrolment of the spire that comes in question, and not
the formation of the whorls themselves, the operculum remains sinistral
on the apparently sinistral shell.

The reverse case to this, when the shell is dextral but the orifices
sinistral, is instanced by the two fresh-water genera _Pompholyx_ (from
N. America), and _Choanomphalus_ (L. Baikal). A similar transition in
the enrolment of the whorls may be confidently assumed to have taken
place, and the shells are styled _ultra-sinistral_.

Yet another variation remains, in which the embryonic form is
sinistral, but the adult shell dextral, the former remaining across the
nucleus of the spire. This is the case with _Odostomia_, _Eulimella_,
_Turbonilla_, and _Mathilda_, all belonging to the Prosobranchiata,
with _Actaeon_, _Tornatina_, and _Actaeonina_ among the Opisthobranchs,
and _Melampus_ alone among Pulmonates.

=Monstrosities of the Shell.=--Abnormal growths of the shell constantly
occur, some of them being scarcely noticeable, except by a practised
eye, others of a more serious nature, involving an entire change in
the normal aspect of the creature. _Scalariform_ monstrosities are
occasionally met with, especially in _Helix_ and _Planorbis_, when
the whorls become unnaturally elevated, and sometimes quite disjoined
from one another; _carinated_ monstrosities develop a keel on a whorl
usually smooth; _acuminated_ monstrosities have the spire produced to
an extreme length (Fig. 158); _sinistral_ monstrosities (see above)
have the spire reversed: dwarfs and giants, as in our own race, are
occasionally noticed among a crowd of individuals.

More serious forms of monstrosity are those which occur in individual
cases. Mr. S. P. Woodward once observed[332] a specimen of an adult
_Helix aspersa_ with a second, half-grown individual fixed to its
spire, and partly embedded in the suture of the body whorl. The younger
snail had died during its first hibernation, as was shown by the
epiphragm remaining in the aperture, and its neighbour, not being able
to get free of the incubus, partially enveloped it in the course of
its growth. In the British Museum two _Littorina littorea_ have become
entangled in a somewhat similar way (Fig. 160 B), possibly as a result
of embryonic fusion. Double apertures are not uncommon[333] in the
more produced land-shells, such as _Cylindrella_ and _Clausilia_ (Fig.
160 A). In the Pickering collection was a _Helix hortensis_ which had
crawled into a nutshell when young, and, growing too large to escape,
had to carry about this decidedly extra shell to the end of its days.
A monstrosity of the cornucopia form, in which the whorls are uncoiled
almost throughout, is of exceedingly rare occurrence (Fig. 161).

  [Illustration: FIG. 158.--Monstrosities of _Neptunea antiqua_ L.,
  and _Buccinum undatum_ L., with a greatly produced spire (from
  specimens in the Brit. Mus.).]

  [Illustration: Fig. 159.--Monstrosities of _Littorina rudis_ Mat,
  The Fleet, Weymouth. (After Sykes.)]

Some decades ago ingenious Frenchmen amused themselves by creating
artificial monstrosities. _H. aspersa_ was taken from its shell, by
carefully breaking it away, and then introduced into another shell of
similar size (_H. nemoralis_, _vermiculata_, or _pisana_). At the end
of several days attachment to the columella took place, and then growth
began, the new shell becoming soldered to the old, and the spiral
part of the animal being protected by a thin calcareous envelope. A
growth of from one to two whorls took place under these conditions.
The individuals so treated were always sordid and lethargic, but they
bred, and naturally produced a normal _aspersa_ offspring.[334] In the
British Museum there is a specimen of one of these artificial unions of
a _Helix_ with the shell of a _Limnaea stagnalis_.

  [Illustration: FIG. 160.--Monstrosities with two apertures: =A=,
  _Cylindrella agnesiana_ C. B. Ad., Jamaica; =B=, _Littorina
  littorea_ (from specimens in the British Museum).]

  [Illustration: FIG. 161.--Cornucopia-shaped monstrosity of _Helix
  aspersa_, from Ilfracombe. (British Museum.)]

=Composition of the Shell.=--The shell is mainly composed of pure
carbonate of lime, with a very slight proportion of phosphate of lime,
and an organic base allied to chitin, known as _conchiolin_. The
proportion of carbonate of lime is known to vary from about 99 p.c. in
_Strombus_ to about 89 p.c. in _Turritella_. Nearly 1 p.c. of phosphate
of lime has been obtained from the shell of _Helix nemoralis_, and
nearly 2 p.c. from that of _Ostrea virginica_. The conchiolin forms a
sort of membranous framework for the shell; it soon disappears in dead
specimens, leaving the shell much more brittle than it was when alive.
Carbonate of magnesia has also been detected, to the extent of ·12 p.c.
in _Telescopium_ and ·48 p.c. in _Neptunea antiqua_. A trace of silica
has also occasionally been found.

When the shell exhibits a crystalline formation, the carbonate of lime
may take the form either of _calcite_ or _aragonite_. The calcite
crystals are rhombohedral, optically uniaxal, and cleave easily, while
the aragonite cleave badly, belong to the rhombic system, and are
harder and denser, and optically biaxal. Both classes of crystal may
occur in the same shell.

Two main views have been held with regard to the formation and
structure of the shell--(1) that of Bowerbank and Carpenter, that the
shell is an organic formation, growing by interstitial deposit, in the
same manner as the teeth and bones of the higher animals; (2) that
of Réaumur, Eisig, and most modern writers, that the shell is of the
nature of an excretion, deposited like a cuticle on the outside of the
skin, being formed simply of a number of calcareous particles held
together by a kind of ‘animal glue.’ Leydig’s view is that the shell
of the Monotocardia is a secretion of the epithelium, but that in the
Pulmonata it originates within the skin itself, and afterwards becomes
free.[335]

According to Carpenter, when a fragment of any recent shell is
decalcified by being placed in dilute acid, a definite animal basis
remains, often so fine as to be no more than a membranous film, but
sometimes consisting of an aggregation of ‘cells’ with perfectly
definite forms. He accordingly divides all shell structure into
_cellular_ and _membranous_, according to the characteristics of the
animal basis. Cellular structure is comparatively rare; it occurs most
notably in _Pinna_, where the shell is composed of a vast multitude of
tolerably regular hexagonal prisms (Fig. 162 B). Membranous structure
comprises all forms of shell which do not present a cellular tissue.
Carpenter held that the membrane itself was at one time a constituent
part of the mantle of the mollusc, the carbonate of lime being secreted
in minute ‘cells’ on its surface, and afterwards spreading over the
subjacent membrane through the bursting of the cells.

The iridescence of _nacreous_ shells is due to a peculiar lineation
of their surface, which can be readily detected by a lens. According
to Brewster, the iridescence is due to the alternation of layers of
granular carbonate of lime and of a very thin organic membrane, the
layers very slightly undulating. Carpenter, on the other hand, holds
that it depends upon the disposition of a single membranous layer
in folds or plaits, which lie more or less obliquely to the general
surface, so that their edges show as lines. The nacreous type of
shell occurs largely among those Mollusca which, from other details
in their organisation, are known to represent very ancient forms
(_e.g._ _Nucula_, _Avicula_, _Trigonia_, _Nautilus_). It is also the
least permanent, and thus in some strata we find that only casts of
the nacreous shells remain, while those of different constitution are
preserved entire.

_Porcellanous_ shells (of which the great majority of Gasteropoda are
instances) usually consist of three layers, each of which is composed
of a number of adjacent plates, like cards on edge. The inclination of
the plates in the different layers varies, but that of the plates in
the inner and outer layer is frequently the same, thus if the plates
are transverse in the middle stratum, they are longitudinal in the
inner and outer strata, and, if longitudinal in the middle, they are
transverse in the other two. Not uncommonly (Fig. 163 B) other layers
occur. In bivalves the disposition and nature of the layers is much
more varied.

  [Illustration: FIG. 162.--=A=, Section of shell of _Unio_:
  _a_, periostracal layer; _b_, prismatic layer; _c_, nacreous
  layer. =B=, Horizontal section of shell of _Pinna_, showing the
  hexagonal prisms.]

In _Unio_ the periostracal or uppermost layer is very thin; beneath
this is a prismatic layer of no great depth, while the whole remainder
of the shell is nacreous (Fig. 162 A). Many bivalves show traces of
tubular structure, while in the Veneridae the formation and character
of the layers approaches closely to that of the Gasteropoda. Further
details may be gathered from Carpenter’s researches.[336]

=Formation of Shell.=[337]--The mantle _margin_ is the principal agent
in the deposition of shell. It is true that if the shell be fractured
at any point, the hole will be repaired, thus showing that every part
of the mantle is furnished with shell-depositing cells, but such new
deposits are devoid of colour and of periostracum, and no observation
seems to have been made with regard to the layers of which they are
composed. As a rule the mantle, except at its margin, only serves to
thicken the innermost layer of shell.

It is probable that the carbonate of lime, of which the shell is mainly
composed, is separated from the blood by the epithelial cells of the
mantle margin, and takes the crystalline or granular form as it hardens
on exposure after deposition. The three layers of a porcellanous shell
are deposited successively, and the extreme edge of the mouth, when
shell is forming, will contain only one layer, the outermost; a little
further in, two layers appear, and further still, three. The pigment
cells which colour the surface are situated at the front edge of the
mantle margin.

  [Illustration: FIG. 163.--Sections of shells. =A=, _Conus_:
  _a_, outer layer; _b_, middle prismatic layer, with obliquely
  intersecting laminae above and below; _c_, inner layer. =B=,
  _Oliva_: _a_, outer layer; _b_, layer of crossed and curved
  laminae; _c_, prismatic layer, succeeded by layer of laminae at
  right angles to one another; _d_, inner layer. =C=, _Cypraea_:
  _a_, outer layer; _b_, middle layer; _c_, inner layer.]

Shelly matter is deposited, and probably secreted, not only by the
mantle, but also in some genera by the foot. This is certainly the case
in _Cymbium_, _Oliva_, _Ancillaria_, _Cassis_, _Distortio_, and others,
in several of which the foot is so large that the shell appears to be
quite immersed in it.[338]

The deposition of shell is not continuous. Rest periods occur, during
which the function is dormant; these periods are marked off on the
edge of the shell, and are known as _lines of growth_. In some cases
(_Murex_, _Triton_, _Ranella_), the rest period is marked by a decisive
thickening of the lip, which persists on the surface of the shell as
what is called a _varix_ (see p. 263).

  [Illustration: FIG. 164.--_Murex tenuispina_ L., Ceylon. × ⅔.]

  [Illustration: FIG. 165.--_Neritina longispina_ Récl., Mauritius.
  (Operculum removed.)]

The various details of sculpture on the exterior surface of the shell,
the striae, ribs, nodules, imbrications, spines, and other forms
of ornamentation are all the product of similar and corresponding
irregularities in the mantle margin, and have all been originally
situated at the edge of the lip. Spines, _e.g._ those of _Murex_ and
_Pteroceras_, are first formed as a hollow thorn, cleft down its lower
side, and are afterwards filled in with solid matter as the mantle
edge withdraws. What purpose is served by the extreme elaboration
of these spiny processes in some cases, can hardly be considered as
satisfactorily ascertained. Possibly they are a form of sculptural
development which is, in the main, protective, and secures to its
owners immunity from the attacks of predatory fishes.

‘Attached’ genera (e.g. _Chama_, _Spondylus_) when living on smooth
surfaces have a flat shell, but when affixed to coral and other uneven
surfaces they become very irregular in shape. The sculpture of the base
on which they rest is often reproduced in these ‘attached’ shells,
not only on the lower, but also on the upper valve, the growing edge
of which rests on the uneven surface of the base. Oysters attached to
the branches of the mangrove frequently display a central convex rib,
modelled on the shape of the branch, from which the plaits of sculpture
radiate, while specimens fixed to the smooth trunk have no such rib.
_Crepidula_, a genus which is in the habit of attaching itself to other
shells, varies in sculpture according to that of its host. Sometimes
the fact may be detected that a specimen has lived on a ribbed shell
when young, and on a smooth one when old, or _vice versâ_. A new genus
was actually founded by Brown for a _Capulus_ which had acquired ribs
through adhesion to a _Pecten_. A specimen of _Hinnites giganteus_ in
the British Museum must at one period of its growth have adhered to
a surface on which was a Serpula, the impression of which is plainly
reproduced on the upper valve of the _Hinnites_.[339]

  [Illustration: FIG. 166.--A specimen of _Anomia ephippium_ L.,
  Weymouth, taken upon _Pecten maximus_, the sculpture of which
  is reproduced on the upper valve of the _Anomia_, and even on a
  young _Anomia_ attached to the larger specimen.]

=Growth of the Shell.=--Nothing very definite is known with regard to
the rate of growth of the shell in marine Mollusca. Under favourable
conditions, however, certain species are known to increase very
rapidly, especially if the food supply be abundant, and if there
is no inconvenient crowding of individuals. Petit de la Saussaye
mentions[340] the case of a ship which sailed from Marseilles for
the west coast of Africa, after being fitted with an entirely new
bottom. On arriving at its destination, the vessel spent 68 days in
the Gambia River, and took 86 days on its homeward voyage. On being
cleaned immediately on its return to Marseilles, an _Avicula_ 78 mm.
and an _Ostrea_ 95 mm. long (both being species peculiar to W. Africa)
were taken from its keel. These specimens had therefore attained this
growth in at most 154 days, for at the period of their first attachment
they are known to be exceedingly minute. P. Fischer relates[341] that
in 1862 a buoy, newly cleaned and painted, was placed in the basin
at Arcachon. In less than a year after, it was found to be covered
with thousands of very large _Mytilus edulis_, 100 mm. × 48 mm., the
ordinary size on the adjoining banks being only about 50 to 60 × 30 mm.

Some observations have already been recorded (p. 40) on the growth of
_Helix aspersa_. In the summer of 1858, which was very dry, especially
in the south of France, the young Helices born that year were still
very small in August. About the end of that month abundant rain came
on, and in four or five days young _H. variabilis_, _H. pisana_, and
_H. aspersa_, eating without cessation, as if to make up for lost
time, grew more than a centimetre of shell. The lip of a young _H.
arbustorum_ has been observed to have grown, at the end of the first
week in the season’s growth, 3 mm., at the end of the second week, 6·25
mm., the third, 11·5 mm., and the fourth 12·5 mm., with a finished
lip.[342]

Careful observation has shown that in the growth of the shell of
_Helix aspersa_ the periostracum is first produced; it is covered with
hyaline globules, 10–12 mm. in diameter, which persist even in the
oldest shells. Calcareous matter is deposited on the internal face of
the new periostracum, at some distance from the margin. It is secreted
by a white zone or band of cells bounding the entire breadth of the
mantle as applied to the peristome. Immediately behind the white zone
are a series of pigment cells which not only give the shell its colour
but complete the calcification of the shelly matter laid down by the
white zone. When the animal has attained its full growth and the lip
is finished off, the white band and the periostracum cells completely
disappear, and only such cells persist as contribute to the internal
thickening of the shell. Shell growth, in this species, is very rapid.
If a portion of the pulmonary sac is laid bare, by removing a fragment
of shell, at the end of 1½ or 2 hours there may be detected a delicate
organic membrane covering the hole, and strewn with crystals of
carbonate of lime. This thickens with great rapidity, and soon fills
up the hole with solid matter. For two consecutive months an animal,
deprived of food, has been known to reproduce this membrane daily after
its removal every morning.[343] Prof. Schiedt has found that oysters,
if deprived of the right valve and exposed to the light, not only
develop brown pigment over the whole exposed surface of mantle and
branchiae, but actually succeed in part in reproducing the valve and
hinge.[344]

_Deposit of Additional Layers of Shell._--Mollusca possess the power
of thickening the interior of the shell, by the deposit of successive
layers. This is frequently done in self-defence against the attacks
of boring Mollusca, sponges, and worms. Cases may often be noticed of
_Ostrea_, _Spondylus_, and other sedentary molluscs, which, unable
to escape the gradual assaults of their foes, have provided against
them by the deposit of fresh shelly matter. A somewhat similar plan is
adopted to provide against intrusion by way of the aperture. Pearls
are, in many cases, the result of shell deposition upon the eggs or
even the body of some intrusive parasite (_Distoma_, _Filaria_, etc.),
and are, in some countries, artificially produced by the introduction
of fragments of sand, metal, etc., into living _Unio_ and _Anodonta_.
Little joss images are made in India and China, the nacre on which is
produced by thrusting them inside living Unionidae.

A specimen of _Helix rosacea_, in the British Museum, into whose shell
a piece of grass somehow became introduced, has partitioned it off
by the formation of a sort of shelly tunnel extending throughout its
entire length (Fig. 167).

  [Illustration: FIG. 167.--A specimen of _Helix rosacea_ Müll.,
  Cape of Good Hope, into which a piece of grass has by some means
  become introduced. The animal has protected itself by covering
  the grass with a shelly layer. (From a specimen in the British
  Museum.)]

=Absorption of Internal Portions.=--Certain genera have the remarkable
property of absorbing, when they become adult, the internal portions
of the whorls and the greater part of the columellar axis. The
effect of this is to make the shell, when the process is complete,
no longer a spiral but a more or less produced cone, and it is found
that in such cases the viscera of the spire lose their spiral form,
and take the shape of the cavity in which they lie. Amongst the
genera in which this singular process takes place are _Nerita_,[345]
_Olivella_, and _Cypraea_ amongst marine forms, and nearly the whole
of the Auriculidae[346] (Fig. 168). _Conus_ reduces the internal
subdivisions of the spire to extreme thinness. It is noticeable that
these genera are all of considerable thickness of shell, and it is
perhaps the result of the whole energy of the animal being directed to
the formation of its external protection that the internal walls of the
spire become atrophied and eventually disappear.

  [Illustration: FIG. 168.--_Auricula Judae_ Lam., showing the
  disappearance of the partitions of the whorls, which are
  represented by dotted lines. (After Fischer.)]

  [Illustration: FIG. 169.--=A=, Decollated (adult) form, and =B=,
  perfect (young) form of _Cylindrella nobilior_ Ad., Jamaica; the
  dotted line shows where decollation takes place.]

  [Illustration: FIG. 170.--Development of _Coecum_: =A=, showing
  the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_,
  first septum; _s´s´_, second septum. (After de Folin.) =B=, Adult
  form of _C. eburneum_ Ad., Panama. × 8.]

=Decollation.=--In certain genera, when the shell becomes adult, the
animal ceases to occupy the upper whorls, which accordingly die and
drop off, the orifice at the top having meanwhile been closed by a
shelly deposit. Such shells are termed _decollated_. In some land
genera decollation is the rule, _e.g._ in _Cylindrella_ (Fig. 169),
_Eucalodium_, and _Rumina_, as well as in many species of the brackish
water genera, _Truncatella_, _Cerithidea_, and _Quoyia_. _Stenogyra_
(_Rumina_) _decollata_, a common shell in the south of Europe, has
been noticed to bang its upper whorls violently against some hard
substance, as if to get rid of them.

  [Illustration: FIG. 171.--Four stages in the growth of
  _Fissurella_, showing how the spire gradually disappears and
  the marginal slit becomes an apical hole, =A=, =B=, =C=, highly
  magnified, =D=, natural size. (After Boutan.)]

  [Illustration: FIG. 172.--Three stages in the growth of _Cypraea
  exanthema_ L. (From specimens taken at Panama.)]

=Special Points in the Growth of Certain Genera.=--In the young of
_Coecum_ the apex is at first spiral, but as growth proceeds and the
long tube begins to form, a septum is produced at the base of the apex,
which soon drops off. Soon afterwards, a second septum forms a little
farther down, and a second piece drops off, leaving the shell in the
normal cylindrical form of the adult (Fig. 170). The development of
_Fissurella_ is of extreme interest. In an early stage it possesses
a spiral shell, with a slit on the margin of the outer lip of the
last whorl. As growth advances, shelly matter is deposited on both
margins, which results in the slit becoming a hole and the spire a
mere callosity, until at last they appear to coalesce in the apex of
the adult shell (Fig. 171). The singular formations of _Magilus_ and
_Rhizochilus_ have already been described (pp. 75, 76). _Cypraea_, in
the young stage, is a thin spiral shell with a conspicuous apex. As
growth proceeds, the surface of the whorls, which are nearly enveloped
by two large lobes of the mantle, becomes overlaid with new layers
of shelly matter, until eventually the spire becomes embedded, and
ultimately disappears from view (Fig. 172).

_Patella_, when young, has a nautiloid shell (see Fig. 45, p. 134),
but it is a remarkable fact that we are entirely ignorant, in this
commonest of molluscs, of the transition stages which convert the
nautiloid into the familiar conical shell. The young shell of
_Pteroceras_ is deceptively unlike the adult, and is entirely devoid
of the finger-like processes which are so characteristic of the genus
(chap. xiv.).

Among the bivalve Mollusca, _Anomia_ in a young stage is not to be
distinguished from Ostrea. Soon a small sinus appears on the ventral
margin, which gradually deepens and, as the shell grows round it, forms
a hole for the byssus, eventually becoming fixed beneath the umbones
(see Fig. 173). In _Teredo_ the two valves of the shell proper, which
is very small, become lodged in a long calcareous tube or cylinder,
which is generally open at both ends (see chap. xvi.). In _Aspergillum_
a somewhat similar cylinder is developed, but the valves are soldered
to the tube, and form a part of it, the tube itself being furnished, at
the anterior end, with a disc, which is perforated with holes like the
rose of a watering-pot. In _Clavagella_ the left valve alone becomes
soldered to the tube, while the right valve is free within it (see
chap. xvi.). _Fistulana_ encloses the whole of its shell in a long
tapering tube, which is not at any point adherent to the shell.

  [Illustration: FIG. 173.--Development of the byssus or the
  plug-hole in _Anomia_. (After Morse.)]

=Terms employed to denote various Parts of the Univalve Shell.=--The
_apex_ is the extreme top of the spire, and generally consists of
the embryonic shell, which may often be recognised by its entire
want of sculpture. When the embryonic shell happens to be large, the
apex is often mammillated, _e.g._ in _Fusus_, _Neptunea_, and some
_Turbinella_; in the _Pyramidellidae_ it is sinistral.

The _suture_ is the line of junction between any two successive
whorls. It may be deep, and even channelled, or very shallow, as in
Fig. 150 B (p. 246).

The _spire_ is the whole series of whorls except the last or _body
whorl_. A _whorl_ is a single revolution of the spiral cone round
the axis. The spire may be _subulate_ (as in _Terebra_, Fig. 150 C),
_turreted_ (_Scalaria_), _depressed_ (_Polygyratia_, Fig. 150 A),
_conical_ (_Trochus_), _globose_ (_Ampullaria_, _Natica_, Fig. 150
B), with almost all conceivable gradations between these types. The
number of whorls is best counted by placing the shell mouth downwards,
and reckoning _one_ for every suture that occurs between the extreme
anterior point of the shell and the apex.

  [Illustration: FIG. 174.--Illustrating the technical terms
  applied to the various parts of a univalve shell.]

The _mouth_ or aperture may be (_a_) entire, as in _Helix_, _Natica_,
_Ampullaria_, when its _peristome_ or margin is not interrupted by any
notch or canal, or (_b_) prolonged at its anterior and sometimes also
at its posterior end into a _canal_. The _anterior canal_ serves as a
protection to the siphon,[347] the _posterior canal_ is mainly anal
in function, and corresponds, in part, to the hole of _Fissurella_,
the slit in _Pleurotoma_ and _Emarginula_, and the row of holes in
_Haliotis_. The mouth presents every variety of shape, from the perfect
circle in _Cyclostoma_ and _Trochus_, to the narrow and prolonged slit
in _Conus_ and _Oliva_.

  [Illustration: FIG. 175.--Anal slit in _Pleurotoma_.]

The right margin of the mouth (the left, in sinistral shells) is
termed the _outer lip_ or _labrum_, the left margin the _inner lip_,
_labium_, or _columella lip_.[348] In young shells the outer lip
is usually thin and unfinished, while in the adult it is generally
thickened into a rib, or furnished with more or less prominent teeth,
or given an inward or outward curve. In some genera, especially the
Strombidae, the outer lip of the adult develops long finger-like
processes, which sometimes attain an extraordinary size (chap.
xiv.). As growth proceeds, these marginal teeth and ribs are either
dissolved and disappear, or are permanently incorporated, in the shape
of _varices_, with the framework of the shell. Some shells, _e.g._
_Natica_, _Turritella_, _Actaeon_, have a permanently unfinished
outer lip, even in the adult stage. The columella lip varies in shape
with the mouth as a whole; thus it may be straight, as in _Conus_, or
excavated, as in _Sigaretus_, _Struthiolaria_, and _Bulla_. Frequently
it is continued by part of the body whorl, as in _Ficula_, _Dolium_,
and _Fasciolaria_.

  [Illustration: FIG. 176.--_Solarium perspectivum_ Lam., from the
  under side.]

  [Illustration: FIG. 177.--Section of _Turbinella pyrum_ L.,
  showing the plicae on the columella and the growth of successive
  whorls.]

The folds or plaits on the columella, which are often characteristic
of the genus or even family (_e.g._ Fasciolariidae, Mitridae,
Turbinellidae) are not merely external, but continue down the whole
spire (see Fig. 177, which also shows how successive fresh growths have
thickened the columella).

The whorls may be wound in a spiral, which is either hollow, as in
_Solarium_, or quite compact, as in _Oliva_, _Terebra_, _Cypraea_,
with every possible intermediate grade. This concavity, which
varies in depth and width, is known as the _umbilicus_, and shells
are accordingly spoken of as _deeply_ (_e.g._ most Trochidae and
Naticidae), _narrowly_ (_e.g._ _Lacuna_, _Littorina_), or _widely_
(_e.g._ _Solarium_) _umbilicated_. When the spiral is quite flat, as in
_Planorbis_ and some _Helix_, the umbilicus vanishes entirely. Shells
in which the whorls are so compactly coiled on an ascending spiral that
there is no umbilicus, are termed _imperforate_.

  [Illustration: FIG. 178.--The slit in =A=, _Hemitoma_, =B=,
  _Emarginula_, =C=, _Macroschisma_, =D=, _Craniopsis_, =E=,
  _Puncturella_, =F=, _Fissurella_.]

_The Slit._--Many shells are furnished with a slit in the last whorl,
which opens, in most cases, on the outer lip, and is sometimes of
considerable depth, at others a mere notch. In the patelliform shells
it is always in front of the apex. The function of the slit appears to
be mainly anal, the excretory products being thus allowed to escape
by a passage of their own, without soiling the clean water taken in
by the branchiae. The posterior canal of some Gasteropoda probably
performs a similar function. In the adult _Fissurella_ the slit
becomes an apical hole (see Fig. 178 F), in the allied genera it is
either immediately in front of the spire (_Puncturella_), or half-way
between the spire and the anterior margin (_Rimula_), or on the margin
and well marked (_Emarginula_), or a mere indentation of the margin
(_Hemitoma_). In _Pleurotomaria_ it is exceptionally long, and is well
marked in _Bellerophon_, _Schismope_, _Scissurella_, _Murchisonia_, and
_Pleurotoma_ (where it is sutural). In _Haliotis_ and _Polytremaria_
it is replaced by a series of holes, which are closed up as the
animal grows past them. Some of these holes (at least in _Haliotis_)
certainly serve the purpose of admitting water to the branchiae, while
others are anal. In _Trochotoma_ there are only two holes, united by a
narrow fissure.

_The Tubed Land Operculates._--A group of the Cyclophoridae, which is
restricted to Further India and the great Malay Islands, has developed
a remarkable _sutural tube_ on the exterior of the last whorl, near
the aperture, A similar tube, but more obscure, exists in _Alycaeus_.
Several stages in the development of this tube may be noticed,
beginning with the elevation of part of the peristome into a simple
irregular shelly plate, which is continued, first into a short, and
then into a long tube, which becomes soldered to the shell; finally,
the tube becomes free, and the anterior part of the last whorl is
disconnected from the spire (Fig. 180 A-D).

  [Illustration: FIG. 179.--The slit in =A=, _Bellerophon_, =B=,
  _Pleurotomaria_, =C=, _Schismope_, =D=, _Polytremaria_, =E=,
  _Haliotis_ (not drawn to scale).]

  [Illustration: FIG. 180.--Development of the tube in the
  tube operculates: =A=, _Pterocycus rupestris_ Bens.; =B=,
  _Opisthoporus birostris_ Pfr.; =C=, _Spiraculum travancoricum_
  Bedd.; =D=, _Rhiostoma Housei_ Pfr.]

It is singular that the tube does not appear to be of any use to
the animal, since its internal extremity, in the complete form, is
closed, and does not communicate with the interior of the whorl. It
may be presumed, however, that in origin the tube served as a means of
conveying air to the animal when the operculum was closed. The holes in
the peristome of _Pupina_, _Cataulus_, and _Anostoma_ (Fig. 154) may be
compared.

  [Illustration: FIG. 181.--_Eburna spirata_ Lam., E. Indies. =F=,
  foot; =OP=, operculum; =P=, penis; =S=, siphon; =T=, tentacles,
  with eyes at their base. (After Souleyet.)]

=The Operculum.=--The operculum is a cuticular development of a
group of cells situated on the dorsal side of the foot, exactly over
the terminal point of the fibres of the columellar muscle. It is so
situated that in crawling it is generally carried free of the shell,
sometimes at the extreme upper end of the foot, more usually somewhat
nearer to the shell (Fig. 181). In _Pterocyclus_ it is pushed back into
the umbilicus when the animal is in motion.

The operculum is present in nearly all land, fresh-water, and marine
Prosobranchiata, absent in all Opisthobranchiata in the adult state,
except _Actaeon_, and in all Pulmonata, except _Amphibola_. It has
been lost in the following marine Prosobranchiata: many Cancellariidae
and Conidae, _Oliva_ (though present in _Olivella_ and _Ancilla_),
Harpidae, Marginellidae, _Voluta_ proper (though present in _V.
musica_), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and,
of land genera, in Proserpinidae. It is evident, therefore, that its
presence or absence is of limited value in classification. In some
species of _Ampullaria_ and _Natica_ it is horny, in others shelly.
Dall found that in a number of specimens of _Volutharpa ampullacea_,
15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but
no operculum, the rest no trace of either. Monstrosities of _Buccinum
undatum_ sometimes occur, which have two, or in rare case three
opercula.

As a rule, the operculum exactly fits the mouth of the shell. But
in cases where the mouth is very large (_e.g._ _Conus_, _Strombus_,
_Concholepas_, some _Bullia_), it only covers a very small portion and
is quite inadequate as a protection (Fig. 62, p. 155). Again, when the
shell has assumed a more or less limpet-shaped form, and habitually
adheres to flat surfaces without much occasion for locomotion, the
operculum becomes degraded and is probably on the way to being lost
altogether. This is the case with _Navicella_ (a modified _Nerita_, see
Fig. 13, p. 17), _Concholepas_ (a modified _Purpura_), _Sigaretus_ (a
modified _Natica_). Probably the more completely patelliform shells of
_Crepidula_, _Haliotis_, _Fissurella_, and _Patella_ have reached the
stage at which the operculum has been lost entirely. In _Navicella_,
besides becoming degraded, the operculum has actually become partly
internal, and apparently serves the purpose of separating the viscera
from the upper part of the foot, something like the shelly plate in
_Crepidula_. This explains why the operculum in this genus is polished
on both sides.[349]

Some authors have imagined that the operculum is homologous (_a_)
to the second valve in Pelecypoda, (_b_) to the byssus. It differs,
however, morphologically from the former in the essential point of not
being produced by the mantle, and from the latter in not being produced
by a special gland.

  [Illustration:

      Turbo         Turbo    Livona    Ampullaria    Natica
   Sarmaticus)   (Callopoma)

  FIG. 182.--Various forms of opercula.]

As regards shape and formation, the operculum has usually a more or
less well-marked nucleus which may be central (_e.g._ _Livona_),
sub-central (_Ampullaria_), lateral (_Purpura_), or terminal
(_Pyrula_). As a rule, both the inner and outer surfaces are fairly
flat, but in _Torinia_, _Cyathopoma_, and _Pterocyclus_ the outer
surface is elevated and conically spiral, in some _Turbo_ (_e.g._
_Sarmaticus_) it is covered with raised tubercles resembling coral,
while in others (_e.g._ _Callopoma_) it is scored with a deep trench.
_Aulopoma_, a land genus peculiar to Ceylon, has a paucispiral
operculum with hollow whorls, deceptively like a _Planorbis_; it fits
over the aperture instead of into it. In _Livona_ and most Trochidae
the operculum is cartilaginous and multispiral. In _Strombus_ it is
narrow, curved, and often serrated like a leaf on one of the edges; in
_Conus_ it is narrowly oblong and rather featureless; in _Littorina_,
paucispiral and always cartilaginous. In many cases (_e.g._
_Paludina_) there is no true spiral form, but the striae are concentric
to a nearly central nucleus, and thus give the appearance of a spiral.
The evolution of the operculum in _Navicella_ from _Nerita_ has already
been illustrated (p. 10). _Neritopsis_ has a very remarkable operculum,
the striated appendage of which locks behind the columella of the
shell, like the tooth in the opercula of the Neritidae.

  [Illustration:

    Pyrula    Purpura    Littorina    Aulopoma    Torinia
                                       × 3/2        × 3

    Neritopsis    Strombus    Conus
                                × 2

  FIG. 183.--Various forms of opercula.]

=Terms employed to denote various parts of the Bivalve Shell.=--The
_umbo_, or _beak_, is the apex of the hollow cone, of which each valve
may be regarded as consisting. This apex is usually more or less
twisted: it is markedly spiral in _Isocardia_, _Diceras_, some _Chama_,
and especially _Requienia_, while in _Pecten_, _Lepton_, and others the
spiral is altogether absent. As a rule the umbones point _forward_,
_i.e._ towards the anterior end of the shell. In _Donax_, _Nucula_, and
_Trigonia_, however, they point backward. The umbones are generally
more or less approximated, but in _Arca_ they are widely separated.

An _equilateral_ shell is one in which the umbones are more or less
central with regard to its anterior and posterior portion, while in
an _inequilateral_ shell the umbones are much nearer one end than the
other. On the other hand, _equivalve_ and _inequivalve_ are terms used
to express the relation of the two valves to one another as a whole.
Thus nearly all bivalve shells are more or less inequilateral, but a
comparatively small proportion are inequivalve.

The _dorsal margin_ is adjacent to, the _ventral margin_ opposite to,
the umbones. The _anterior_ and _posterior margins_ are respectively
the front and hinder edges of the shell.

The muscles which serve to close the valves leave _impressions_ on the
inner surface of each valve. These, when both muscles are present, are
known as the _anterior_ and _posterior adductor impressions_. The
impression produced by the muscular edge of the mantle, which curves
downwards and backwards from the anterior adductor impression, is known
as the _pallial line_. In shells with only one muscle it is represented
by an irregular row of small marks, or disappears altogether
(_Ostrea_). The _pallial sinus_ is produced by the muscles which
retract the siphons, and is most marked in those genera in which the
muscles are powerful and the siphons large (_e.g._ _Tellina_, _Mya_).
It is entirely absent in genera possessing no retractile siphons.

  [Illustration: FIG. 184.--Left valve of _Venus gnidia_ L.: =A=,
  anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin, =AB=,
  length, =CD=, breadth of shell.

  _a.m_, anterior; _p.m_, posterior adductor muscle; _p_, pallial
  line; _p.s_, pallial sinus; _l_, ligament; _lu_, lunule; _u_,
  umbo; _c_, cardinal teeth; _a.l_, anterior lateral tooth; _p.l_,
  posterior lateral tooth.]

  [Illustration: FIG. 185.--Right valve of _Lucina tigerina_ L.:
  =A=, anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin;
  =AB=, length, =CD=, breadth of shell.

  _a.m_, anterior; _p.m_, posterior adductor muscles; _p_, pallial
  line; _l_, ligament; _u_, umbo; _c_, cardinal teeth; _a.l_,
  _p.l_, anterior and posterior lateral tooth.]

_Right and Left Valve._--The simplest way of distinguishing the
valves as right and left is to hold the shell in such a way that the
siphons point towards the observer, and the mouth away from him; in
this position the valve to the right is called the _right valve_, and
the valve to the left the _left valve_. If, however, the animal is
not present, it may be remembered that the ligament is nearly always
_behind_ the beaks, and that the beaks, as a rule, point _forward_,
thus the right and left valves can generally be named by observation
of the beaks and ligament. When the ligament is median to the valves
(_e.g._ _Ostrea_, _Pecten_), and the beaks are not curved, the
valves may be recognised by noting the fact that the impression of
the adductor muscle (in these cases always single) is nearer to the
posterior than to the anterior side. In a similar way the pallial
impression, which only forms a sinus on the posterior side, furnishes a
guide to the valves of _Donax_, in which the beaks point backward, and
of _Tellina_, in which the beaks are frequently central.

In the fixed inequivalves (_e.g._ _Chama_) it is sometimes the right,
sometimes the left valve which is undermost, but the fixed valve,
whether right or left, is always deep, and the free valve flat.
_Ostrea_ and _Anomia_ are always fixed by the _left_ valve.

The _lunule_ is a well-marked area _in front of_ and close to the
umbones, usually more or less heart-shaped, and limited by a ridge.
Generally it is shallow, but sometimes, as in _Dosinia_, _Opis_, and
some _Cardium_, deeply impressed. A corresponding area _behind_ the
umbones, enclosing the ligament, is called the _escutcheon_ (Fig. 186),
but it seldom occurs.

The _ligament_ is a more or less elastic band, which unites the two
valves along a line adjacent to the umbones. As a rule, the greater
part of the ligament is external to the shell, but it may be entirely
internal. It is placed, normally, _behind_ the umbones, but in a few
cases, when the hinge line is very long (_Arca_, _Pectunculus_),
it extends in front of them as well. The edges of the valves, when
the ligament is mainly external, are more or less excavated for its
reception. When internal it is generally contained in a groove or
spoon-shaped pit, known as the _fossette_ (compare Fig. 187).

  [Illustration: FIG. 186.--_Venus subrostrata_ Lam.: _es_,
  escutcheon; _li_, ligament; _lu_, lunule; _u_, _u_, umbones.]

The ligament consists of two distinct parts, which may occur together
or separately, the external, or _ligament proper_, and the internal, or
_cartilage_. Only the external portion can be seen when the valves are
closed. As a rule, the two portions are intimately connected with one
another, the ligament folding over the cartilage, but in some cases,
_e.g._ _Mya_, _Mactra_, where the cartilage is lodged within the hinge,
they are completely disconnected (Fig. 187).

In _Pecten_ the external ligament is very thin, and runs along the
dorsal margin, while the internal ligament is large, solid, and
situated in a shallow pit. In _Perna_, where the hinge is toothless,
the ligament is folded into a number of transverse ridges, which fit
into corresponding grooves in the shell.

The ligament proper is _inelastic_ and insoluble in caustic potash.
The cartilage is very elastic, composed of parallel fibres, slightly
iridescent, and soluble in caustic potash.

The operation of the ligament--using the word as including the whole
ligamental process--is in opposition to that of the adductor muscles.
When the latter close the valves, they compress the ligament, an action
which its elasticity resists: thus its operation tends in part towards
keeping the valves open. But when ligament and cartilage are both fully
developed, they work in opposition to one another, the ligament, by its
resistance to compression, preventing any straining of the adductor
muscles when the valves are open, and the cartilage, for the same
reason, preventing the ventral margins of the shell from closing too
rapidly upon one another when the valves are being shut.

  [Illustration: FIG. 187.--Hinge of =A=, right valve, and =B=,
  left valve of _Mulinia edulis_ King; _ca_, cardinals; _l.a_,
  anterior laterals; _l.p_, posterior laterals; _f_, fossette; _c_,
  cartilage; _l_, ligament.]

_The Hinge._--The valves of _Pelecypoda_ are generally articulated,
below the umbones, by a _hinge_ which is furnished, in the majority of
cases, with interlocking teeth, small pits or depressions in each valve
corresponding to the teeth in the other. The teeth are distinguished
as _cardinal_, or those immediately below the umbo, and _lateral_,
or those to either side of the cardinals, the latter being also
distinguished as _anterior_ and _posterior laterals_, according as they
are before or behind the umbo (Fig. 184). In shells which are tolerably
equilateral there is no difficulty in distinguishing between cardinal
and lateral teeth. But when they are very inequilateral, the whole
hinge may share in the inequality of growth, and an anterior lateral
may be thrown backward and simulate a cardinal, or a cardinal may be
thrown backward and simulate a posterior lateral (_e.g._ _Cardita_,
_Unio_, Fig. 188). In many _Chama_ the cardinals are pushed up into
the umbo and become a mere ridge, while the strong anterior lateral
becomes nearly central and simulates a cardinal.

  [Illustration: FIG. 188.--Hinges of =A=, _Cardita semiorbiculata_
  Brug., and =B=, _Unio rectus_ Lam., showing how, in inequilateral
  shells, the lateral teeth tend to shift their position. _a.m_,
  anterior adductor, _p.m_, posterior adductor muscle; _c_, _c_,
  cardinal teeth; _p.l_, posterior lateral teeth; _l_, ligament.]

Some bivalves, _e.g._ _Anodonta_, _Ostrea_, _Pedum_, many _Mytilus_,
have no hinge teeth at all, in others the laterals are wanting
(_Psammobia_, _Diplodonta_). In the Arcadae the hinge consists of a
number of very similar denticles, which are often serrated like the
teeth of a comb (Fig. 189).

  [Illustration: FIG. 189.--The hinge in _Arcadae_: =A=, _Nacula
  Loringi_ Ang. × 4/3; =B=, _Arca granosa_ L.; _u.a_, umbonal area.]

  [Illustration: FIG. 190.--=A=, _Tridacna scapha_ Lam.; =B=,
  _Cardium enode_ Sowb., showing the interlocking of the ventral
  margins.]

Hinge-teeth are probably, in origin, derived from the crenulations or
ribbings of the surface of the shell, the upper ends of which impinge
upon the dorsal margin and mark it in a way which is quite recognisable
when the shell is thin. Similar crenulations, resulting in interlocking
of the valves, are not uncommon upon the ventral margin in certain
genera (Fig. 190). The mechanical effect of these continued riblets,
when fitted together on the opposing valves, would be to prevent the
valves sliding upon one another while closing, or after being closed.
Thus there would be a probability of their surviving, even after the
ribbing had disappeared from the surface of the shell, the increased
strength given by the hinge compensating for, and making it possible to
do without, the extra strength supplied by the ribs. It is therefore
possible that the teeth of the Nuculidae and Arcadae, which have no
distinction between cardinals and laterals, represent a very ancient
type, from which have been evolved the various forms of hinge in which
cardinals and laterals are distinguished. Even in some forms of Arcadae
(comp. _Pectunculus_) we get a hint how the transverse teeth of the
typical _Arca_ may have become transformed into the longitudinal tooth
of the normal lateral.[350]

The developed hinge-teeth, then, ensure the opening of the valves in
one direction; they also secure their accurate closure upon one another
in exactly the same plane. Exposed shells and gaping siphons matter
little to animals which are protected by their burrowing propensities,
but to those which live in material which can be easily penetrated
by their foes, it must be of advantage to be able to buckle their
armour absolutely tight. The edentulous hinge of _Anodonta_
is a degeneration from a dentate type, which retains its teeth (in
_Unio_, etc.) when subject to the jar of rapid streams, but tends
to lose them in the stiller waters of canals, lakes, and ponds.

_Other processes in the bivalve shell._--In _Anatina_ each umbo is
fissured and strengthened on the inside by a kind of umbonal plate
which carries the ligament. Some forms of _Liligna_ develop a strong
internal umbonal rib, which serves as a buttress to strengthen the
shell. In _Pholas_, the so-called falciform process serves as a point
of attachment for the muscles of the foot and viscera. There is no
ligament or hinge-teeth, the place of the latter being taken by the
anterior adductor muscle, which is attached to the hinge-plate, the
latter being reflected back into the shell.

In _Septifer_ the anterior adductor muscle is carried on a sort of
shelf or _myophore_, and in _Cucullaea_ the posterior adductor is
partly raised on a similar and very prominent formation.

_Length and breadth_ of bivalve shells is variously measured. Most
authorities measure _length_, or ‘antero-posterior diameter,’ by a
straight line drawn from the extreme anterior to the extreme posterior
margin, and _breadth_ by a similar line, drawn from the umbones to a
point, not very clearly marked, on the opposite ventral margin (see
Figs. 184 and 185). Others, less correctly, reverse these terms.
_Thickness_ is measured by the extreme distance of the opposite faces
of the closed valves. As a rule, the length exceeds, and often greatly
exceeds, the thickness, but in a few cases--_e.g._ the _Cardissa_
section of _Cardium_--this is reversed.

_The periostracum._--Nearly all shells are covered, at some period of
their growth, by a _periostracum_,[351] or surface skin, which serves
the purpose of protecting the shell against the destructive effects of
the chemical action set up by water or air. It also, in some cases (see
p. 258), acts as a kind of base upon which the shell is deposited. In
old shells it is commonly worn away, especially at those parts which
are likely to become abraded.

The form and composition of the periostracum varies greatly. Sometimes
(_e.g._ _Oliva_) it is a mere transparent film, at others (_Zonites_)
it is transparent, but stout and solid. It is corneous in _Solenomya_,
covered with fine hairs in many _Helicidae_, in _Conus_, _Velutina_,
and _Cantharus_ it is thick, fibrous, and persistent; in _Trichotropis_
and some _Triton_ it is furnished with long bristles on a thick ground
(Fig. 191). In fresh-water shells it is usually rather thick, in order
to protect the shell from the erosive powers of certain kinds of water.
In some cases (_Mya_, _Anatina_) the periostracum is continued over the
siphons, so as to form a protection throughout their whole length.

   [Illustration: FIG. 191.--_Triton olearium_ L., Mediterranean,
  an example of a shell with a stout and hairy periostracum. × ½.]

_Erosion._--The fresh-water Mollusca generally, and marine mollusca in
a few rare cases (_Purpura_, _Littorina_) are subject to _erosion_,
or decay in the shell substance. In univalves erosion usually sets
in near the apex (Fig. 192), where the life of the shell may be
regarded as weakest, and in bivalves near the umbones. It is commonest
in old shells, and rarely occurs in the very young. So long as the
periostracum is present to protect the shell, erosion cannot set in,
but when once it has been removed the shell is liable to the chemical
changes set up in its substance by water. There is abundant evidence
to show that erosion is caused by pollution of water. Out of many
instances one must suffice. In a certain stream near Boston, U.S.,
numbers of Mollusca occurred, the shells of which were very perfect and
free from disease. Some little way down stream an alkaline manufactory
drained its refuse into the water. At and below this point for some
distance every shell was more or less eroded, most of them seriously.
Farther down, when the alkali refuse became diluted away, the shells
retained their normal condition.[352]

  [Illustration: FIG. 192.--Example of an eroded fresh-water shell
  (_Melania confusa_ Dohrn, Ceylon).]

A small percentage of lime in the water appears to produce erosion. The
result of some experiments by G. W. Shrubsole, in the investigation of
this point, may be tabulated as follows:[353]--

         Water from             Lime present        Result
                                per gall.

    River Dee, near Chester     3·00 grs.     acted strongly on shells
    Wrexham                     4·00 grs.       „      „         „
    River Dee, near Llanderyel  0·53 grs.       „      „         „
    Trent Canal                 8·33 grs.     no action          „




                               CHAPTER X

    GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA--THE
            PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS


The Mollusca afford specially valuable evidence on problems of
geographical distribution. This fact is largely due to their extreme
susceptibility to any change in the conditions of life. Genera which
are accustomed to live in a certain temperature and on certain food,
cannot sustain life if the temperature falls or rises beyond certain
limits, or if the required food be not forthcoming. There is therefore
a marked contrast between the Mollusca of the tropics and of the
temperate zones, while different regions in the same latitude, whether
within or without the tropics, often show great diversity in their
fauna. Every region is thus _characterised_ by its Mollusca. The
Mollusca, for instance, of Australia or of South Africa characterise
those countries quite as much as do the kangaroo and the emu, the
hartebeest and the ostrich; there is nothing like them anywhere else in
the world. In the Greater Antilles the Mollusca stand out beyond all
other forms of life as characteristic of the islands as a whole, and of
each separate island in particular.

The geographical distribution of the land and fresh-water Mollusca must
be considered quite apart from that of the marine Mollusca. The sea
offers no such serious barriers to the spread of the latter as the land
does to the spread of the former. If we were to journey to the Azores,
and turn our attention to the land-snails, we should find them almost
wholly peculiar, while amongst the sea-shells we should recognise many
as occurring also on our southern coasts, and few that were different
from those of the Mediterranean. The marine Mollusca of the Sandwich
Islands, in spite of the enormous intervening distance, are not very
different from those of Natal, but the land Mollusca of the two
countries are as widely different as is possible to imagine.

Land Mollusca are, as has been remarked, fettered to the soil.
Quadrupeds, birds, fishes, and reptiles are provided with organs of
motion which enable them to overpass barriers of various kinds. Even
plants, although themselves incapable of motion, may be conveyed in
every direction by means of seeds, which are either wafted by the wind
or adhere to the skin of animals. But the Mollusca have no such regular
means of transport, and are, in a large number of instances, limited to
districts of a certain character of soil, or producing certain kinds of
vegetation.

The localisation, both of genera and species, occurs all over the
world. The genus _Achatinella_, which is peculiar to the Sandwich
Islands, is found there in a profusion of species. It lives in the
mountain valleys which radiate from the central ridge of each island,
and each valley is characterised by its own peculiar set of species.
The great carnivorous _Glandina_ is restricted to Central America and
the adjacent parts of the two continents, with one or two species in
Southern Europe. _Bulimus_ proper is restricted to South America;
_Achatina_ to Africa south of the Sahara; _Tornatellina_ to the
Pacific Islands; _Cochlostyla_ to the Philippines; _Cylindrella_ and
_Bulimulus_ are peculiar to the New World; _Buliminus_, _Nanina_,
_Scarabus_, and _Cassidula_ to the Old.

Extreme cases of this restriction of habitat sometimes occur. Thus
_Limnaea involuta_ is found only in a single small mountain tarn in
Ireland; _Clausilia scalaris_ along a narrow strip of limestone in
Malta; _Strophia nana_ is confined to a few square rods on an island
that is itself a mere dot in the Caribbean Sea; the genus _Camptonyx_
occurs only in the neighbourhood of Mt. Girnar, in Gujerat; and
_Lantzia_ in moss on the top of a mountain in Bourbon.

Attempts to colonise snails in strange localities have usually resulted
in failure, especially when the attempt has involved serious changes
of environment. The common _Cochlicella acuta_ of our coasts resists
all endeavours to establish it beyond a certain distance from the
sea. Snails brought from the Riviera and placed under almost similar
conditions of climate on our own southern coasts have lived for a
while, but have very rarely taken permanent root. Mr. H. W. Kew[354]
has collected a good many of these attempts to acclimatise species,
the general success of which seems to depend almost entirely on a
restoration of the old conditions of life.

At the same time there are certain species which exhibit a curiously
opposite tendency, and which seem capable of flourishing in almost
any part of the world, and under the most varied surroundings. Our
own common garden snail (_Helix aspersa_) is a striking instance of
this adaptability to new conditions. It has been established, by art
or by accident, in Nova Scotia, Maine, South Carolina, New Orleans,
California, Mexico city, Cuba, Hayti, Cayenne, Brazil, Valparaiso,
Cape Town, the Azores, St. Helena, Mauritius, Loyalty Islands, and
Australia. The great _Achatina fulica_ of East Africa has been
established first in Mauritius, and from thence has been carried to
the Seychelles and Calcutta. _Helix lactea_, a common Mediterranean
species, has been carried to Teneriffe and Montevideo; _Helix
similaris_, whose fatherland is Eastern Asia, has been transported to
Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia;
_Ennea bicolor_ (Eastern Asia) to India, Bourbon, Mauritius, West
Indies; _Stenogyra decollata_ (Mediterranean basin) to South Carolina;
_S. Goodallii_ (West Indies) to British pineries; _Helix Hortensis_ to
New Jersey. Seven common English species (_Limax gagates_, _Hyalinia
cellaria_, _H. alliaria_, _Helix aspersa_, _H. pulchella_, _Pupa
umbilicata_) have become naturalised in St. Helena,[355] and as many as
nineteen in Australia.[356]

Cases of artificial transport of this kind are readily detected; they
follow the lines of trade. The snails themselves or their ova have
been accidentally enclosed with plants or mould, or have adhered
to packing-cases, or to hay and grass used in packing. Thus they
constitute no disturbance to the general rule of the persistent
localisation of species and genera, and there is little fear that the
evidence which the geographical distribution of the Mollusca brings to
bear upon the general problems of distribution will be confused by any
intermixture of fauna naturally distinct.

_Land Mollusca: Barriers to Dispersal._--The chief natural barriers
to dispersal are extremes of temperature, the sea, mountain ranges,
and deserts. Rivers, however large, seem of little effect in checking
dispersal. There is no appreciable difference between the land Mollusca
north and south of the Ganges, or north and south of the Amazon. Living
snails, or their ova, are no doubt transported from one bank to another
on floating débris of various kinds. The barrier offered by the sea is
obvious, and at first sight appears insurmountable; but the facts with
regard to oceanic groups of islands like the Azores and Canaries (see
p. 297) show that even a stretch of salt water many hundred miles in
breadth may be ineffectual in preventing the dispersal of Mollusca.

Mountain ranges, provided they are too high to be scaled, and too
long to be turned in flank, offer a far more effective barrier than
the sea. Every thousand feet upward means a fall of so many degrees
in the mean temperature, and a change, more or less marked, in the
character of the vegetation. There is generally, too, a considerable
difference in the nature of the climate on the two sides of a great
mountain range, one side being often arid and cold, the other rainy and
warm. The combined effect of these influences is, as a rule, decisive
against the dispersal of Mollusca. Thus the Helices of California are
almost entirely peculiar; one or two intruders from states farther east
have succeeded in threading their way through the deep valleys into
the Pacific provinces, but not a single genuine Californian species
has been able to scale the heights of the Cascade Mountains. The land
Mollusca of India are numbered by hundreds; not one penetrates north
of the Himalayas. According to Mr. Nevill,[357] the change from the
Indo-Malayan to the so-called European molluscan fauna at the northern
watershed of the Kashmir valley is most abrupt and distinct; in two
days’ march northward, every species is different. Ranges of inferior
altitude, such as the Pyrenees, the Carpathians, or the Alleghanies,
may be turned in flank as well as scaled, and we find no such marked
contrast between the Mollusca on their opposite sides.

The most effective barrier of all, however, is a desert. Its scorching
heat, combined with the absence of water and of vegetable life, check
dispersal as nothing else can. The distribution of the Mollusca of the
Palaearctic Region is an excellent instance of this. Their southern
limit is the great desert which stretches, with scarcely a break,
from the west coast of Africa to the extreme east coast of Asia. The
Mediterranean offers no effectual barrier; shells of southern Europe
are found in profusion in Morocco, Tunis, and Egypt, while all through
Siberia to the extreme of Kamschatka the same types, and even the same
species, of Mollusca occur.

A detailed examination of the means, other than voluntary, by which
Mollusca are transported from one place to another hardly comes within
the scope of this work. Ocean currents, rivers, floods, cyclonic
storms of wind, birds, and even beetles and frogs, play a part, more
or less considerable, in carrying living Mollusca or their ova, either
separately or in connexion with floating débris of every kind, to a
distance from their native home. Accidental locomotion, of one or
other of these kinds, combined with the well-known tenacity of life
in many species (p. 37), may have contributed to enlarge the area of
distribution in many cases, especially in the tropics, where the forces
of nature are more vigorous than in our latitudes. The ease with which
species are accidentally spread by man increases the probability of
such cases occurring without the intervention of human agency, and
numbers of instances may be collected of their actual occurrence.[358]

A point, however, which more concerns us here is to remark on the
exceedingly wide distribution of the prevailing forms of fresh-water
Mollusca. It might have been expected that the area of distribution in
the fresh-water forms would be greatly restricted, since they cannot
migrate across the land from one piece of water to another, and since
the barriers between pond and pond, lake and lake, and one river system
and another are, as far as they are concerned, all but insuperable. We
might have expected, therefore, as Darwin and Wallace have remarked, to
find a great multiplicity of species confined to very restricted areas,
since the possibility of communication with the parent stock appears,
in any given case, to be so exceedingly remote.

As is well known, the exact reverse occurs. The range, not merely of
genera, but even of individual species, is astonishingly wide. This is
especially the case with regard to the Pulmonata and Pelecypoda. The
genera _Limnaea_, _Planorbis_, _Physa_, _Ancylus_, _Unio_, and _Cyclas_
are world-wide. Out of about ten genera of fresh-water Mollusca in
New Zealand, one of the most isolated districts known, only one is
peculiar. In South Africa and the Antilles no genus is peculiar. In the
latter case, this fact is remarkable, when we consider that the same
sub-region has at least ten peculiar genera of operculate land Mollusca
alone.

To give a few instances of the distribution of particular species:--

_Limnaea stagnalis_ L. occurs in the whole of Europe, and northern
Asia to Amoorland, Turkestan, Afghanistan, North Persia, and Kashmir;
Greenland, North America from the Atlantic to the Pacific, and
from North Canada and British Columbia as far south as Texas. The
distribution of _L. peregra_ Müll., _L. truncatula_ Müll., and _L.
palustris_ Müll, is almost equally wide.

_Planorbis albus_ occurs in the whole of Europe, and northern Asia to
Amoorland, Kamschatka, and Japan; Turkestan, the Altai-Baikal district,
Alaska and Greenland, North Canada, and the whole of eastern North
America.

The distribution of _Anodonta anatina_ L., _Cyclas cornea_ L., and
_Pisidium pusillum_ Gmel. is almost equally wide.

It is evident that the accidental means of transport mentioned above
are insufficient to account for the facts as we find them; we are
therefore compelled to seek for further explanation. Anything in the
nature of a current furnishes a ready means of transport for Mollusca
which have obtained a footing in the upper waters of a river, and there
is no difficulty in imagining the gradual spread of species, through
the agency of floods or otherwise, over a whole river system, when
once established at any point upon it. The feeble clinging power of
newly-hatched _Limnaea_ has often been noticed as contributing to the
chances of their range of distribution becoming extended. Fresh-water
Mollusca, too, or their ova, are exceedingly likely, from their
extreme abundance, to be transported by water-birds, which fly without
alighting from one piece of water to another. Again, the isolation
of one river system from another is, in many instances, by no means
well marked or permanent, and a very slight alteration of level will
frequently have the effect of diverting the supplies of one watershed
into another. When we know what enormous oscillations in level have
taken place over practically the whole surface of the globe, we can
recognise the probability that the whole river system of the earth has
been mixed up and reconstructed again and again, with a very thorough
blending of adjacent fauna.

It is possible that the very uniform conditions under which fresh-water
Mollusca live may have something to do with the uniformity of their
distribution and the comparative sameness in their development. There
can scarcely be any question that the environments of fresh-water
species are in themselves less varied and less liable to fluctuation
than those of species whose home is the land. Water is very like
water, all the world over; it may be running or motionless, warm or
cold, clear or muddy, but the general tendency is for it to be free
from extremes of any kind. Even if the surface water of a lake or
river freezes, or becomes unusually hot, there is generally plenty of
water at a lower stratum which maintains a less extreme temperature,
and to which creatures can retire on the first symptoms of a change.
From this two results will follow. Not only will the inhabitants of a
piece of water not be inclined to vary much from the type, since their
whole surroundings, food, etc., continue very much the same, but,
if transported by any accident or cataclysm elsewhere, they will be
exceedingly likely to arrive at a place which closely resembles their
former home in all essentials. Thus the tendency for new types to be
formed would be constantly checked, or rather would very seldom arise.

Mr. Belt, while recognising the importance of changes of level as
affecting the distribution of fresh-water species, appears to regard
the operations of such changes from a rather different point of view
to that described above. “I think it probable,” he writes,[359] “that
the variation of fresh-water species of animals and plants has been
constantly checked by the want of continuity of lakes and rivers in
time and space. In the great oscillation of the surface of the earth,
of which geologists find so many proofs, every fresh-water area has
again and again been destroyed.... Thus species of restricted range
were always exposed to destruction, because their habitat was temporary
and their retreat impossible, and only families of wide distribution
could be preserved.”

       *       *       *       *       *

The terrestrial surface of the globe has been divided, as indicating
the facts of geographical distribution, into six regions--the
Palaearctic, Oriental, Australasian, Ethiopian, Nearctic, and
Neotropical. To these is sometimes added a seventh, the Neantarctic,
consisting of Chili and Patagonia (and certain islands of the south
Atlantic); but since the Mollusca of Chili unmistakably form a part
of the Neotropical fauna, it seems hardly worth while to recognise a
separate region for those of the extreme south of South America, which
have no peculiar characteristics.

In certain points the exact limits of these regions, as indicated by
the Mollusca, will probably not correspond to those which are marked
out by other zoological classes. Wallace’s line, for instance, does not
exist, as far as the Mollusca are concerned.

These regions may be further subdivided into sub-regions, thus:--

    Regions          Sub-regions

                   { Septentrional
    =Palaearctic=  { Mediterranean
                   { Central Asiatic

    =Oriental=     { Indo-Malay
                   { Chinese

                   { Papuan
    =Australasian= { Australian
                   { Polynesian

                   { Central African
    =Ethiopian=    { South African
                   { Malagasy

                   { American
    =Nearctic=     { Californian

                   { Antillean
                   { Central American
    =Neotropical=  { Colombian
                   { Brazilian
                   { Chilian


                      =A. The Palaearctic Region=

The southern boundary of this region is the northern limit of the
African Sahara, the Mediterranean forming no break whatever in its
continuity. In Asia this boundary is less well marked, but roughly
corresponds to the southernmost of the vast ranges of mountains
which border the great tablelands of central Asia. Across Africa the
line of desert is well defined; but in the north-east, as the desert
approaches more nearly to the sea, the African extent of the region
is correspondingly narrowed until it becomes little more than a strip
of coast land, scarcely widening even in Lower Egypt. On the Morocco
coast, Palaearctic land forms penetrate as far south as Cape Nun.[360]
At its eastern extremity the line becomes less well defined, but
probably proceeds along the snowy mountains west of Setchouan, the
Pe-ling and Tan-sia-shan ranges, so as to include all the high ground
of Thibet and of the upper waters of the Hoang-ho, and ultimately
reaches its eastern limit at some point on the shores of the Sea of
Japan.

The region thus includes all Europe, Africa north of the Sahara, with
the Atlantic islands (the Azores, Canaries, etc.), North Arabia,
Asiatic Turkey, the greater part of Persia, Afghanistan, Thibet, all
Asiatic Russia, and a very large portion of the Chinese empire.

The principal characteristics of the region as a whole are:--

(1) The rich development of _Helix_, _Arion_, _Limax_, _Buliminus_, and
_Clausilia_.

(2) The comparative absence of land operculates (see map,
_frontispiece_).

(3) The uniform character of the fresh-water fauna.

It is in the southern portion of the region that _Helix_ (in the
sub-genera _Macularia_, _Iberus_, _Pomatia_, and _Xerophila_) and
_Buliminus_ (_Zebrina_, _Chondrula_, _Ena_) attain their maximum.
In the north, _Fruticicola_ is the characteristic group; in the
mountainous districts of the south-east, _Campylaea_, with _Clausilia_.
The Arionidae have their headquarters in the damp and warm regions
of western Europe, but are rare in the south. They only approach the
Mediterranean coast in Algeria, near Gibraltar, and in the region
between the base of the Pyrenees and the Maritime Alps, and are very
poor in species throughout Italy and Sardinia. They are absent from
almost the whole of northern Africa, the Mediterranean islands (except
Sardinia), the whole Balkan district, the Crimea, Caucasus, and western
Asia.[361]

The uniformity of the fresh-water fauna is disturbed only in the
extreme south. A few species of _Melanopsis_, with _Neritina_, occur
in southern Spain and Austria, Galicia, and southern Russia, while a
_Melania_ or two (absent from Spain) penetrate the south-eastern parts
of Europe as far as Germany. _Cyrena_ begins to replace _Cyclas_ in
southern Russia and the Caucasus.

The Palaearctic region falls into three sub-regions:--

(1) The =Northern or Septentrional Sub-region=, _i.e._ the district
north of the line formed by the Pyrenees,[362] Alps, Carpathians, and
which, passing to the northward of the Aralo-Caspian district, follows
the great central mountain range of Asia until it reaches the Sea of
Japan, perhaps somewhere in the neighbourhood of Vladivostok.

(2) The =Mediterranean Sub-region=, _i.e._ the countries bordering
on the Mediterranean, the Black and Caspian Seas, with the Atlantic
Islands.

(3) The =Central Asiatic Sub-region=, _i.e._ Turkestan, Afghanistan,
Thibet, and probably the districts of Mongolia and Manchuria.[363]

(1) The =Septentrional Sub-region= has been divided by some writers
into two provinces, the European and the Siberian. There seems, on
the whole, but little occasion to separate off northern Asia, the
characteristic of which is, as will be seen below, rather the gradual
disappearance, as we proceed eastward, of European species and genera,
than the development of any new and peculiar groups. The remarkable
fauna of Lake Baikal stands apart, not only from European, but also
from the Siberian types occurring in its immediate neighbourhood.

On the whole, the Septentrional Sub-region is poor in species
except those which inhabit fresh water. This fact is probably due
to the extreme vicissitudes of temperature which prevail, and it is
interesting to notice that the number of land Mollusca appears to touch
its lowest point in districts where the annual range of temperature is
greatest. On the other hand, in the western portions of the region,
where the climate is moist and temperature more equable, the Mollusca
are considerably more abundant and varied.

The line which separates the Septentrional from the Mediterranean
Sub-region must of necessity be very roughly drawn, and stragglers from
the south will be found to make their way northward, and _vice versâ_,
under favouring circumstances of temperature and geological formation.
Jordan has noticed[364] that species which in southern countries are
not confined to any particular quality of soil are in more northern
latitudes found only on limestone, which absorbs more heat than other
formations. Conversely, the higher elevations of the Alps, Pyrenees,
and even Carpathians are like islands in a sea, and support a
thoroughly northern fauna, quite strange to that of the plains below.
Thus _Helix harpa_ Say, a completely boreal shell, which is at home in
Canada, Sweden, Lapland, and the Amoor district, is found on the Riffel
Alp, at a height of 6000 feet.[365] _Vertigo arctica_ Wall., a species
abundant in Lapland, North Siberia, Iceland, and Greenland, occurs on
the high Alps of the Tyrol.

_Circumpolar Species._--A certain number of species are common to
the extreme north both of the Palaearctic and Nearctic regions, and
are, in fact, circumpolar. The number of these species, however, is
so small, not exceeding about 40 species (= 16 genera), that it seems
hardly worth while creating a special sub-region for their reception,
particularly as no genus is peculiar. At the same time the fact is
instructive as illustrating the close connexion of the northern
districts of the two regions, a connexion which was no doubt more
intimate in recent geological times than it is now.

The circumpolar genera are as follows. The list decisively sets forth
the superior hardiness of the fresh-water as compared with the land
genera:--

    Valvata   1 sp.
    Bithynia  1  „
    Vitrina   1  „
    Hyalinia  4  „
    Helix     2  „
    Patula    2  „
    Pupa      3  „
    Cionella  1  „
    Succinea  1  „
    Limnaea   7  „
    Planorbis 5  „
    Aplecta   1  „
    Physa     1  „
    Anodonta  1  „
    Unio      1  „
    Pisidium  1  „

_Great Britain._--There are in all about 130 species--83 land, 46
fresh-water; _Limnaea involuta_ (mountain tarn near Killarney) appears
to be the only peculiar species. There are 11 _Hyalinia_, 5 _Arion_,
and 25 _Helix_, the latter belonging principally to the sub-genera
_Xerophila_, _Tachea_, _Trichia_, and _Fruticicola_. Three _Testacella_
are probably not indigenous, but are now so well established as to
reckon in the total. Of the four _Clausilia_ two reach Ireland and
one Scotland; two do not occur north of the Forth. There are only
two land operculates, one of which (_Cyclostoma elegans_) occurs in
Ireland but not in Scotland, while the other (_Acicula lineata_)
reaches the southern counties of Scotland. Several species, _e.g._
_Helix pomatia_, _H. obvoluta_, _H. revelata_, _H. cartusiana_, _H.
pisana_, _Buliminus montanus_, are restricted to the more southern or
western counties; _Geomalacus maculosus_ is confined to a district in
south-western Ireland.

The Pleistocene beds of East Anglia contain a number of species now
extinct in these islands, whose occurrence appears to indicate a warmer
climate than the present. Such are _Helix ruderata_, _H. fruticum_,
_H. incarnata_, _Clausilia pumila_, _Unio littoralis_, _Hydrobia
marginata_, and _Corbicula fluminalis_.

_Scandinavian Peninsula._--From Norway 121 species in all are recorded,
and 148 from Sweden. The milder climate of Norway allows many species
to reach a considerably higher latitude than in Sweden, thus in Sweden
_Limax maximus_ only reaches 62°, but in Norway 66° 50´. Similarly
_Arion hortensis_ and _Balea perversa_ only reach 63° and 61°
respectively in Sweden, but in Norway are found as far north as 69° and
67° 50´. _Clausilia_ is represented by 9 species in southern Norway;
one of these is found north of the Arctic circle. There are 4 _Pupa_,
9 _Vertigo_, and 11 _Hyalinia_, but _Helix_ dwindles to 14, 9 of which
occur north of the Arctic circle. No land operculates are found;
_Cyclostoma elegans_, however, occurs in Jutland and Zealand, which
practically form a part of this district.

_Iceland._--Eleven species, all Scandinavian, occur. These are _Arion_
2, _Limax_ 1, _Helix_ 2 (_arbustorum_ L. and _hortensis_ Müll., the
latter being found only on the warmer southern coast), _Limnaea_ 1,
_Planorbis_ 1, _Pisidium_ 4.

_France._--The northern, central, and eastern districts belong to
this sub-region, while the southern and western, in which an entirely
new element occurs and many northern forms disappear, belong to the
Mediterranean. Thus, for instance, _Helix pomatia_ L., _H. incarnata_
Müll., _H. fruticum_ Müll., _H. cantiana_ Mont., _H. strigella_ Drap.,
_H. rufescens_ Penn., _H. plebeia_ Drap., are not found in southern
France. No detailed enumeration of species is at present possible,
the efforts of a large number of the leading French authorities
being directed to indiscriminate species-making rather than to the
careful comparison of allied forms. Perhaps the principal difference
between the Mollusca of northern France and those of our own islands
is the occurrence of two species of _Pomatias_. In the more elevated
districts of eastern France (the Vosges, Jura, western Alps), a certain
number of species occur which are confined to the high grounds of
south central Europe. Among these are _Helix holoserica_ Stud.,
_H. personata_ Lam., _H. bidens_ Chem., _H. depilata_ Drap., _H.
cobresiana_ Alt., _H. alpina_ Faure.

The Pleistocene deposits of the valley of the Somme tell the same
tale as those of eastern England, containing as they do species
and even genera whose northern range is now much more limited. The
Eocene fossils from the Paris beds show most remarkable relationships
to genera now existing in the West Indies and Central America.
Others again indicate affinities with India. Thus we find _Ceres_,
_Megalomastoma_, and _Tudora_ by the side of _Leptopoma_, _Faunus_, and
_Paludomus_.

_Germany._--The Mollusca of the plains of northern Germany are
few and not striking, and exhibit little difference from those of
our own islands. In the mountainous districts of the south and
south-east, a number of new forms occur, amongst which are 3 species
of _Daudebardia_, a remarkable carnivorous form, with the general
appearance of a _Vitrina_; 24 of _Clausilia_, many _Pupa_, several
_Buliminus_, 3 of the _Campylaea_ group of _Helix_, stragglers from
the Italo-Dalmatian fauna, and 1 of _Zonites_ proper. Our familiar
_Helix aspersa_ is entirely absent from Germany. There are only 4 land
operculates--_Pomatias_ 2, _Acicula_ 1, _Cyclostoma_ 1, all of which
occur exclusively in the south. _Bithynella_ and _Vitrella_, two minute
forms of fresh-water operculates akin to _Hydrobia_, occur throughout
the district.

  [Illustration: FIG. 193.--=A=, _Daudebardia brevipes_ Fér.: _sh_,
  shell; _p.o_, pulmonary orifice. (After Pfeiffer.) =B=, shell of
  _D. rufa_ Pfr., S. Germany.]

_Northern Russia and Siberia._--This vast tract extends from eastern
Germany to the Amoor district. It is exceedingly poor in Mollusca, and
is chiefly characterised by the gradual disappearance, as we proceed
eastward, of European species. There are a few characteristic Siberian
Mollusca, closely allied to European forms, and in the extreme east a
new element is introduced in the appearance of types which indicate
Chinese affinities. The whole district may be regarded as bounded to
the south by a line drawn from Lemberg to Moscow, and thence to Perm;
passing south of the Ural mountains, it includes the whole basins of
the rivers Obi, Yenesei, and Lena, coinciding with the vast mountain
ranges which terminate to the north the table-land of central Asia, at
the eastern extremity of which it dips sharply southwards, so as to
include the Amoor basin and Corea.

All the larger Helices are wanting, and no land operculates occur.
_Helix arbustorum_ L., _H. nemoralis_ Müll., _H. lapicida_ L., _H.
aculeata_ Müll., and _Hyalinia nitidula_ Drap., do not appear to occur
east of the Baltic; _Arion fuscus_ Müll., _Helix strigella_ Drap.,
_Buliminus obscurus_ Müll., _Clausilia laminata_ Mont., _C. bidentata_
Bttg., _C. plicatula_ Drap., _Viviparus fasciatus_ Müll., and _Neritina
fluviatilis_ L., do not pass the Urals.

In the Obi district (West Siberia) a further batch of European species
find their easterly limit. Among these are _Helix hispida_ L.,
_Bithynia tentaculata_ L., _Vivipara vivipara_ L., _Pisidium amnicum_
Müll., and _Unio tumidus_ Retz. A few distinctly Siberian species now
appear, _e.g._ _Ancylus sibiricus_ Gerst., _Valvata sibirica_ Midd.,
and _Vitrina rugulosa_ Koch.

The following are among the European species which reach eastern
Siberia: _Hyalinia nitida_ Müll., _Succinea oblonga_ Drap., _Planorbis
vortex_ L., _spirorbis_ L., _marginatus_ Drap., _rotundatus_ Poir.,
_fontanus_ Light., _Valvata piscinalis_ Müll., _Bithynia ventricosa_
Leach, and _Anodonta variabilis_ Drap. Here first occur such
characteristic species as _Physa sibirica_ West., _P. aenigma_ West.,
_Helix pauper_ Gld., _H. Stuxbergi_ West., _H. Nordenskiöldi_ West.,
_Planorbis borealis_ Lov., _Valvata aliena_ West., _Cyclas nitida_
Cless., and _C. levinodis_ West. In the Amoor district a decided
Chinese element makes its appearance in a few hardy forms which have
penetrated northward, _e.g._ _Philomycus bilineatus_ Bens., and a few
each of the _Fruticicola_ (Chinese) and _Acusta_ groups of _Helix_. Out
of 53 species, however, enumerated from this district, as many as 33,
belonging to 18 genera, occur also in Great Britain.

_Lake Baikal._--The Mollusca of Lake Baikal exhibit distinct
characteristics of their own, which seem to indicate the long-continued
existence of the lake in its present condition. Several entirely
peculiar genera occur, which are specialised forms of _Hydrobia_,
_e.g._ _Baikalia_, _Liobaikalia_, _Gerstfeldtia_, _Dybowskia_, and
_Maackia_; _Benedictia_ alone extends to the basin of the Amoor.
_Choanomphalus_, another peculiar and ultra-dextral (p. 250) genus
belonging to the Limnaeidae, appears to be related to the West American
_Carinifex_.

(2) The =Mediterranean Sub-region= is divided into four provinces:
(_a_) The Mediterranean province proper; (_b_) the Pontic; (_c_) the
Caucasian; and (_d_) the Atlantidean province.

(_a_) The _Mediterranean province proper_ is best considered by further
subdividing it, with Fischer and others, into separate districts, each
of which has certain peculiar characteristics.

(i) The _Hispano-Algerian_ district includes the greater part of
the Iberian peninsula, the Balearic Islands, and northern Africa
from Morocco to Tunis. The extreme western parts of these districts,
including West Morocco, Portugal, Asturias, and south-west France,
under the influence of the moist climate caused by the Atlantic, show
some peculiar features which, in the view of some, are sufficient to
justify their separation from the rest of the Hispano-Algerian portion.
Among these is a marked development of the slugs, _Testacella_,
_Arion_, and _Geomalacus_, the latter of which occurs even in
south-western Ireland.

  [Illustration: FIG. 194.--=A=, _Parmacella Valenciensii_ W. and
  B. × ⅔. (After Moquin-Tandon.) =A´=, shell of the same, natural
  size.]

_Spain._--The principal features are the development of the
_Macularia_, _Iberus_, and _Gonostoma_ groups of _Helix_, and the
occurrence of the remarkable slug _Parmacella_, which is found in
many other parts of the sub-region, and extends eastward as far as
Afghanistan. _Clausilia_ has but few species, mostly in the north.
There are four species of land operculates, one of which is referred
to a genus (_Tudora_) now living only in the West Indies, but which
occurs in the Eocene fossils of the Paris basin. In the south there are
several species of _Melanopsis_ and _Neritina_.

The _States of Northern Africa_ have a thoroughly Mediterranean fauna,
whose facies on the whole shows rather more affinity to Spain than to
Sicily. The Helices of Morocco and Algeria belong to the same groups as
those of southern Spain. Many are of a dead white colour, the better
to resist the scorching effect of the sun. _Ferussacia_ is abundant,
_Geomalacus_ and _Parmacella_ are represented by a single species
each, and there is one _Clausilia_. According to Kobelt,[366] the
original land connexion between southern Spain and Morocco must have
been much more extensive than is usually assumed, and probably reached
at least to the meridian of Oran and Cartagena. The Mollusca of Oran
and Cartagena are, according to him, much more closely related than
those of Oran and Tangier, or those of Cartagena and Gibraltar, but at
Cartagena some species, which are characteristic of the Mediterranean
coasts from Syria westward, disappear, are absent from the rest of
Spain and from Morocco, but reappear on the south-western coasts of
France. These species may possibly have pushed along that arm of the
sea which, when the Straits of Gibraltar were closed as far as the
latitude of Oran and Cartagena, united in comparatively recent times
the Bay of Biscay with the Gulf of Lions.

The following genera, which do not occur in Spain, have probably spread
into northern Africa as far as Algeria, _via_ Sicily and Tunis, namely,
_Glandina_ (1 sp.), _Daudebardia_ (1 sp.), _Pomatias_ (2 sp.). Tunis
shows strong traces of Sicilian influence, and Kobelt found a colony of
snails, of Sicilian affinities, as far west as Tetuan.

_The Sahara._--The Algerian Sahara contains, in many places, a
sub-fossil Molluscan fauna which appears to show that the district
has, in quite recent times, undergone a gradual desiccation. The
species are mainly fresh-water, including _Melania_, _Melanopsis_,
and _Corbicula_, with here and there valves of _Cardium edule_, and
indicate, on the whole, an affinity with recent Egyptian, rather than
North African species. It is probable that a vast series of _étangs_,
or brackish-water lakes, once stretched along this region, and were
ultimately connected with the sea somewhere between Tunis and Egypt.

  [Illustration: FIG. 195.--Characteristic shells of S. France:
  =A=, _Helix_ (_Macularia_) _niciensis_ Fér.; =B=, _Leucochroa
  candidissima_ Drap.]

(ii) _Southern France._--The southern portion of France bordering on
the Mediterranean contains many species, especially of _Helix_, which
do not occur in the centre and north. Amongst these are--

    Leucochroa candidissima Drap.
    Hyalinia olivetorum Gmel.
    Zonites algirus L.
    Helix rangiana Desh.
      „   serpentina Fér.
      „   niciensis Fér.
      „   splendida Drap.
      „   vermiculata Müll.
      „   melanostoma Drap.
      „   aperta Born.
      „   ciliata Ven.
      „   explanata Müll.
      „   apicina Lam.
      „   cespitum Drap.
      „   Terverii Mich.
      „   pyramidata Drap.
      „   trochoides Poir.
    Ferussacia folliculus Gron.
    Rumina decollata L.
    Pupa megacheilos C. and J.

Several species of fresh-water _Hydrobia_ (_Bithynella_) occur. The
district, on the whole, unites certain characteristics derived from
northern Italy with those of eastern Spain.

(iii) The _Italo-Dalmatian_ district includes Italy and the
neighbouring islands (Corsica, Sardinia, Sicily, Malta), and the
regions at the head and north-eastern shores of the Adriatic
(Carinthia, Carniola, Croatia, and Dalmatia), the line which separates
these latter districts from the fauna of southern Austria, Bosnia, and
Servia being very difficult to define.

  [Illustration: FIG. 196.--_Helix_ (_Pomatia_) _aperta_ L., S.
  France, showing epiphragm.]

  [Illustration: FIG. 197.--_Helix_ (_Campylaea_) _zonata_ Stud.,
  Piedmont.]

  [Illustration: FIG. 198.--_Helix_ (_Iberus_) _strigata_ Müll.,
  Florence.]

_Italy_, with the neighbouring islands, has a rich molluscan fauna.
In the sub-Alpine districts of northern Italy the prominent _Helix_
groups are _Campylaea_, _Pomatia_, and _Anchistoma_, which in the south
are generally replaced by _Iberus_, which here attains its maximum
development. Large _Hyalinia_ are abundant in the north, and _Pomatias_
and _Clausilia_ are frequent all along the Apennines. _Sicily_ has
about 250 species, half of which are peculiar. Helices of the _Iberus_
type abound, but _Campylaea_ is reduced to two species. Many peculiar
forms of _Clausilia_ occur, especially a latticed type of great beauty.
_Ferussacia_ and _Pupa_ are well represented, and there are one each
of _Glandina_ and _Daudebardia_.

_Dalmatia_ and the adjacent districts are chiefly remarkable for the
rich development of _Clausilia_, which here attains its maximum (nearly
100 species). The _Campylaea_ section of _Helix_ is represented by its
handsomest forms, many of which are studded with short hairs. Here too
is the headquarters of _Zonites_ proper, which stretches westward as
far as Provence, and eastward to Asia Minor; and also of the single
European _Glandina_, which has a similar eastward range, but spreads
westward through Italy and Sicily to Algeria, not occurring in southern
France. The land operculates are chiefly represented by _Pomatias_, and
among the fresh-water operculates are a _Melania_ and a _Lithoglyphus_,
the latter having probably spread from the basin of the Danube.

  [Illustration: FIG. 199.--=A=, _Clausilia crassicosta_ Ben.,
  Sicily; =B=, _Clausilia macarana_ Zieg., Dalmatia; =B´=,
  clausilium of same.]

(iv) The _Egypto-Syrian_ district extends along the south-eastern
shores of the Mediterranean from Tripoli to North Syria, and eastward
to the Euphrates valley. Lower Egypt alone belongs to this portion,
the fauna of Upper Egypt being of an entirely tropical character, and
belonging to the Ethiopian Region.

_Lower Egypt._--The Mollusca of Lower Egypt stand in the unique
position of belonging, half to the Palaearctic, and half to the
Ethiopian Region. The land Mollusca are of a distinctly Mediterranean
type, while the fresh-water, directly connected as they are by the
great highway of the Nile with regions much farther south, contain
a large admixture of thoroughly tropical genera (_Ampullaria_,
_Lanistes_, _Melania_, _Cleopatra_, _Corbicula_, _Cyrena_, _Iridina_,
_Spatha_, _Mutela_). The _Helices_, which are not numerous, are
rather a mixture of circum-Mediterranean species than of a specially
distinctive character. _H. desertorum_, however, belonging to the group
_Eremophila_, is characteristic. There is a single _Parmacella_, but
the physical features of the country are unfavourable to the occurrence
of such genera as _Clausilia_, _Pupa_, _Hyalinia_, and the land
operculates.

_Syria._--The Mollusca, especially in the more mountainous regions
of the north, are much more varied and numerous than those of
Egypt. _Clausilia_ is again fairly plentiful, and the Helicidae are
represented by some striking forms of the sections _Levantina_,
_Pomatia_, and _Nummulina_. _Leucochroa_ has several curious types with
a constricted aperture, and the Agnatha are represented by _Libania_, a
peculiar form of _Daudebardia_. A prominent feature is the occurrence
of a number of large white _Buliminus_ of the _Petraeus_ section (Fig.
200). Land operculates appear to be absent, but _Melanopsis_ and
_Neritina_ are abundant. The Dead Sea contains no Mollusca, but Lake
Tiberias has a rich fauna, including the above-mentioned genera, with a
_Corbicula_ and several _Unio_.

  [Illustration: FIG. 200.--=A=, _Buliminus_ (_Petraeus_)
  _labrosus_ Oliv., Beyrout; =B=, _Buliminus_ (_Chondrula_)
  _septemdentatus_ Roth., Palestine.]

_Upper Mesopotamia_ appears to possess a mixture of Syrian and
Caucasian forms, including a _Parmacella_. Lower Mesopotamia has an
exceedingly poor land fauna, but is comparatively rich in fresh-water
species, the growing eastern character of which is shown by the
occurrence of several _Corbicula_ and _Pseudodon_, and of a _Neritina_
of a distinctly Indian type.

(_b_) The _Pontic province_ extends from Western Austria to the Sea of
Azof, and includes Austria, Hungary, Roumania, the Balkan peninsula
(so far as it does not form part of the Mediterranean sub-region),
the islands of the Greek Archipelago, southern Russia and the Crimea,
and Asia Minor. It thus practically corresponds to the whole Danube
basin, together with the lands bordering on the Black Sea, except at
the extreme east, which belongs to the Caucasian sub-region. Fischer
separates off Greece, Asia Minor (except the northern coast-line),
and the intervening islands, with Crete and Cyprus, as constituting
a portion (Hellado-Anatolic) of the Mediterranean sub-region proper.
These districts, however, appear to possess scarcely sufficient
individuality to warrant their separate consideration.

A prominent characteristic of the Pontic Mollusca is the great
abundance of _Clausilia_ and _Buliminus_. In the islands east and
west of Greece _Clausilia_ forms a large proportion of the fauna,
each island, however small, possessing its own peculiar forms. The
Helices belong principally to the groups _Campylaea_ (which is very
abundant in Austro-Hungary), _Pomatia_ (Greece and Asia Minor), and
_Anchistoma_. _Macularia_ is comparatively scarce, but is represented
in Greece by one very large form (_Codringtonii_ Gray). _Zonites_
proper has its metropolis in this sub-region, and the Danube basin
contains one or two species of _Melania_ and _Lithoglyphus_.
_Buliminus_ is abundant throughout the sub-region, in the sub-genera
_Zebrina_, _Napaeus_, _Mastus_, and _Chondrula_. Several striking forms
of _Zebrina_ (_Ena_) are peculiar to the Crimea.

(_c_) _The Caucasian Province._--The limits of this province can hardly
be exactly defined at present. It appears, however, to include the
whole line of the Caucasus range, Armenia, and North Persia.

The land Mollusca are abundant and interesting. Among the carnivorous
genera are four species of _Daudebardia_, a _Glandina_, and three
peculiar forms of naked slug, _Pseudomilax_, _Trigonochlamys_, and
_Selenochlamys_. There is a single _Parmacella_, the same species as
the Mesopotamian, and a good many forms of _Limax_. _Vitrina_ and
_Hyalinia_ are well represented, and the predominant groups of _Helix_
are _Euloto_, _Cartusiana_, _Xerophila_, and _Fruticocampylaea_, the
last being peculiar. _Clausilia_ and _Pupa_ are rich in species,
together with _Buliminus_ of the _Chondrula_ type. One _Clausilia_ of
the _Phaedusa_ section, together with a _Macrochlamys_ (Transcaspian
only), a _Corbicula_, and a _Cyclotus_, show marked traces of Asiatic
affinity. There is one species each of _Acicula_ and _Cyclostoma_, and
one of _Pomatias_.

The Caspian Sea, like Lakes Baikal and Tanganyika, is distinguished
by the possession of several remarkable and peculiar genera. The sea
itself, the waters of which are brackish, is 80 feet below the level
of the Black Sea, and is no doubt a relict of what formed, in earlier
times, a very much larger expanse of water. Marine deposits containing
fauna now characteristic of the Caspian, have been found as far north
as the Samara bend of the Volga. It is probable, therefore, that
in Post-pliocene times an arm of the Aralo-Caspian Sea penetrated
northward up the present basin of the Volga to at least 54° N. The
Kazan depression of the Volga (55° N.) also contains characteristic
Caspian fossils.[367] According to Brusina,[368] the Caspian fauna, as
a whole, is closely related to the Tertiary fauna of southern Europe.

Twenty-six species of univalve Mollusca, the majority being modified
forms of _Hydrobia_, have been described from the Caspian, namely,
_Micromelania_ (6), _Caspia_ (7), _Clessinia_ (3), _Nematurella_ (3),
_Lithoglyphus_ (1), _Planorbis_ (1), _Zagrabica_ (1), _Hydrobia_ (2),
_Neritina_ (2). The bivalves are mostly modified forms of _Cardium_
(_Didacna_, _Adacna_, _Monodacna_), which also occur in estuaries along
the north of the Black Sea. A form of _Cardium edule_ itself occurs,
and numberless varieties of the same species are found in a semi-fossil
condition in the dry or half dry lake-beds, which are so abundant
throughout the Aral district.

(_d_) _The Atlantidean province_ consists of the four groups of
islands, the Madeiran group, the Canaries, the Azores, and the Cape
Verdes.

_The Madeiran group_ contains between 140 and 150 species of Mollusca
which may be regarded as indigenous, the great majority of which are
peculiar. Only 11 species are common to Madeira and to the Azores,
and about the same number, in spite of their much greater proximity,
to Madeira and the Canaries. No less than 74 species, or almost
exactly one-half, belong to _Helix_, and 9 to _Patula_. A considerable
number of the Helices are not only specifically but generically
peculiar, the genera bearing close relationship to those occurring
in the Mediterranean region. As a rule they are small in size, but
often of singular beauty of ornamentation. Various forms of _Pupa_
are exceedingly abundant (28 sp.), as is also _Ferussacia_ (12 sp.).
There are also 3 _Clausilia_ (which genus occurs on this group alone),
and 3 _Vitrina_ (a genus which occurs on all the groups). The land
operculates are represented solely by 4 _Craspedopoma_, which is common
to all the groups except the Cape Verdes.

  [Illustration: FIG. 201.--Characteristic land Mollusca from the
  Madeira group: =A=, _Helix_ (_Irus_) _laciniosa_ Lowe, Madeira;
  =B=, _Helix_ (_Hystricella_) _turricula_ Lowe, Porto Santo; =C=,
  _Helix_ (_Iberus_) _Wollastoni_ Lowe, Porto Santo; =D=, _Helix_
  (_Coronaria_) _delphinuloides_ Lowe, Madeira.]

_The Canaries_ have about 160 species, only about a dozen of which are
not peculiar. As many as 75 of these belong to _Helix_ (the sub-genera
being very much the same as in the Madeiran group), and 11 to _Patula_.
There is 1 species of _Parmacella_ (which occurs in this group
alone), and 6 of _Vitrina_, of considerable size. A remarkable slug
(_Plectrophorus_) was described from Teneriffe by Férussac many years
ago, but it has never been rediscovered, and is probably mythical, or
wrongly assigned. _Buliminus_ (_Napaeus_) has as many as 28 species,
all but one being peculiar, and _Ferussacia_ 7. _Cyclostoma_ has two
indigenous species, which, with one _Hydrocena_ and one _Craspedopoma_,
make up the operculate land fauna.

_The Azores_ are comparatively poor in Mollusca, having only 52
species, nearly two-thirds of which are peculiar. _Helix_ has 15
species, _Patula_ 4, and _Pupa_ 8. _Ferussacia_, so abundant in
Madeira and the Canaries, is entirely absent, its place being taken by
_Napaeus_ (7 sp.), which is curiously absent from Madeira, but richly
represented in the Canaries. There are 7 _Vitrina_, while the land
operculates consist of one each of _Craspedopoma_ and _Hydrocena_.
A singular slug (_Plutonia_), with an ancyliform internal shell, is
said to occur. The group was long believed to possess no fresh-water
Mollusca, but two species (one each of _Pisidium_ and _Physa_) have
recently been discovered.

_The Cape Verdes_, owing to the extreme dryness of their climate, are
poor in land Mollusca. There are 11 _Helix_, nearly all of which belong
to the group _Leptaxis_, which is common to Madeira and the Canaries.
_Ferussacia_ is absent, _Buliminus_ is represented by a single species,
and there are no land operculates. Ethiopian influence, however, as
might be expected from the situation of the group, is seen in the
occurrence of an _Ennea_, a _Melania_, and an _Isidora_.

It will be noticed how little countenance the molluscan fauna of these
island groups gives to any theory of an Atlantis, any theory which
regards the islands as the remains of a western continent now sunk
beneath the ocean. Had ‘Atlantis’ ever existed, we should have expected
to find a considerable proportion of the Mollusca common to all the
groups, and perhaps to Europe as well, and there would apparently be
no reason why a genus which occurred in one group should not occur in
all. As a fact, we find the species extremely localised throughout,
and genera occur and fail to occur in a particular group without any
obvious reason. All the evidence tends to show that the islands are
purely oceanic, and have been colonised from the western coasts of the
Mediterranean, _i.e._ from the direction of the prevailing currents and
winds.

(3) =Central-Asiatic Sub-region.=--The countries included in this vast
sub-region are Turkestan, Songaria, Afghanistan, including the Pamirs,
Western Thibet, and probably Mongolia. Kashmir belongs to the Indian
fauna. At present the whole district is very imperfectly known; indeed,
it is only at a few points that anything like a thorough investigation
of the fauna has been made. It is therefore almost premature to
pronounce any decided opinion upon the Mollusca, but all the evidence
at present to hand tends to show that they belong to the Palaearctic
and not to the Oriental system. This is especially the case with regard
to the fresh-water Mollusca, many of which are specifically identical
with those occurring in our own islands. A slight admixture of such
widely distributed types as _Corbicula_ and _Melania_ occurs, but it
is not sufficient to disturb the general European facies of the whole.
It is possible that eventually the whole district may be regarded as a
sub-region combining certain characteristics of the eastern portions of
the Mediterranean basin with an extension of the septentrional element,
due to higher elevation and more rigorous climate. The principal
features in the land Mollusca appear to be the occurrence of a number
of _Buliminus_ of the _Napaeus_ group, a few _Parmacella_ (Afghanistan
being the limit of the genus eastward), _Clausilia_, _Pupa_, _Limax_,
and _Helix_, with several stray species of _Macrochlamys_.


              =B. The Oriental or Palaeotropical Region=

This region includes all Asia to the south of the boundary of the
Palaearctic region, that is to say, India, with Ceylon, Burmah, Siam,
and the whole of the Malay Peninsula, China proper, with Hainan and
Formosa, and Japan south of Yesso. It also includes the Andamans and
Nicobars, and the whole of Malaysia, with the Philippines, as far
eastward as, and including Celebes with the Xulla Is., and the string
of islands south of the Banda Sea up to the Ké Is. The Moluccas,
in their two groups, are intermediate between the Oriental and
Australasian regions.

In this vast extent of land two distinct centres of influence are
prominent--the Indian and the Chinese. Each is of marked individuality,
but they differ in this essential point, that while the Chinese element
is decidedly restricted in area, being, in fact, more or less confined
to China itself and the adjacent islands, the Indian element, on the
other hand, extends far beyond continental Asia, and embraces all the
Malay islands to their farthest eastward extent, until it becomes
overpowered by the Papuan and Australian fauna. Upper Burmah, with
Siam, forms a sort of meeting-point of the two elements, which here
intermingle in such a way that no very definite line of demarcation can
be drawn between them.

Thus we have--

                   {                             { (_a_) Indian Province
                   { 1. _Indo-Malay Sub-Region_  { (_b_) Siamese Province
 =Oriental Region= {                             { (_c_) Malay Province
                   {                             { (_d_) Philippine Province
                   {
                   {                             { (_a_) Chinese Province
                   { 2. _Chinese Sub-Region_     { (_b_) Japanese Province

The Indo-Malay fauna spreads eastward from its metropolis, but has
practically no westward extension, or only such as may be traced on the
eastern coasts of Africa and the off-lying islands. There appears to
exist no other case in the world where the metropolis of a fauna is so
plainly indicated, or where it lies, not near the centre, but at one of
the ends of the whole area of distribution.

Comparing the two sub-regions, the Chinese is distinguished by the
great predominance of _Helix_, while in the Indo-Malay sub-region
_Nanina_ and the related genera are in the ascendancy. In India
itself there are only 6 genera of true Helicidae, poorly represented
in point of numbers; in China there are at least three times this
amount, most of them abundant in species. The Indo-Malay sub-region,
on the other hand, is the metropolis of the Naninidae, which abound
both in genera and species. In the Chinese sub-region _Clausilia_
attains a development almost rivalling that of S.E. Europe, while in
India there are scarcely a dozen species. A marked feature of the
Indo-Malay sub-region is the singular group of tubed land operculates
(_Opisthoporus_, _Pterocyclus_, etc.). In China the group is only
represented by stragglers of Indian derivation, while the land
operculate fauna, as a whole, is distinctly inferior to the Indian.
Another characteristic group of the Indo-Malay region is _Amphidromus_,
with its gaudily painted and often sinistral shell; the genus is
entirely absent from China proper and Japan, where its place is taken
by various small forms of the _Buliminus_ group. Fresh-water Mollusca,
especially the bivalves and operculates, are far more abundant in the
Chinese sub-region than in the Indo-Malay.

(1) The =Indo-Malay Sub-region=.--(_a_) _The Indian Province_ proper
includes the peninsula of Hindostan, together with Assam and Upper and
Lower Burmah. To the east and extreme north-east, the boundaries of the
province are ill-defined, and the fauna gradually assimilates with the
Siamese on the one hand and the Chinese on the other. Roughly speaking,
the line of demarcation follows the mountain ranges which separate
Burmese from Chinese territory, but the debatable ground is of wide
extent, and Yunnan, the first Chinese province over the border, has
many species common with Upper Burmah.

The gigantic ranges of mountains which bound the sub-region to the
north-west and north limit the extension of the Indian fauna in those
directions in a most decisive manner. There is no quarter of the world,
even in W. America, where a mountain chain has equal effect in barring
back a fauna. In the north of Kashmir, where the great forests end,
there is a most complete change of environment as the traveller gains
the summit of the watershed; but Kashmir itself distinctly belongs to
the Indian and not the Palaearctic system. The great desert to the
south of the Punjab is equally effective as a barrier towards the west.

The Mollusca of India proper include a very large number of interesting
and remarkable genera. India is the metropolis of the great family
of the Naninidae, or snails with a caudal mucus-pore, which are here
represented by no less than 14 genera and over 200 species. The
genera _Macrochlamys_, _Sitala_, _Kaliella_, _Ariophanta_, _Girasia_,
_Austenia_, and _Durgella_ are at their maximum. _Helix_ is scarcely
represented, containing only about 30 inconspicuous species (leaving
Ceylon out of account). _Buliminus_ is abundant, especially in
the north. The Stenogyridae are represented by _Glessula_, which
is exceedingly abundant in India, but has only a few straggling
representatives in the rest of the Oriental region. Among the Pupidae
is the remarkable form _Boysia_, with its twisted upturned mouth,
while _Lithotis_ is a peculiar form allied to _Succinea_, to which
group also probably belongs _Camptonyx_, a limpet-like form with a very
small spire, peculiar to the Kattiawar peninsula. _Camptoceras_, an
extraordinarily elongated sinistral shell, with a loosely coiled spire,
is peculiar to the N.W. Provinces.

Among the fresh-water pulmonates is an _Ampullarina_, a genus only
known elsewhere from the Fiji Is. and E. Australia. _Cremnoconchus_ is
a form of _Littorina_, peculiar to the W. Ghâts, which has habituated
itself to a terrestrial life on moist rocks many miles from the sea.
The fresh-water operculates include the peculiar forms _Mainwaringia_,
from the mouth of the Ganges (intermediate between _Melania_ and
_Paludomus_), _Stomatodon_, _Larina_, _Fossarulus_, _Tricula_, and
others. The bivalves are neither numerous nor remarkable; _Velorita_, a
genus of the Cyrenidae, is peculiar.

  [Illustration: FIG. 202.--Characteristic Indian Mollusca: =A=,
  _Hypselostoma tubiferum_ Blanf.; =B=, _Camptoceras terebra_
  Bens.; =C=, _Camptonyx Theobaldi_ Bens.]

  [Illustration: FIG. 203.--_Streptaxis Perroteti_ Pfr., Nilghiri
  Hills: =A=, adult; =A´=, young form.]

The land operculate fauna of India is singularly rich and varied. About
25 genera, and at least 190 species, occur. Here we find the metropolis
of _Cyclophorus_ among the larger forms, and of _Diplommatina_ and
_Alycaeus_ among the smaller. A large proportion of the operculate
genera are quite peculiar to the extreme south of India and Ceylon. The
appearance of a few species of the European genus _Pomatias_ is very
remarkable.

The carnivorous genera are poorly represented. A few _Ennea_ occur,
while _Streptaxis_ is practically confined to the extreme south and
north-east.


            _Land and Fresh-water Mollusca of India proper_

    Streptaxis        9
    Ennea             8
    Helicarion       15
    Girasia          14
    Austenia         11
    Ibycus            1
    Africarion        2
    Durgella          4
    Ariophanta       15
    Xesta             8
    Macrochlamys     78
    Microcystis       7
    Sitala           20
    Kaliella         35
    Hemiplecta       15
    Sesara            3
    Trochomorpha      5
    Trochomorphoides  1
    Parmacella (?)    1
    Tebennophorus     1
    Anadenus          4
    Plectopylis      11
    Plectotropis      3
    Trachia          12
    Thysanota         1
    Camaena           1
    Amphidromus       2
    Boysia            1
    Petraeus         14
    Cerastus          6
    Rachis            5
    Cylindrus         1
    Pupa             15
    Hapalus           4
    Clausilia        10
    Subulina          2
    Opeas             6
    Glessula         49
    Geostilbia        3
    Succinea         11
    Lithotis          2
    Vaginula          1
    Camptonyx         1
    Coelostele        1
    Carychium         3
    Ancylus           1
    Limnaea           7
    Camptoceras       3
    Planorbis        10
    Ampullarina       1
    Melania          17
    Mainwaringia      1
    Paludomus        10
    Stomatodon        1
    Larina            1
    Cremnoconchus     3
    Fairbankia        2
    Tricula           1
    Bithynia          9
    Fossarulus        1
    Stenothyra        3
    Vivipara          4
    Valvata           1
    Ampullaria        4
    Assiminea         9
    Acmella           2
    Pomatias          4
    Diplommatina     63
    Pupina            1
    Streptaulus       1
    Coptochilus       3
    Alycaeus         49
    Lagochilus        1
    Cyclophorus      12
    Scalaina          1
    Micraulax         2
    Jerdonia         10
    Spiraculum        4
    Otopoma           1
    Cyclotopsis       2
    Georissa          1
    Modiola           1
    Scaphula          1
    Unio             40
    Solenaia          1
    Cyrena           13
    Sphaerium         1
    Pisidium          5
    Velorita          2
    Tanysiphon        1
    Novaculina        1
    Nausitora         1

_The Cingalese district_, which almost approaches the character of a
distinct province, presents several remarkable points of dissimilarity
from the rest of India. It consists of the island of Ceylon, and of
a portion of S. India whose exact limits have yet to be defined. It
appears, however, that the Western or Malabar coast, with the hills
parallel to it, is more akin to Ceylon than the Eastern or Coromandel
coast. The Travancore, Malabar, and S. Canara districts, with the
Palnai, Anamalai, and Nilghiri Hills, are markedly Cingalese, while
there seems to be no distinct evidence of similar relationship on the
part of the Madras or even the Cuddalore district.

Among the principal features of the Cingalese district is the
occurrence of three peculiar genera of _Helix_, one (_Acavus_) large
and finely coloured, another (_Corilla_) smaller, with a singularly
toothed aperture. While the _Corilla_ group shows relations with
_Plectopylis_ and other Burmese and Siamese sub-genera _Acavus_ (Fig.
204) is totally distinct from any other Indian form, and shows signs
of close relationship, in the great size of the embryonic shell, to
the Helices of Madagascar (p. 335). In Ceylon the group is entirely
isolated, and its occurrence, besides decisively separating that
island from India, Burmah, and Siam, forms a most interesting problem
in the history of distribution. _Eurystoma_, with a single species (_E.
vittata_ Müll.), is also peculiar.

As usual when _Helix_ gains ascendancy, the Naninidae retrogress.
_Durgella_, _Austenia_, and _Girasia_ are absent altogether, while
_Macrochlamys_, _Sitala_, _Kaliella_, etc., are present in greatly
diminished numbers. The sub-genus _Beddomea_ is peculiar, a form
directly related to _Amphidromus_ (Siam and Malacca). The fresh-water
operculate _Philopotamis_ is peculiar, but for one species found in
Sumatra; while _Tanalia_ is quite peculiar. But the forms which, next
to the _Helices_, most emphasise the separation of the Cingalese
district are the land operculates. There are eleven genera or
sub-genera of land operculates which do not occur in the rest of India
proper. Two (_Aulopoma_ and _Cataulus_) are quite peculiar, while the
other nine are represented in Burmah, Siam, and the Malay islands, but
not in India. On the other hand, _Diplommatina_ and _Alycaeus_, so
profusely abundant in India, have not yet been discovered in Ceylon.
Among the slugs, _Tennentia_ is a peculiar genus, whose nearest
relation occurs in the Seychelles.

  [Illustration: FIG. 204.--_Helix_ (_Acavus_) _Waltoni_ Reeve,
  Ceylon, showing embryonic shell (_emb_). × ⅔.]


_Genera and Subgenera occurring in the Cingalese District, but not in
N. and Central India_

    Streptaxis
    Tennentia
    Acavus
    Eurystoma
    Corilla
    Beddomea
    Philopotamis
    Tanalia
    Theobaldius
    Leptopomoides
    Craspedotropis
    Pterocyclus
    Aulopoma
    Ditropis
    Cyathopoma
    Mychopoma
    Cataulus
    Nicida
    Opisthostoma

The district consisting of _Upper Burmah_, _Pegu_, _Tenasserim_, and
_Aracan_, while essentially a part of the Indian province, contains
several Siamese genera which are not found in India proper, as well
as several which are at present peculiar. Amongst the former category
are, of Helicidae, a single representative each of the genera _Camaena_
(Siamese and Chinese) and _Aegista_ (Chinese). Influence of the same
kind is seen in the increased numbers of _Plectopylis_ (14 sp.) and
_Plectotropis_ (5 sp.), of _Clausilia_ (10 sp.) and _Amphidromus_ (5
sp.), and of the large tubed operculates (11 sp. in all). _Sesara_ and
_Sophina_ among the Naninidae are strange to India, while _Hyalimax_ is
common only to the Andamans, Nicobars, and Mascarene Is. _Hypselostoma_
(Fig. 202, A) is a most remarkable genus of the Pupidae, reminding one
of _Anostoma_ of the New World. It is peculiar to the peninsula, but
for one species in the Philippines. Among the Pupinidae, we have the
peculiar _Raphaulus_ and _Hybocystis_ (Fig. 205), a very remarkable
form, of which another species occurs at Perak. Two _Helicina_ mark the
most westward extension of the genus on the mainland. In the extreme
north of Upper Burmah, Indian and Chinese forms intermingle.

  [Illustration: FIG. 205.--_Hybocystis gravida_ Bens. Young and
  adult.]

The Burmese district, together with the Indian and Siamese provinces,
is pre-eminently the home of a group of Mollusca, originally of marine
origin, which have permanently habituated themselves to a brackish
or fresh-water existence. They belong to widely different families,
and even Orders. Besides _Cremnoconchus_ mentioned above, we have,
among the bivalves, _Novaculina_, a _Solen_ living in fresh water in
the Ganges, Irawadi, and Tenasserim estuaries; _Scaphula_, an _Arca_,
one species of which occurs in the Ganges hundreds of miles above
the tide-way (see Fig. 9, p. 14); and _Martesia_, a _Pholas_ from
the Irawadi Delta. _Clea_ (which also occurs in Java and Sumatra) is
probably an estuarine _Cominella_; a _Tectura_ has earned the name
_fluminalis_ from its exclusive residence in the Irawadi R.; _Iravadia_
is probably a _Rissoina_ of similar habits, occurring from Ceylon round
to Hong-Kong; _Brotia_ is a _Cerithium_ from an affluent of the River
Salwin, and _Canidia_ is a _Nassa_, occurring in the embouchures of
rivers from India to Borneo. Nowhere else in the world is there such
a collection--not exhausted by this list--of marine forms caught in
process of habituation to a fresh-water or even a land existence.

The _Andaman_ and _Nicobar_ Islands possess no peculiar features in
their land Mollusca. They are closely related to the adjacent coasts of
Lower Burmah. _Amphidromus_ (2 sp.) occurs in the Andamans alone, and
_Clausilia_ (2 sp.) in the Nicobars alone, while _Hyalimax_ occurs in
both groups. A remarkable _Helix_ (_codonodes_ Fér.) from the Nicobars
appears to find its nearest relations in the isolated group from
Busuanga and Mindoro (p. 315). Land operculates are abundant, in the
Nicobars actually outnumbering the pulmonates (28 to 22). _Helicina_
and _Omphalotropis_, genera characteristic of small islands, are found
on both groups.

(_b_) The _Siamese Province_ includes the area occupied by the
districts known as Siam, Laos, Cambodia, Cochin China, Annam, and
Tonquin. Along the whole of its northern frontier, the zoological is
no more than a political boundary, while on the east the mountain
ranges which part Siam from Pegu and Tenasserim are not of sufficient
height to offer any effective barrier to distribution. The province is
accordingly qualified to a considerable extent by Indian and Chinese
elements.

_Streptaxis_ is, but for three _Ennea_, the sole representative of the
carnivorous genera, and attains its maximum in the Old World. Partly
owing to Chinese influence, the Helicidae, with 11 genera and 46
species, begin to regain their position as compared with the Naninidae
(12 genera, 54 species). Of the Helicidae, _Acusta_ and _Hadra_ appear
now for the first time, and, with _Plectotropis_, _Stegodera_, and
_Clausilia_, form a marked Chinese element. _Amphidromus_, with 33
species, is the most characteristic land pulmonate. Several genera,
whose nucleus of distribution lies among the islands farther east,
appear to have penetrated as far as these coasts. Such are _Chloritis_,
_Camaena_, and _Obbina_ among the Helicidae, _Trochomorpha_, and, of
the operculates, _Helicina_.

  [Illustration: FIG. 206.--_Cyclophorus siamensis_ Sowb., Siam.]

Land operculates are very richly developed. In all, there are 17 genera
and 104 species known. The tubed operculates attain their maximum,
and _Cyclophorus_ is even more abundant than in India. Fresh-water
bivalves abound. _Dipsas_ and _Pseudodon_ are common to China, and
_Unio_ and _Anodonta_ are profusely represented. A curious resemblance
to S. America appears in this group, a single _Mycetopus_ occurring,
the only species not Brazilian, while _Arconaia_ appears very closely
to approach the _Hyria_ of the same locality. Several genera of the
_Hydrobia_ type (_Pachydrobia_, _Jullienia_, _Chlorostracia_) are
peculiar.


        _Land and Fresh-water Mollusca of the Siamese Province_

    Streptaxis       20
    Ennea             3
    Helicarion        7
    Microcystis       3
    Sesara (?)        1
    Medyla            1
    Xesta             4
    Macrochlamys      6
    Kaliella          5
    Hyalinia (?)      1
    Hemiplecta       14
    Rhysota           2
    Trochomorpha      6
    Trochomorphoides  3
    Plectopylis       5
    Stegodera         2
    Plectotropis     12
    Trachia           3
    Fruticicola       2
    Acusta            2
    Chloritis         8
    Dorcasia          1
    Camaena           5
    Hadra             5
    Obbina            1
    Amphidromus      33
    Bocourtia         2
    Buliminus         4
    Hypselostoma      2
    Tonkinia          1
    Clausilia        15
    Opeas             7
    Spiraxis (?)      2
    Subulina          1
    Succinea          4
    Vaginula          7
    Limnaea           7
    Planorbis         6
    Canidia          13
    Melania          39
    Faunus            1
    Bithynia          9
    Wattebledia       1
    Stenothyra        4
    Hydrobia          1
    Pachydrobia       9
    Jullienia         6
    Lacunopsis        6
    Chlorostracia     4
    Vivipara         39
    Valvata           1
    Ampullaria       15
    Assiminea         7
    Procyclotus       6
    Dasytherium       2
    Opisthoporus      5
    Rhiostoma         7
    Myxostoma         1
    Pterocyclus       7
    Cyclophorus      28
    Leptopoma        10
    Lagochilus        6
    Pupina            8
    Hybocystis        3
    Alycaeus          6
    Cataulus (?)      1
    Diplommatina      2
    Helicina          4
    Georissa          2
    Modiola (f. w.)   2
    Dreissensia       3
    Anodonta         17
    Mycetopus         1
    Pseudodon        18
    Dipsas            4
    Unio             64
    Arconaia          1
    Cyrena            6
    Batissa           1
    Corbicula        35

(_c_) _The Malay Province_ includes the peninsula of Malacca south
of Tenasserim, and the series of islands beginning with Sumatra and
stretching eastward up to the Ké Is., besides Borneo and Celebes. The
Philippines form a separate province.

The Malay province is singularly poor in representative forms, whether
we regard it as a whole or consider the islands separately. Not a
single genus, with the exception of _Rhodina_ (Malacca), appears to
be peculiar. The contrast with the West Indies is in this respect
very striking. Java, for instance, which is well explored, and almost
exactly eleven times the size of Jamaica, has about 100 species of land
Mollusca, while Jamaica has about 460.

This want of individuality in the land Mollusca of the Malay islands is
accounted for by a consideration of the sea depths which separate them
from the Asiatic mainland. The accompanying map, the red line on which
is intended to show what would be the result of an elevation of the sea
bottom for no greater amount than 100 fathoms, exhibits clearly the
fact that these islands are practically a part of Asia, a large stretch
of very shallow sea extending between Siam and the greater part of the
north-west coast of Borneo.

In all probability the three great islands of Sumatra, Java, and
Borneo were united with the mainland of Asia, and with one another, at
a period, geologically speaking, comparatively recent. This follows
from the general uniformity of their land Mollusca, both as regards
one another and as regards the mainland. Nor do the smaller members
of the island series--Bali, Lombok, Sumbawa, Flores, Timor, and Timor
Laut--show any marked individuality in the possession of peculiar
genera. Wallace’s line is absolutely non-existent, so far as the land
Mollusca are concerned. The really noticeable break in distribution
comes with the Aru Is., for while the Tenimber group (Timor Laut, etc.)
are decidedly Malay, and the Ké Is., in the poverty of our information,
uncertain, the Aru Is. are as Papuan as New Guinea itself. The profound
depths of the Banda Sea to the north, and the Timor Sea to the south,
appear to have kept the islands from Flores to Timor Laut free from the
intrusion of any Moluccan or any considerable Australian element. The
Moluccas, as has been already remarked, besides possessing considerable
peculiarities of their own, unite a mixture of the Malay and Papuan
elements, and serve as a sort of debatable ground for the meeting of
the two.

  [Illustration: FIG. 207.--_Ariophanta Rumphii_ v. d. B., Java.]

The Malay peninsula is practically another island of somewhat the same
shape and general trend as Sumatra, and about one-half the size. Its
general relations--and the remark applies to the great Sunda Islands as
well--appear to be rather more with Burmah, Tenasserim, and even the
Cingalese district, than with Siam. Points of connexion between Ceylon
and Sumatra, and Ceylon and Borneo, have already (p. 304) been brought
out.

  [Illustration:

    Map to illustrate the
    GEOGRAPHICAL DISTRIBUTION
    of the Land Mollusca of the
    EAST INDIAN ARCHIPELAGO

  _The red line marks the 100 fathom line_

    London: Macmillan & Co.  _London Stanford’s Geog^l. Estab^t_.]

It seems not impossible, from the point of view of the land Mollusca
only, that the Sunda Islands may at one time have stretched much
farther into the Bay of Bengal, prolonged, perhaps, into what are now
the Andaman and Nicobar groups, while Ceylon and the western side of
the Deccan, united into one continuous piece of land, and possibly
separated from N. India by a wide stretch of sea, extended farther
eastward in a long island, or series of islands.

Java, from its Mollusca, does not appear to hold the comparatively
isolated position which its mammals and birds seem to indicate. Borneo,
on the other hand, is more Siamese than Java or Sumatra in respect of
a group whose metropolis is Siam, namely, the tubed operculates; for
while that section is represented by 3 species in Sumatra and only 2
in Java, in Borneo it has as many as 19, _Rhiostoma_ not occurring in
the two former islands at all. _Alycaeus_, _Lagochilus_, _Pupina_,
and _Cyclophorus_ are found throughout, but _Hybocystis_ (Malacca, 1
sp.) does not quit the mainland. Borneo is remarkably rich in land
operculates, especially noticeable being the occurrence (11 sp.) of
_Opisthostoma_ (Fig. 208), a most extraordinary form of land shell
(Ceylon, Siam), of _Diplommatina_ (17 sp.), and _Raphaulus_. The
occurrence of a single _Papuina_ (Moluccas eastward) is very remarkable.

  [Illustration: FIG. 208.--=A=, _Opisthostoma Cookei_ E. A. Smith,
  Borneo; =B=, _Opisthostoma grandispinosum_ G.-A., Borneo. Both ×
  8.]

_Amphidromus_ is a genus characteristic of the great Sunda Islands,
attaining its maximum in Java (12 sp.). The Indian _Glessula_ still
has one species each in Sumatra, Java, and Borneo. One species of
_Streptaxis_[369] occurs in Malacca, but _Ennea_ (3 sp.) reaches as
far east as Borneo and the Philippines. _Parmarion_, _Helicarion_,
_Ariophanta_, and other groups of the Naninidae are well represented.
_Hemiplecta_ and _Xesta_ are abundant and large, while the _Rhysota_
of Borneo contain some huge sinistral forms. _Rhodina_ is a remarkable
form from Malacca, whose exact generic position is not yet settled.
_Clausilia_ has a few species on all the islands, the last occurring
on Ternate, and a single _Papuina_ (Moluccas and N. Guinea) occurs in
Borneo.

The _Island of Celebes_ marks the beginning of a distinct decrease in
the Indo-Malay element. The Naninidae lose ground, in proportion to the
Helicidae, _Macrochlamys_, for instance, being represented by only one
species, and _Hemiplecta_ by four. Other characteristic genera of the
Indian region dwindle, such as _Amphidromus_, _Clausilia_, the tubed
operculates, and _Cyclophorus_, while _Sitala_, _Kaliella_, _Glessula_,
and _Plectotropis_ disappear altogether. Comparing the total numbers
of Naninidae and Helicidae from Sumatra to New Guinea, we obtain this
interesting result:--

                        Sumatra  Java  Borneo  Celebes  Moluccas  N. Guinea
 _Nanina_ (all genera)     26     32     51       22       36       40
 _Helix_ (all genera)       7     11     13       14       55       91

It will be noticed that the proportion of Naninidae to Helicidae,
which has been nearly 4 to 1 in Sumatra, falls to 3 to 1 in Java, and
rises again to 4 to 1 in Borneo (showing the essentially continental
character of the island); in Celebes it further falls to 3 to 2, while
in the Moluccas the scale turns and _Helix_ has the advantage by about
8 to 5, and in N. Guinea by more than 2 to 1.

  [Illustration: FIG. 209.--_Amphidromus perversus_ L., Java.]

There is the same absence of marked features of individuality in
Celebes as in the islands dealt with above. Not a single genus is
peculiar. The nature of the sea bottom between Borneo and Celebes,
with its indications of a somewhat broad bridge over an otherwise deep
channel of separation, would seem to account for and suggest the true
explanation of the facts as they stand. At the same time, there are
indications of a certain amount of contrast between N. and S. Celebes.
The Indian element, which constitutes the preponderating majority of
the fauna, is common to north and south alike. But the north part
of the island, in which _Obba_ and _Obbina_ occur, shows decided
relationship with the Philippines, while the occurrence of three
_Chloritis_ and one _Planispira_ tend to approximate S. Celebes rather
with the Moluccas.

The _islands eastward of Java_, from Bali to Timor Laut and the
Tenimber Is., present no trace of individual peculiarities; they simply
carry on the Indo-Malay fauna as though along a great peninsula. Even
Timor, surrounded as it is on all sides by sea of profound depth, shows
no sign of possessing even one peculiar genus. _Amphidromus_, perhaps
the most characteristic of all Indo-Malay genera, occurs throughout,
diminishing in numbers as we go eastward (Bali, Lombok, and Sumbawa 4
sp., Timor 2 sp., Timor Laut 1 sp.), while _Plectotropis_ reaches no
farther than Flores and Timor. The tubed operculates are altogether
wanting. In Timor Laut we have Moluccan influence appearing in 3
_Chloritis_, and there is one (supposed) _Corasia_. Two _Helices_ of a
marked Australian type (_Rhagada_) occur, one in Flores, the other on
Dama I., south-west of Timor. The configuration of the sea bottom (see
map) would lead us to believe that the north-west coast of Australia
once stretched a good deal nearer to these islands.

The _Moluccas_, taken as a whole, constitute a transition region
between the Indo-Malay and the Papuan faunas, uniting, to a very
considerable extent, the features of both. They fall into two
well-defined groups. The northern, or Ternate group, consists of Gilolo
(Halmahera), Batchian, and the outlying islands as far south as and
including Obi major. The southern, or Amboyna group, consists of Buru,
Ceram, Amboyna, and the chain of islands to the south-east of Ceram, as
far as, and including the Ké Is.

The Ternate group shows decidedly closer relations with New Guinea than
the Amboyna group. Thus, among the _Helices_, the markedly Papuan genus
_Papuina_ is represented by 7 species in the Ternate group, but by 1 in
the Amboyna group. Again, the _Cristigibba_ section of _Planispira_,
which is a Papuan form, has 4 representatives in the northern group,
but only 1 in the southern. Certain points of connexion with Celebes
come out in the southern group which are wanting in the northern; thus
of _Chloritis_ there are 8 species in Amboyna, 0 in Ternate, 3 in
Celebes.

In the Moluccas the Helicidae, for the first time as we move eastward
from India, gain the ascendancy over the Naninidae, the numbers being,
_Helix_ 55, _Nanina_ 36. If we take the groups separately, we find that
in the Amboyna group the proportion is 22 to 23, while in the Ternate
group it is 33 to 13, an additional proof that the Amboyna group is far
less Papuan than the Ternate. Of _Planispira_, the most characteristic
sub-genus of _Helix_, there are 12 species in the Ternate group, and 5
in the Amboyna. The section _Phania_, which contains 4 species of the
finest _Helices_ known, is quite peculiar to the Ternate group. One
species of _Obbina_, a sub-genus markedly Philippine, occurs in each
group. Several of the Indo-Malay land operculates (_e.g._ _Ditropis_)
reach their limit here, and here too we have the last _Clausilia_
(strangely absent from the Amboyna group). _Amphidromus_ is not
reported on sufficient authority to warrant its insertion in the list.


_Land Mollusca of the Moluccas._ (T = Ternate, A = Amboyna[370] group)

    Helicarion            1 A
    Euplecta              1 A
    Xesta            6 A, 4 T
    Macrochlamys          1 A
    Lamprocystis     4 A, 2 T
    Macrocycloides        4 A
    Sitala                1 A
    Kaliella         3 A, 1 T
    Trochomorpha     3 A, 3 T
    Endodonta             1 A
    Patula                1 A
    Plectotropis          1 T
    Eulota                1 A
    Chloritis             8 A
    Planispira      5 A, 12 T
    Cristigibba      1 A, 4 T
    Obbina           1 A, 1 T
    Phania                4 T
    Albersia              3 T
    Camaena               1 T
    Papuina          1 A, 7 T
    Pupa                  3 A
    Vertigo               2 A
    Clausilia             1 T
    Opeas            4 A, 4 T
    Subulina              1 A
    Tornatellina          1 A
    Vaginula              1 A
    Melania         18 A, 4 T
    Faunus                1 A
    Vivipara              1 A
    Acmella               1 A
    Diplommatina     4 A, 2 T
    Registoma             1 T
    Pupinella             1 A
    Callia                2 A
    Leptopoma        4 A, 5 T
    Lagochilus       1 A, 1 T
    Ditropis              3 A
    Cyclotus         4 A, 6 T
    Omphalotropis         3 A
    Georissa              1 T
    Helicina         6 A, 3 T

(_d_) _The Philippine Province._--In the extraordinarily rich
development of their Mollusca, the Philippines form a remarkable
contrast with the poverty of the adjacent Malay islands. No less
than 727 species of land Mollusca alone are known from the group,
amongst which are included some of the finest and handsomest forms
yet discovered. The main features of the fauna are Indo-Malay, with
the addition of a certain Australasian element, and a remarkable
development of individual characteristics.

The principal indigenous feature is the profuse abundance of the
genus _Cochlostyla_, a group of large and elegant land shells, partly
helicoid, partly bulimoid in shape, many of the species of which are
covered with a curious hydrophanous epidermis. They are in the main of
arboreal habits, living in the tops of the enormous forests which cover
the greater part of the islands. As many as 247 species, belonging to
15 sub-genera, have been described.

  [Illustration: FIG. 210.--_Cochlostyla_ (_Chrysalis_)
  _mindoroensis_ Brod., Mindoro, Philippines.]

  [Illustration: FIG. 211.--_Cochlostyla_ (_Orthostylus_) _Portei_
  Reeve, Luzon. × ⅔.]

The distribution of the sub-genera of _Cochlostyla_ on the different
islands of the Philippine group affords important evidence on the
geological relation of the islands to one another. Thus we find
_Orthostylus_ and _Hypselostyla_ occurring in the central islands and
S. Luzon, but not in Mindanao or Mindoro; we find _Chrysalis_ peculiar
to Mindoro, _Prochilus_ to Mindoro and the Cuyos, _Ptychostyla_ to
Luban, all these being sub-genera of very marked characteristics.
Six out of the fifteen sub-genera are entirely absent from Mindanao,
although occurring on the islands in the immediate vicinity. The
little group Tablas-Romblon-Sibuyan are entirely deficient in certain
sub-genera which occur on the islands surrounding them on all
sides.[371]

Other forms peculiar to the Philippines are _Diaphora_, a section of
_Ennea_ with a curiously produced mouth, and several sub-genera of the
Naninidae (_Vitriniconus_, _Vitrinoidea_, _Hemitrichia_). The great
_Rhysota_ here find their metropolis. Another very marked group of
_Helix_ is _Obbina_, 19 of the 25 known species being peculiar.

The Helicidae proper of the Philippines are still held in check, as in
the greater part of the Indian region, by the Naninidae. The single
_Trachia_ and _Plectotropis_, and the 2 species each of _Plectopylis_
and _Satsuma_, indicate affinities with Indo-China. Further important
Indian relationships are seen in the great _Nanina_ and _Cyclophorus_,
which here attain almost Indian dimensions; in _Kaliella_ (8 sp.),
_Sitala_ (2), _Clausilia_ (1). Among the operculates we still have
1 _Alycaeus_ and 1 _Coptochilus_. Singularly enough, several Indian
genera which occur here are not found in the intervening islands
of Borneo, Sumatra, or Java, _e.g._ _Streptaxis_, _Hypselostoma_,
_Ditropis_, _Acmella_, and _Cyathopoma_. The curiously tubed Malay
operculates, _Opisthoporus_, etc., fail to reach the Philippines
proper, although occurring in Borneo and N. Celebes; one of them
reaches Palawan. The strikingly Malay genus _Amphidromus_ reaches
Palawan, but no farther (1 sp.), while 2 species reach Mindanao, and
one of these penetrates as far as Bohol and S. Leyte. Amongst the
slugs, _Mariaella_ occurs again only in the Seychelles, and _Tennentia_
only in Ceylon.

  [Illustration: FIG. 212.--_Helix_ (_Obbina_) _rota_ Brod.,
  Philippines.]


          _Land and Fresh-water Mollusca of the Philippines_

    Streptaxis        1
    Ennea            10
    Mariaella         3
    Tennentia         1
    Helicarion       21
    Vitrinopsis       5
    Vitrinoidea       1
    Rhysota          17
    Trochonanina      2
    Euplecta         28
    Hemiplecta       11
    Hemitrichia      15
    Xesta             2
    Macrochlamys      5
    Microcystis       3
    Lamprocystis     17
    Bensonia          4
    Vitriniconus     16
    Sitala            2
    Kaliella          8
    Trochomorpha     21
    Endodonta         1
    Plectopylis       3
    Plectotropis      1
    Aulacospira       3
    Pupisoma          1
    Satsuma           2
    Dorcasia          2
    Chloritis         7
    Obbina           19
    Papuina           1
    Phoenicobius      7
    Cochlostyla     247
    Amphidromus       2
    Hapalus (?)       4
    Hypselostoma      1
    Pupa              4
    Clausilia         1
    Subulina          3
    Prosopeas         2
    Opeas             4
    Geostilbia        1
    Tornalellina      1
    Succinea          3
    Vaginula          2
    Ancylus           1
    Limnaea           3
    Planorbis         3
    Physa             2
    Melania          50
    Pirena            2
    Bithynia          1
    Vivipara          7
    Ampullaria        5
    Acmella           2
    Diplommatina     41
    Arinia            6
    Pupina            5
    Registoma         7
    Hargreavesia      1
    Callia            2
    Pupinella         3
    Helicomorpha      4
    Coptochilus       1
    Alycaeus          1
    Leptopoma        42
    Lagochilus       11
    Cyclophorus      31
    Ditropis          7
    Cyathopoma        5
    Cyclotus         19
    Omphalotropis     3
    Helicina         18
    Georissa          3
    Anodonta          1
    Cyrena            3
    Corbicula         7

_Islands adjacent to the Philippines._--The Philippines are connected
with Borneo by two distinct ridges or banks of elevation, which enclose
between them the Soo-loo or Mindoro Sea. There can be little doubt
that these ridges represent the ancient highway of transit, by which
Indo-Malay species passed into the Philippines. The depth of the sea on
either side is profound, ranging from an average of about 1000 fathoms
west of Palawan to 2550 off the south-west coast of Mindanao.

It appears that the fauna of the Soo-loo ridge is definitely Philippine
up to and including Bongao, Sibutu, and Bilatan, the last islands
at the Bornean end of the ridge. On these are found two species of
_Cochlostyla_ and an _Obbina_.

The Palawan ridge may also be described as more or less Philippine
throughout. One species of _Cochlostyla_ occurs on Balabac, just north
of Borneo, and two on Palawan, but these are perhaps counterbalanced
by the definitely Indo-Malay _Amphidromus_ and _Opisthoporus_ (1 sp.
each). At the northern end of the ridge, on Busuanga and Calamian, the
Philippine element predominates.

Representatives of two remarkable groups of _Helix_ (_Camaena_ and
_Phoenicobius_) occur along the Palawan ridge and in Mindoro. The
_Phoenicobius_ find their nearest allies in the curious small group
known as _Obba_, from N. Celebes, the _Camaena_ possibly in a type
of _Helix_ (_Hadra_) occurring in New Guinea and N.E. Australia. The
only other _Helix_ from the whole of the E. Indies which bears any
resemblance to the _Phoenicobius_ group is _H. codonodes_ Pfr., which
is peculiar to the Nicobars. A few forms assigned to _Camaena_ also
occur in Further India and Siam. It would appear possible, therefore,
that these two isolated groups are a sort of survival of a fauna which
perhaps had once a much more extended range.

(2) The =Chinese Sub-region=.--The _Chinese Sub-region_ includes the
whole of China from its southern frontier up to and including the
basin of the Blue or Yang-tse River, together with the coast district,
including Corea, perhaps as far north as Vladivostok, and the outlying
islands of Hainan, Formosa, the Loo-Choo and Bonin groups, and Japan to
the north of Niphon. It may be divided into two provinces, the Chinese
and the Japanese.

(_a_) The fauna of the _Chinese province proper_ bears, in many
respects, strong marks of relationship to that of India and Siam. Thus
_Streptaxis_, _Helicarion_, _Macrochlamys_, _Kaliella_, _Sitala_,
_Ariophanta_, _Rhysota_, _Hemiplecta_, _Diplommatina_, _Opisthoporus_,
_Pterocyclus_, _Lagochilus_, and _Alycaeus_ all occur, especially in
Southern China. The two points in which the sub-region bears special
marks of individuality are _Helix_ and _Clausilia_. The sub-genera of
_Helix_ which have their metropolis in China are _Satsuma_, _Cathaica_,
_Aegista_, _Acusta_, _Euhadra_, _Plectotropis_, and _Plectopylis_.
Sinistral forms (compare Fig. 213) are rather prevalent. In several
cases--_e.g._ _Trichia_, _Gonostoma_, _Fruticicola_--there is a
reappearance of forms which appear to belong to well-known European
sub-genera. _Clausilia_ here attains a kind of second centre of
distribution, and is represented by its finest forms, which belong to
several peculiar sub-genera. The carnivorous Mollusca are not abundant,
and are represented by _Rathouisia_ (a peculiar genus of naked slug),
_Ennea_, and _Streptaxis_. In the western provinces _Buliminus_ is
abundant in several sub-genera, one of which appears to be the European
_Napaeus_.

  [Illustration: FIG. 213.--_Helix_ (_Camaena_) _cicatricosa_
  Müll., China.]

There is little which is striking in the operculates, which are most
abundant in the south, and appear to be mainly derived from Indian and
Siamese sources. The occurrence of _Helicina_ (3 sp.), _Omphalotropis_
(1), _Leptopoma_ (2), and _Realia_ (2), is evidence of some influence
from the far East. _Heudeia_ is a very remarkable and quite peculiar
form of _Helicina_ with internal plicae, perhaps akin to the Central
American _Ceres_.

Fresh-water genera are exceedingly abundant, especially _Melania_,
_Unio_, and _Anodonta_. The occurrence of _Mycetopus_ (a South-American
genus) is remarkable. There are several peculiar forms of fresh-water
operculates, whose exact position is hardly yet assured.


        _Land and Fresh-water Mollusca of the Chinese Province_

    Rathouisia      1
    Streptaxis      7
    Ennea          12
    Parmarion       2
    Helicarion     15
    Euplecta        3
    Macrochlamys   19
    Microcystina    2
    Microcystis     7
    Kaliella       16
    Sitala          8
    Ariophanta      1
    Rhysota         5
    Hemiplecta      1
    Trochomorpha    2
    Limax           1
    Philomycus      1
    Patula          2
    Gonostoma       4
    Metodontia      2
    Vallonia        1
    Plectotropis    9
    Fruticicola    11
    Satsuma        14
    Trichia        10
    Cathaica       22
    Aegista        10
    Armandia        3
    Acusta         15
    Obbina          1
    Camaena         5
    Euhadra        14
    Plectopylis    19
    Stegodera       6
    Chloritis       1
    Hel. Inc. sed. 39
    Buliminus      21
    Buliminopsis    3
    Buliminidius    3
    Napaeus        14
    Rachis (?)      4
    Pupa           10
    Clausilia     102
    Opeas          12
    Euspiraxis      1
    Subulina        5
    Stenogyra (?)  12
    Succinea        8
    Vaginula        7
    Limnaea         2
    Planorbis       6
    Melania        44
    Paludomus       3
    Bithynia       12
    Lithoglyphus    3
    Melantho (?)    1
    Pachydrobia     1
    Prososthenia    2
    Stenothyra      2
    Hydrobia        2
    Mecongia        1
    Oncomelania     9
    Margaracya      1
    Rivularia       4
    Delavaya        1
    Fenouillia      1
    Vivipara       34
    Diplommatina   20
    Pupina          6
    Alycaeus       23
    Leptopoma       2
    Lagochilus     10
    Cyclophorus    18
    Coelopoma       1
    Pterocyclus     3
    Opisthoporus    4
    Cyclotus       10
    Scabrina        4
    Ptychopoma     12
    Omphalotropis   1
    Realia          2
    Pseudopomatias  1
    Helicina        3
    Georissa        4
    Heudeia         1
    Cyclas          1
    Corbicula      50
    Unio           53
    Monocondylaea   1
    Anodonta       55
    Mycetopus      12
    Pseudodon       1
    Dipsas          4

The island of _Hainan_, in the extreme south of the sub-region, has 40
species of Mollusca, 22 of which are peculiar, but there is no peculiar
genus.

The Mollusca of _Formosa_, although in many cases specifically
distinct, show close generic relationship with those of China. The
characteristic Chinese groups of _Helix_ and _Clausilia_ occur, and
there is still a considerable Indian element in several species
of _Streptaxis_, _Macrochlamys_, _Kaliella_, and _Alycaeus_. The
occurrence of two _Amphidromus_, a genus which, though Siamese, is not
found in China or Hainan, is remarkable.

The peninsula of _Corea_ must undoubtedly be included in the Chinese
sub-region. It is true that the land operculates scarcely occur,
but there are still a number of _Clausilia_, and several of the
characteristic Chinese groups of _Helix_ are reproduced. In some points
Corea appears to show more affinity to Japan than to China, four of
the _Helices_ being specifically identical with those of Japan, but the
peninsula is at present too little explored for any generalisations to
be made as to its fauna in this respect.

(_b_) _Japanese Province._--Kobelt distinguishes four groups of
Mollusca inhabiting Japan (_a_) circumpolar species, actually occurring
in Europe, Siberia, or N. America, or represented by nearly allied
species (these of course do not belong to the Japanese province as
such); (_b_) Indo-tropical species; (_c_) species which are Chinese
or akin to Chinese; (_d_) peculiar species, a mixture of two forms,
southern and northern, the latter being chiefly _Hyalinia_, _Patula_,
and _Fruticicola_. Out of a total of 193 Japanese species, at least 164
are peculiar.

The Japanese _Helices_ belong to sub-genera common to China
(_Plectotropis_ 8, _Euhadra_ 21, _Acusta_ 23?); but the Naninidae
scarcely occur at all. The principal feature of the fauna is the
development of _Clausilia_, which presents some extraordinarily fine
forms. One slug (_Philomycus_) is identical with an Indian species.
The operculates, which consist mainly of a few species each of
_Diplommatina_, _Cyclophorus_, _Pupinella_, _Pupina_, _Helicina_, and
_Georissa_, belong almost exclusively to the southern islands Kiu-siu,
Sikoku, and southern Niphon. The three species usually reckoned as
_Japonia_ are probably forms of _Lagochilus_.


                     =C. The Australasian Region=

This region includes all the islands of the Pacific east of the
Moluccas, and falls into three sub-regions--the Papuan, the Australian,
and the Polynesian.

1. The =Papuan Sub-region= may be divided into--(_a_) the _Papuan
Province_ proper, which includes New Guinea, with the Aru Is. and
Waigiou, the Admiralty Is., New Ireland, New Britain, and the
d’Entrecasteaux and Louisiade Groups; (_b_) the _Queensland Province_,
or the strip of N.E. Australia from C. York to the Clarence R. (about
29° S. lat.); (_c_) the _Melanesian Province_, which includes the New
Hebrides, New Caledonia, with the Loyalty Is. and the Viti Is. The
Solomons form a transition district between the Papuan and Melanesian
provinces, abounding on the one hand in characteristic Papuan
_Helices_, while on the other they form the north-western limit of
_Placostylus_, the group especially characteristic of the Melanesian
province.

(_a_) _The Papuan Province._--The molluscan fauna of New Guinea is the
richest and by far the most original of all the Australasian region.
We find ourselves, almost in a moment, in a district full of new and
peculiar forms. New Guinea may be regarded as the metropolis of the
rich Helicidan fauna, which is also characteristic of the Moluccas to
the west, of N. and N.E. Australia to the south and south-east, and of
the Solomons and other groups to the north-east. Here abound species
of _Papuina_ and _Insularia_ (the latter being quite peculiar), among
which are found, if not the largest, certainly the most finished forms
of all existing _Helices_. _Chloritis_ (13 sp.), _Planispira_ (5),
and _Cristigibba_ (9) are common with the Moluccas, while a tropical
Australian element is shown in _Pedinogyra_ (1) and _Hadra_ (4). Very
remarkable, too, is the occurrence of one species of _Obbina_ and
_Rhysota_, genera which culminate in the Philippines and here find
their most eastward extension; while a single _Corasia_ serves to
form a link between the _Corasia_ of the Philippines and those of the
Solomon Is., if the latter are true _Corasia_.

We naturally find considerable traces of a Polynesian element, which
appears to be principally characteristic of the eastern part of the
island. Most noteworthy in this respect is the occurrence of _Partula_
(3), _Tornatellina_ (1), _Charopa_ (1), _Thalassia_ (3). As compared
with the true _Pulmonata_, the operculates are feebly represented,
and the great majority are of a markedly Polynesian type. Not a
single _Cyclophorus_ occurs; _Lagochilus_, _Alycaeus_, and all the
tubed operculates, so marked a feature of the Indo-Malay fauna, are
conspicuous by their absence, and the prevailing genera are _Cyclotus_,
_Helicina_, and a number of sections of _Pupina_. _Leptopoma_, as
in the Philippines, is strongly represented. Not that an Indo-Malay
element is altogether absent. We still have _Xesta_ (5), _Hemiplecta_
(8), and even _Sitala_ (2), but the great predominance of _Helix_
seems to have barred the progress, for the greater part, of the Indian
Naninidae.

The slugs appear to be represented by a solitary _Vaginula_. A single
_Perrieria_ is a very marked feature of union with Queensland, where
the only other existing species (_P. australis_) occurs. The solitary
_Rhytida_, so far the only representative of the carnivorous group of
snails, emphasises this union still further. Little is known of the
fresh-water fauna. _Melania_ (28 sp.) is predominant, but on the whole
the relations are Australian rather than Indo-Malay. _Ampullaria_ is
wanting, while a decisive point of similarity is the occurrence of
_Isidora_ (3 sp.), a genus entirely strange to the Oriental region, but
markedly characteristic of the Australasian.


             _Land and Fresh-water Mollusca of New Guinea_

    Rhytida         1
    Helicarion      2
    Rhysota         1
    Hemiplecta     11
    Xesta           2
    Microcystis     3
    Microcystina    2
    Sitala          2
    Oxytes (?)      2
    Conulus         1
    Trochomorpha    8
    Nanina (?)      3
    Charopa         1
    Thalassia       3
    Ochthephila(?)  1
    Chloritis      13
    Planispira      5
    Cristigibba     9
    Insularia      17
    Obbina          1
    Albersia        3
    Hadra           4
    Pedinogyra      1
    Papuina        35
    Corasia (?)     1
    Bulimus (?)     1
    Calycia         4
    Partula         3
    Pupa            1
    Stenogyra       1
    Tornatellina    1
    Perrieria       1
    Succinea        1
    Vaginula        1
    Limnaea         2
    Isidora         3
    Melania        28
    Faunus          1
    Vivipara        4
    Diplommatina    1
    Pupina          4
    Pupinella       3
    Omphalotropis   2
    Bellardiella    2
    Leptopoma      16
    Cyclotus        5
    Cyclotropis     5
    Helicina       15
    Unio            4
    Cyrena          3
    Corbicula       1
    Batissa         8

_Waigiou_ is practically a part of New Guinea. Twelve genera and twenty
species of Mollusca are known, eight of the latter being peculiar. The
occurrence of _Papuina_, _Insularia_, and _Calycia_ sufficiently attest
its Papuan relationship. Two species each of _Albersia_, _Chloritis_,
and _Planispira_ occur.[372]

The _Aru Is._ are, as we should expect from their position, and
particularly from the configuration of the adjacent sea bottom (see
map), markedly Papuan. At the same time they show unmistakable signs
of long-continued separation from the parent island, for of their 36
land Mollusca 15, and of their 20 fresh-water Mollusca 9 are peculiar.
The Papuan element consists in the presence of _Papuina_, _Albersia_,
and _Cristigibba_. Moluccan influence is not absent, for the three
_Helicina_, the _Albersia_, and one _Cyclotus_ are all Moluccan
species. The fresh-water fauna appears to be a mixture of varied
elements. The single _Segmentina_ is common to India, the _Glaucomya_
to Malacca and the Philippines, while the single _Batissa_ is also
found in New Zealand.


_Land and Fresh-water Mollusca of the Aru Islands_

    Xesta         4
    Microcystis   1
    Hyalinia(?)   1
    Trochomorpha  1
    Patula        1
    Eulota        1
    Chloritis     5
    Cristigibba   2
    Albersia      1
    Papuina       4
    Pupa          2
    Stenogyra     2
    Planorbis     1
    Segmentina    1
    Melania      14
    Leptopoma     3
    Moussonia     1
    Realia        1
    Cyclotus      3
    Helicina      3
    Cyrena        2
    Glaucomya     1
    Batissa       1

The _Louisiades_, the _d’Entrecasteaux_, and _Trobriand Is._,
and _Woodlark I._, are closely related to New Guinea, containing
no peculiar genera. Each group, however, contains a considerable
proportion of peculiar species, an indication that their separation
from New Guinea dates from a very distant period. From the Louisiades
are known 34 species in all, 22 of which are peculiar.

The fauna of the _Admiralty Is._, of _New Hanover_, and _New Ireland_
is markedly Papuan, without any especial feature of distinction. The
Admiralty Is. contain 15 sp. _Papuina_, 7 _Chloritis_, 1 _Planispira_,
and 1 _Corasia_. A single _Janella_ shows relationship with the New
Hebrides and with New Zealand. In New Ireland _Planispira_ (which
is specially characteristic of W. New Guinea and the Moluccas)
has disappeared, but there are 7 _Papuina_ and 6 _Chloritis_. The
essentially Polynesian _Partula_ is present in both groups.

The prominent feature of the Mollusca of the _Solomon Is._ is the
extraordinary development of _Papuina_, which here culminates in a
profusion of species and singularity of form. The genus is arboreal,
crawling on the branches and attaching itself to the leaves of trees
and underwood. Of the 140 land _Pulmonata_ known from the group, no
less than 50, or 36 per cent, are _Papuina_. Ten species of _Corasia_
occur, but whether the shells so identified are generically identical
with those of the Philippines, is not satisfactorily determined.
_Trochomorpha_, with 22 species, here attains its maximum. _Chloritis_
begins to fail, but still has 3 species. Indo-Malay influence still
appears, though feebly, in _Hemiplecta_ (3), _Xesta_ (1), and possibly
even _Macrochlamys_ (1). The _Rhytida_, the 3 _Hadra_, and possibly
the _Paryphanta_ represent the Australian element. The growing numbers
of _Partula_ (13), the small and inconspicuous land operculates (only
22 in all, with _Helicina_ very prominent), and the almost complete
absence of fresh-water bivalves, show signs of strong Polynesian
affinities. An especial link with the New Hebrides, New Caledonia,
and the Viti Is. is the occurrence of _Placostylus_ (16 sp.). It is
very remarkable that this genus should occur in the Solomon Is. and not
in New Ireland. The occurrence of _Streptaxis_, if authentic, is very
noteworthy, the nearest species being from the Philippines.


        _Land and Fresh-water Mollusca of the Solomon Islands_

    Streptaxis (?) 1
    Rhytida        1
    Paryphanta (?) 1
    Helicarion     2
    Xesta          1
    Macrochlamys   1
    Hemiplecta     3
    Microcystis    2
    Trochomorpha  22
    Nanina (?)     2
    Patula         1
    Thalassia      2
    Chloritis      3
    Philina        2
    Hadra          3
    Papuina       50
    Merope         1
    Corasia (?)   10
    Placostylus   16
    Partula       13
    Succinea       1
    Melania       18
    Diplommatina   2
    Pupina         4
    Leptopoma      4
    Omphalotropis  2
    Cyclotus       1
    Cyclotropis    2
    Helicina       7
    Unio           1

(_b_) _The Queensland Province._--The strip of coast-line from Cape
York to the Clarence R. stands apart from the rest of Australia, and
is closely connected with New Guinea. There can be little doubt that
it has been colonised from the latter country, since an elevation
of even 10 fathoms would create (see map) a wide bridge between the
two. Many of the genera are quite strange to the rest of Australia.
Land operculates are abundant, and of a Papuan type. Several of the
characteristic Papuan genera of _Helix_ (_Papuina_, _Chloritis_,
_Planispira_) occur, while _Hadra_ attains its maximum. _Panda_,
_Pedinogyra_, and _Thersites_ are three remarkable groups in a rich
_Helix_ fauna. _Parmacochlea_ is a peculiar form akin to _Helicarion_.
The carnivorous Mollusca are represented by _Rhytida_, _Diplomphalus_
(New Caledonia), and _Elaea_. One species of _Janella_, a slug peculiar
to this region, occurs. The predominant fresh-water genus is _Bulinus_
(_Isidora_). _Ampullaria_ and _Anodonta_ are entirely absent from
Australia and New Zealand.

  [Illustration: FIG. 214.--Characteristic Australian _Helices_:
  =A=, _H._ (_Hadra_) _pomum_ Pfr.; =B=, _H._ (_Thersites_)
  _richmondiana_ Pfr. × ⅔.]

  [Illustration: Map D. _To face page 322._

    MAP
    to illustrate the relations
    OF THE LAND MOLLUSCA OF
    NEW GUINEA WITH THOSE
    OF NORTH AUSTRALIA.

  _The red line marks the 100 fathom line_

  London: Macmillan & Co.]


              _Land Mollusca of the Queensland Province_

    Diplomphalus       1
    Rhytida           10
    Elaea              1
    Parmacochlea       1
    Helicarion         7
    Nanina             3
    Hyalinia          10
    Thalassia          4
    Charopa            5
    Patula (?)         4
    Macrocyclis (?)    1
    Helicella         10
    Planispira         8
    Hadra             51
    Chloritis          5
    Pedinogyra         1
    Thersites          1
    Papuina            6
    Panda              2
    Helix (inc. sed.)  6
    Bulimus (?)        1
    Stenogyra          1
    Tornatellina       4
    Pupa               3
    Vertigo            4
    Perrieria          1
    Succinea           3
    Vaginula           1
    Janella            1
    Georissa           1
    Pupina            16
    Hedleya            1
    Callia             1
    Diplommatina       3
    Ditropis           2
    Dermatocera        1
    Helicina           8

(_c_) _The Melanesian Province_ includes those islands on which
the remarkable group _Placostylus_ occurs, the metropolis of whose
distribution is New Caledonia. These islands are very possibly the
remains of what was once a much wider extent of land. A single species
of _Placostylus_ occurs both on Lord Howe’s I. and in the North I. of
New Zealand, but this fact, while highly interesting as indicating a
possible former extension of land in a south-easterly direction, is
hardly sufficient to bring these islands within the province as now
limited. The Solomon Is., although containing _Placostylus_ as far to
the west as Faro I., form, as has been already stated, a transitional
district to the Papuan province.

_New Caledonia._--The chief features of the Mollusca are the remarkable
development of the helicoid carnivorous genera _Rhytida_ (30 sp.) and
_Diplomphalus_ (13 sp.), and of _Placostylus_ (45 sp.). There is a
stray _Papuina_, and a peculiar form _Pseudopartula_, but _Helix_ has
almost entirely disappeared. Polynesian influence is represented by
_Microcystis_ (3 sp.), the so-called _Patula_ (13 sp., many of which
are probably _Charopa_), _Tornatellina_ (2 sp.), and _Helicina_ (20
sp.). _Partula_ does not reach so far south, but there are two species
of _Janella_. The recurrence of _Melanopsis_ (19 sp.), absent from
the whole Oriental region, is curious, and forms another link with
New Zealand. The curious sinistral _Limnaea_ (_Isidora_), common with
Australia and New Zealand, is abundant.

  [Illustration: FIG. 215.--_Placostylus caledonicus_ Pet., New
  Caledonia, × ⅔.]

_The New Hebrides_ link New Caledonia and the Solomons by their
possession of the typical heavy _Placostylus_ (5 sp.) of the former,
and the lighter and more elegant _Charis_ (2 sp.) of the latter. There
are 4 _Papuina_, and _Partula_ is abundant (18 sp.), but there is no
evidence at present that the carnivorous genera or the _Melanopsis_ and
_Isidora_ of New Caledonia occur.

_The Fiji Is._, by the possession of 14 _Placostylus_ of the _Charis_
section, which is entirely absent from the adjacent Tonga group,
form the eastern limit of the province. There appears to be only a
single _Partula_, but the Polynesian element, especially as seen in
_Navicella_ (8 sp.), _Neritina_ (20 sp.), _Helicina_ (11 sp.), and
_Omphalotropis_ (11 sp.), is very strong. The _Microcystis_ (9 sp.) and
_Trochomorpha_ (14 sp.) are also of a Polynesian type.

(2) The =Australian Sub-region= includes the whole of Australia (with
the exception of the Queensland province) and Tasmania, with New
Zealand and the off-lying islands. The fauna, from the prevalence of
desert, is scanty, especially in genera. Land operculates are almost
entirely wanting. _Limax_ is not indigenous, though several species
have become naturalised. The bulk of the fresh-water species belong to
_Isidora_, and it is doubtful whether _Physa_ occurs at all. _Unio_
has a few species, and also _Vivipara_, but neither _Anodonta_ nor
_Ampullaria_ occur. There are a few _Melania_ and _Neritina_.

_Tropical South Australia._--The Mollusca are scanty, and occur chiefly
in the neighbourhood of the rivers, the soil being arid, with no
shelter either of trees or rocks. Fresh-water species predominate, and
the rich land fauna of Queensland is totally wanting. There are no land
operculates, 6 _Hadra_, 1 _Bulimus_ (?), 1 _Stenogyra_.

_West Australia._--Owing to the deserts which bound it, the Mollusca
are very isolated, only one species being common with N., S., and E.
Australia. The chief characteristics are _Liparus_, a form intermediate
between _Helix_ and _Bulimus_, and, among the _Helices_, the group
_Rhagada_. There are no slugs, no carnivorous snails, and only three
land operculates.


                   _Land Mollusca of West Australia_

    Lamprocystis  1
    Hyalinia      1
    Patula        7
    Chloritis     2
    Gonostoma     2
    Trachia       3
    Xerophila     1
    Rhagada       8
    Hadra         5
    Liparus      10
    Pupa          4
    Succinea      3
    Cyclophorus   2
    Helicina      1

In _Eastern and Southern Australia_ (New South Wales, Victoria, and
South Australia) the tropical element, so abundant in Queensland,
almost entirely disappears, the last operculate (a _Helicina_) only
reaching Port Macquarie, though several species of _Helicarion_ occur
in the extreme south. _Hadra_ is still abundant in New South Wales (18
sp.) and S. Australia (10 sp.), but becomes scarce in Victoria (2 sp.);
New South Wales has also one _Panda_ and two _Thersites_. _Cystopelta_
is common with Tasmania, and one of the Janellidae (_Aneitea_) with
Queensland. The carnivorous snails are represented by _Rhytida_.
_Caryodes_, a bulimoid group perhaps akin to _Liparus_, is common with
Tasmania only.

_Tasmania._--About 80 species of land Mollusca are known, not more than
10 being common with Australia. No land operculates occur; _Endodonta_
and _Charopa_ are rare, and _Hadra_ has entirely disappeared, but
_Pupa_ and _Succinea_ occur. Carnivorous genera are represented by
_Paryphanta_, _Rhytida_, and _Rhenea_. _Anoglypta_ is a peculiar
section of _Helix_, while _Caryodes_, _Cystopelta_, and _Helicarion_
are common with Australia. Among the fresh-water Mollusca are a
_Gundlachia_ (see p. 345), and some forms of _Amnicola_ or _Hydrobia_,
one of which (_Potamopyrgus_) is common only with New Zealand.[373]

_The Neozealanian Province._--The Mollusca of New Zealand, with the
Kermadec, Chatham, and Auckland Is., are remarkably isolated. Such
genera as _Nanina_, _Partula_, _Pupa_, _Stenogyra_, _Succinea_,
_Vaginula_, _Truncatella_, _Helicina_, and _Navicella_, which might
have been expected to occur, are entirely absent. The bulk of the
land Mollusca are small and obscure forms, perhaps remains of a very
early type, and appear to belong to the Zonitidae, neither _Patula_
nor _Helix_ occurring at all. The carnivorous forms are represented
by _Schizoglossa_, a peculiar genus akin to _Daudebardia_, by
_Paryphanta_, an extraordinary group of large shells with a thick
leathery epidermis, and by _Rhytida_ and _Rhenea_. In spite of its
extreme isolation, the general relations of the fauna are partly
with New Caledonia, partly with E. Australia. The occurrence of
_Placostylus_ has already been mentioned (p. 323), and three species of
_Janella_, a genus which also occurs in Queensland and New Caledonia,
indicate the same affinity. _Otoconcha_ is peculiar. The fresh-water
Mollusca, besides the _Isidora_ characteristic of the sub-region, are
partly related to New Caledonia through the occurrence of _Melanopsis_,
partly to Tasmania through _Potamopyrgus_, while the peculiar _Latia_
is possibly akin to _Gundlachia_ (Tasmania). The land operculates
number only 5 genera and 14 species in all, excluding a doubtful
_Diplommatina_.[374]


     _Land and Fresh-water Mollusca of the Neozealanian Province_

    Schizoglossa  1
    Paryphanta    5
    Rhytida       6
    Rhenea        2
    Helicarion    1
    Otoconcha     1
    Microcystis   1
    Trochonanina  1
    Phacussa      3
    Thalassohelix 5
    Gerontia      2
    Allodiscus   10
    Pyrrha        1
    Therasia      7
    Phenacohelix  3
    Suteria       1
    Flammulina   13
    Laoma        23
    Endodonta    10
    Charopa      28
    Placostylus   1
    Carthaea      1
    Tornatellina  1
    Janella       3
    Latia         2
    Ancylus       2
    Limnaea       5
    Amphipeplea   2
    Planorbis     1
    Isidora       7
    Melanopsis    2
    Potamopyrgus  4
    Paxillus      1
    Lagochilus    7
    Omphalotropis 1
    Realia        4
    Hydrocena     1
    Unio          9
    Sphaerium     1
    Pisidium      2

_Lord Howe’s I._ is remarkable as containing a _Placostylus_, which
thus links the island with this province. The remainder of the fauna
is Polynesian, with the exception of a species (common to the Fijis)
of _Parmella_, a slug akin to _Helicarion_, _Parmacochlea_, and
_Cystopelta_.

(3) The =Polynesian Sub-region= includes all the island groups of the
central and southern Pacific (except those classified in the Papuan and
Australian sub-regions), from the Pelews and Carolines in the west to
the Marquesas and Paumotus in the east, and from the Tonga group in the
south to the Sandwich Is. in the north. It may be subdivided into (_a_)
the Polynesian province proper, and (_b_) the Hawaiian province, which
includes the Sandwich Is. only.

  [Illustration: FIG. 216.--Characteristic Polynesian Mollusca:
  =A=, _Achatinella vulpina_ Fér., Sandwich Is.; =B=, _Partula
  planilabrum_ Pease, Society Is.]

(_a_) The general features of the _Polynesian province_ are very
similar throughout, although the Mollusca of each island group are
in the main peculiar. The species are mostly small and obscure.
_Helix_ scarcely occurs, its place being taken by small Zonitidae
(_Microcystis_, _Charopa_, _Trochomorpha_, etc.), and by groups of
so-called _Patula_ (_Endodonta_, _Pitys_, etc.), the exact position
of which is not yet settled. _Libera_, remarkable for its method of
ovipositing (p. 128), is peculiar to the Society and Hervey Is.;
_Partula_ is almost universal, attaining its maximum (40 sp.) in the
Society Is.; _Tornatellina_, _Pupa_, and _Vertigo_ occur throughout.

The land operculates consist chiefly of _Omphalotropis_, _Pupina_,
_Realia_, and _Helicina_. _Diplommatina_ and _Palaina_ are abundant
on the Pelews, and a _Moussonia_ occurs in the Samoa Is. _Ostodes_, a
small form of _Cyclophorus_, is found in some of the southern groups.
The fresh-water operculates are _Melania_, _Neritina_ (including
_Clithon_, a sub-genus furnished with spines), and _Navicella_; there
are no Unionidae, while fresh-water _Pulmonata_ are very scarce.

(_b_) The land Mollusca of the _Hawaiian province_ are distinguished
by the possession of four entirely peculiar genera--_Achatinella_,
_Leptachatina_, _Carelia_, and _Auriculella_. More than 300 of the
two former genera have been described, every mountain valley of some
of the islands having its own peculiar species. The destruction of
the indigenous herbage by goats is rapidly extinguishing many forms.
_Partula_, and the small land operculates, so characteristic of the
other groups, are, with the exception of _Helicina_, entirely wanting.
The occurrence of one of the _Merope_ group of _Helix_ (Solomon Is.) is
remarkable, and there is a rich development of _Succinea_. “_Patula_,”
_Microcystis_, _Tornatellina_, and the other small Polynesian land
_Pulmonata_ are well represented. The presence of _Isidora_, absent
from the central Pacific groups, is remarkable, and _Erinna_ is a
peculiar genus belonging to the Limnaeidae.




                              CHAPTER XI

     GEOGRAPHICAL DISTRIBUTION OF LAND MOLLUSCA (_continued_)--THE
             ETHIOPIAN, NEARCTIC, AND NEOTROPICAL REGIONS


                       =D. The Ethiopian Region=

The Ethiopian region includes the whole of Africa south of the Great
Desert, and Southern Arabia, together with the outlying islands,
excepting those of the Atlantidean province (p. 297).

Regarded as a whole, the Ethiopian is poorest in land Mollusca of all
the tropical regions. And yet its characteristics are very remarkable.
The entire _Achatina_ group is peculiar, and takes, especially in W.
Africa, some curious forms (_Columna_, _Perideris_, _Pseudachatina_).
Carnivorous Mollusca (_Ennea_, _Gibbus_, etc.) are highly developed,
especially in the south and east, the largest known helicoid form
(_Aerope_) being from Natal. In the possession of these types of the
Agnatha, Africa is more closely related to the Australasian than to the
Oriental region. The true _Cyclostoma_ are entirely peculiar to the
region, but are absent from West Africa.

Fresh-water Mollusca are abundant and characteristic, especially in and
near the Great Lakes. _Lanistes_, _Cleopatra_, and _Meladomus_, among
the operculates, together with _Mutela_ and _Aetheria_ (Unionidae),
_Galatea_ and _Fischeria_ (Cyrenidae), are peculiar.

In its negative, as well as its positive features, the Ethiopian region
is markedly isolated. Helicidae and Naninidae are equally deficient,
the former, indeed, attaining some numerical predominance in the
extreme south, but the species are nearly all insignificant in size
and colouring. It is only in Madagascar that _Helix_ asserts itself.
_Arion_, _Limax_, _Hyalinia_, _Clausilia_, and a number of other
genera abundant along the Mediterranean, are either altogether absent,
or are very scantily represented. Land operculates, so characteristic
of other tropical countries, are almost entirely wanting. If we
disregard the Malagasy sub-region, there are scarcely forty species of
land operculates on the whole African continent.

The Ethiopian region may be divided into three sub-regions: (1) the
Central African; (2) the South African; (3) the Malagasy.

(1) =The Central African Sub-region= is bounded on the north by the
Great Desert, on the east and west by the ocean, and on the south by a
line roughly drawn between the mouth of the Orange River and Delagoa
Bay; it also includes S. Arabia. No natural features exist which tend
to break up this vast district into areas of independent zoological
development. The absence of long and lofty mountain ranges, the
enormous size of the great river basins, and the general uniformity
of climate, equalise the conditions of life throughout. It will be
convenient to break the sub-region up into provinces, but in most cases
no precise line of demarcation can be laid down.

(_a_) The _Senegambian Province_ may be regarded as extending from
the mouth of the Senegal River to Cape Palmas. Only 8 genera of land
Mollusca are known, including 4 _Limicolaria_ and 3 _Thapsia_, with 1
small _Cyclophorus_. Fresh-water genera are abundant, and include most
of the characteristic Ethiopian forms.

(_b_) The _West African Province_ extends from Cape Palmas to the mouth
of the Congo, and is rich in Mollusca. The great _Achatina_, largest
of land snails, whose shell sometimes attains a length of 6½ in.,
_Limicolaria_, _Perideris_, and _Pseudachatina_ are the characteristic
forms. The Agnatha are represented by _Ennea_, _Streptaxis_, and
_Streptostele_. _Rachis_ and _Pachnodus_, sub-genera of _Buliminus_,
occur also on the east coast. A special feature is the development of
several peculiar slug-like genera, _e.g._ _Oopelta_, perhaps a form of
_Arion_; _Estria_, a slug with an external shell, akin to _Parmacella_;
and _Aspidelus_, a form intermediate between _Helicarion_ and _Limax_.
_Claviger_, a handsome group akin to _Cerithium_, is peculiar to the
estuaries of West African rivers.

About sixteen species are known from the _Cameroons District_, but
no peculiar genera occur. The _French Congo District_ has not yet
been well explored. _Tomostele_, a genus allied to _Streptostele_, is
peculiar, and _Pseudachatina_ attains its maximum.

  [Illustration: FIG. 217.--_Columna flammea_ Müll., Princes I.]

_St. Thomas and Princes Is._, in the Gulf of Guinea, are well known.
Princes I. has 22 species, 14 peculiar, and 2 common to St. Thomas
only, one of the latter being the great sinistral _Achatina bicarinata_
Chem. The remarkable genus _Columna_ (Fig. 217) is peculiar, and
_Streptostele_ (4 sp.) attains its maximum. Peculiar to St. Thomas are
_Pyrgina_, a turreted form of _Stenogyra_; _Thyrophorella_, a sinistral
form of _Zonites_; and _Atopocochlis_, a large bulimoid shell, whose
true relationships are not yet known. _Homorus_, a group of _Achatina_
with an elongated spire, occurring also in the Angola District and on
the east coast, has 4 species. No fresh-water species have as yet been
discovered in either of the islands.

The _Angola and Benguela District_, extending from the Congo to the
Cunene R., probably belongs to the West African Sub-region, but until
its fauna is better known it is advisable to consider it apart.
_Achatina_ continues abundant, but the other characteristic West
African forms (_Pseudachatina_, _Streptostele_, _Perideris_) diminish
or are absent altogether. No _Helix_ and only 1 _Cyclophorus_ occur.

_Ovampo_, _Damara_, and _Great Namaqualand_, lying between the Cumene
and Orange rivers, seem to form a transition district between the West
and South African faunas. _Helix_ reappears, while the characteristic
West African genera are almost entirely wanting.

(_c_) The _East African Province_ extends from about Delagoa Bay to the
Abyssinian shores of the Red Sea. In general out-line the province
consists of a flat marshy district, extending inland for many miles
from the sea; this is succeeded by rising ground, which eventually
becomes a high table-land, often desolate and arid, whose line of slope
lies parallel to the trend of the coast. The Mollusca are little known,
and have only been studied in isolated districts, usually from the
discoveries of exploring expeditions.

The _Mozambique District_, from Delagoa Bay to Cape Delgado, includes
no genus which does not occur on the west coast, except _Cyclostoma_
(2 sp.). _Trochonanina_ (4 sp.), _Urocyclus_, a characteristic African
slug (2 sp.), _Rachis_ (6 sp.), _Pachnodus_ (2 sp.), and _Achatina_ (5
sp.), are the principal groups.

  [Illustration: FIG. 218.--_Urocyclus comorensis_ Fisch.,
  Comoro Is.: =G=, Generative orifice; =M=, mucous gland; =O=,
  orifice leading to internal shell; =P=, pulmonary orifice; =T=,
  tentacles. (After Fischer.)]

The _Zanzibar District_, from Cape Delgado to the Somali country, has
the same general features. _Meladomus_, a large sinistral _Ampullaria_,
is characteristic, while _Cyclostoma_ (5 sp.) becomes more abundant.
_Helix_ is still absent, but the carnivorous forms (_Streptaxis_ 2 sp.,
_Ennea_ 7 sp.) are rather numerous.

The _Somali District_ is characterised by operculate groups of
the _Otopoma_ type (_Georgia_, _Rochebrunia_, _Revoilia_) whose
generic value is rather doubtful. _Petraeus_, in an Arabian type,
supplants _Rachis_ and _Pachnodus_. _Achatina_ is nearly wanting, but
_Limicolaria_ has 9 species. A few _Helix_, said to be of the _Pisana_
group, occur.

The _District between the Great Lakes and the coast region_ is fairly
well known through recent explorations, especially those associated
with Emin Pasha. _Streptaxis_ (6 sp.) and _Ennea_ (24 sp.) are
numerous, _Helix_ is wanting, and the Naninidae are represented by
_Trochonanina_ (7 sp.), and other forms at present grouped under
_Nanina_ or _Hyalinia_. On the high ground _Buliminus_, _Cerastus_, and
_Hapalus_ replace, to some extent, the _Achatina_ and _Limicolaria_ of
the marshy plains. Land operculates (_Cyclophorus_ 1, _Cyclostoma_ 8)
are more numerous; among fresh-water genera we have _Lanistes_ (5 sp.),
_Cleopatra_ (3 sp.), _Meladomus_ (1 sp.), and _Leroya_, a sinistral
form with the facies of a _Littorina_. The characteristic African
bivalves (_Mutela_, _Spatha_, etc.) are few in number.

(_d_) _Province of the Great Lakes._--The Mollusca of the four great
lakes of Eastern Central Africa--Lakes Albert Nyanza (Luta Nzige,
2720 ft.), Victoria Nyanza (Oukéréwé, 3700 ft.), Nyassa (1520 ft.),
and Tanganyika (2800 ft.)--are well known, and supply an interesting
problem in distribution. Those of the three first mentioned lakes
differ in no way from the rest of tropical Africa, but the Mollusca
of Tanganyika include, in addition to the ordinary African element,
a number of peculiar operculate genera, belonging principally to the
Melaniidae and Hydrobiidae. Several of these possess a solidity of form
and compactness of structure which is unusual in fresh-water genera,
and has led to the belief, among some authorities, that they are the
direct descendants of marine species, and that Tanganyika represents
an ancient marine area. This view appears untenable. The Victoria
Nyanza and Nyassa are part of the same system as Tanganyika, and it
is not easy to see how, if Tanganyika were once an arm of the sea,
they were not equally so, especially as they are several hundred miles
nearer the Indian Ocean as at present defined. Nor, as will be seen
from the figures given above, is there anything in the altitudes which
would make us expect anything exceptional in Tanganyika. The similar
case of L. Baikal must be compared (p. 290), where again a number of
specialised forms of _Hydrobia_ occur.

Of the genera concerned, _Paramelania_ and _Nassopsis_ are forms of
Melaniidae; _Tiphobia_ (Fig. 219), which is allied to _Paludomus_, is a
compact shell with angulated spinose whorls; _Lacunopsis_, _Ponsonbya_,
_Limnotrochus_, and _Tanganyicia_ are probably forms of _Lithoglyphus_,
some, as their names denote, being of decidedly marine facies;
_Syrnolopsis_ and _Turbonilla_ (?) look like Pyramidellidae, _Horea_
and _Reymondia_ like _Rissoina_; _Bourguignatia_ appears to belong
to _Vivipara_, with which has now been merged the genus _Neothauma_.
Recently discovered forms from the adjacent L. Mweru are evidently of
kindred origin.

(_e_) The _Afro-Arabian Province_ includes Abyssinia, with S. Arabia,
the African shores of the Gulf of Aden, and Socotra. The province
contains a singular mixture of types. The high ground of Abyssinia
stands like a lofty European island in the midst of a tropical plain,
with Palaearctic genera flourishing like hardy northern plants on a
mountain in low latitudes. _Helix_, _Vitrina_, and _Pupa_ abound,
with a few _Clausilia_ and even a _Limax_. On the lower levels occur
_Limicolaria_ (3 sp.), _Subulina_ (7 sp.), _Helicarion_, and _Homorus_,
but land operculates are entirely wanting. Characteristic of the
province as a whole are various forms of _Buliminus_, which in Socotra
are represented by two peculiar sub-genera, _Achatinelloides_ and
_Passamaiella_. In S. Arabia the mixture of types produces curious
results: the _Helix_, _Clausilia_, and _Vitrina_ being Palaearctic,
the _Limicolaria_ and all the operculates Ethiopian, while the
single _Trochomorpha_ is Indian. Indian influence, indeed, comes out
unmistakably throughout the province. Thus in Socotra there are two
_Cyclotopsis_, in Abyssinia two _Africarion_ (closely related to the
Indian _Girasia_), two _Microcystis_, and a _Glessula_, and in the
Scioa district there is a _Sitala_. The fresh-water Mollusca of Socotra
are Indian forms.

  [Illustration: FIG. 219.--_Tiphobia Horei_ E. A. Smith, L.
  Tanganyika.]

  [Illustration: FIG. 220.--Mollusca characteristic of L.
  Tanganyika: =A=, _Nassopsis nassa_ Woodw.; =B=, _Spekia zonata_
  Woodw.; =C=, _Syrnolopsis lacustris_ E. A. Smith.]

  [Illustration: FIG. 221.--_Achatina zebra_ Lam., S. Africa. × ½.]

(2) =The South African Sub-region.=--The principal characteristic of
the Mollusca of S. Africa is the occurrence of numerous small species
of Helicidae, belonging chiefly to the groups _Pella_, _Phasis_,
_Dorcasia_, and _Sculptaria_, all of which are practically peculiar.
Carnivorous genera are also prominent, _Ennea_ here attaining its
maximum. _Rhytida_ (to which several species still regarded as _Pella_
belong) is common only to the S. Pacific and Australasia, and forms,
with _Isidora_ among the fresh-water pulmonates, a remarkable link of
connexion. _Aerope_, the largest of all helicoid carnivorous genera,
and _Chlamydephorus_, a carnivorous slug with an internal shell, are
peculiar. _Achatina_ is still abundant, but _Limicolaria_ is wanting.
_Livinhacea_, a form with a continuous peristome, perhaps akin to
_Bulimus_; _Apera_, a form of slug; and _Coeliaxis_, a genus perhaps
akin to the Papuan and Queensland _Perrieria_, are all peculiar. The
land operculates, which are not numerous, are of the East African type.


             _Land Mollusca of the S. African Sub-region_

    Chlamydephorus     1
    Ennea             31
    Aerope             5
    Rhytida            3
    Helicarion         3
    Trochonanina       1
    Trochozonites      1
    Limax              1
    Apera              1
    Vitrina            7
    Nanina             6
    Conulus            2
    Patula             2
    Pella             44
    Dorcasia           8
    Phasis             1
    Sculptaria         2
    Helix (inc. sed.)  4
    Rachis             1
    Pachnodus          3
    Buliminus (?)      4
    Pupa              20
    Vertigo            2
    Achatina          18
    Livinhacea         1
    Stenogyra          4
    Coeliaxis          1
    Succinea           3
    Vaginula           2
    Cyclophorus        1
    Cyclostoma         7
    Cyclotus (?)       1
    Blanfordia         1

_St. Helena._--The Molluscan fauna of St. Helena is perhaps the most
puzzling, as regards its geographical affinities, of any in the world.
It consists of 29 peculiar species of land Mollusca (fresh-water
species being unknown), 19 of which are recently extinct, partly
owing to the destruction of the forest, but are found in considerable
abundance in a state of good preservation.[375] The genera are--

    Hyalinia       1
    Patula         4 (3 extinct)
    Endodonta     10 (7 extinct)
    Bulimulus      7 (5 extinct)
    Pachyotus      1 (extinct)
    Tomigerus (?)  1 (extinct)
    Pupa           2 (extinct)
    Succinea       3

The 5 genera which concentrate our attention are _Patula_, _Endodonta_,
_Pachyotus_ (Fig. 222), _Tomigerus_, and _Bulimulus_, all of which
appear utterly strange to an oceanic island in the middle of the S.
Atlantic. _Patula_ and _Endodonta_ are essentially Polynesian forms,
occurring abundantly on all the island groups in the Central Pacific.
_Pachyotus_, _Tomigerus_ (assuming its correct identification), and
_Bulimulus_ are all S. American forms, the two former being especially
characteristic of Brazil. How this mixture of genera now confined to
regions so widely distant, not only from St. Helena itself, but from
one another, became associated here, is a problem obviously not easy
of solution. The fauna is probably a remnant of a very ancient type,
possibly at one time much more widely distributed. _Endodonta_ (an
essentially insular form, like _Omphalotropis_) actually occurs on
Fernando Noronha, off the Brazil coast, and we shall see how an Indian
and even a Polynesian element is present off the eastern coasts of
Africa.

_Ascension I._--One indigenous species, a so-called _Limax_, is all
that has ever been discovered.

(3) =The Malagasy Sub-region= includes Madagascar with its attendant
satellites Bourbon, Mauritius, and Rodriguez, and the Seychelles and
Comoro groups. No land Mollusca are known from the Amirantes, the
Chagos, or from Aldabra. The special characteristics of the sub-region
are the great development of the carnivorous land Mollusca (_Ennea_,
_Gibbus_), the occurrence of a considerable number of true Helicidae of
great size and beauty, and the prominence of the genus _Cyclostoma_.

(_a_) _The Madagascan Province._--The land Mollusca of Madagascar,
although as yet imperfectly known, possess a striking individuality.
Two of the chief characteristics of the Ethiopian region are the
paucity of its land operculate and of its _Helix_ fauna; Madagascar
is especially distinguished by the rich development of both these
groups. For size, colouring, and beauty of shape, the Helicidae of the
two sub-genera _Ampelita_ and _Helicophanta_ rival, if they do not
surpass, any in the world. They are quite peculiar to this sub-region,
not a trace of them occurring on the Mascarenes, Seychelles, or even
on the Comoros. _Helicophanta_ is distinguished by the enormous size
of its embryonic shell, which persists in the adult (Fig. 223), and in
this respect the group appears to be related to _Acavus_ (Ceylon, Fig.
204) and _Panda_ (N.E. Australia). As is usual when _Helix_ is well
developed, _Nanina_ (about 12 sp.) is proportionately scanty.

The African _Bulimini_ (_Pachnodus_ and _Rachis_) are represented by
two species, but _Achatina_, so abundant on the mainland, is scarce.
Two other groups of _Buliminus_, _Leucotaenia_ and _Clavator_, are
peculiar. The presence of a single _Kaliella_, specifically identical
with a common Indian form, is very remarkable.

_Cyclostoma_ proper, of which Madagascar is the metropolis, is richly
developed (54 sp.). Many of the species are of great size and of
striking beauty of ornamentation. Unlike its Helicidae, this genus is
not restricted to Madagascar; several species occur on the mainland,
6 on the Comoros, one on the Seychelles, and 16 in Mauritius. The
sub-genera _Acroptychia_ and _Hainesia_ are peculiar.

  [Illustration: FIG. 222.--_Pachyotus auris vulpina_ Desh., St.
  Helena (sub-fossil).]

  [Illustration: FIG. 223.--_Helix_ (_Helicophanta_) _Souverbiana_
  Fisch., Madagascar, showing embryonic shell. × ⅔.]

  [Illustration: FIG. 224.--_Cyclostoma campanulatum_ Pfr.,
  Madagascar.]

The fresh-water Mollusca of Madagascar contain further traces of Indian
relationship. Thus we find two species of _Paludomus_, a genus whose
metropolis is Ceylon, India, and Further India, and which is barely
represented on the Seychelles and in the Somali district. _Melanatria_,
which is peculiar to Madagascar, has its nearest affinities in the
Cingalese and East Indian faunas. Several of the _Melania_ and the two
_Bithynia_ are of a type entirely wanting in Africa, but common in the
Indo-Malay sub-region. Not a single one of the characteristic African
fresh-water bivalves (_Mutela_, _Spatha_, _Aetheria_, _Galatea_, etc.)
has been found in Madagascar. On the other hand, certain African
Gasteropoda, such as _Cleopatra_ and _Isidora_, occur, indicating, in
common with the land Mollusca, that an ultimate land connexion with
Africa must have taken place, but at an immeasurably remote period.


             _Land and Fresh-water Mollusca of Madagascar_

    Ennea                9
    Urocyclus            2
    Helicarion (?)       1
    Macrocyclis (?)      1
    Kaliella             1
    Nanina (inc. sed.)   9
    Ampelita            35
    Helicophanta        17
    Pachnodus            2
    Rachis               2
    Leucotaenia          2
    Clavator             2
    Achatina             3
    Opeas                2
    Subulina             3
    Vaginula             4
    Limnea               2
    Planorbis            3
    Isidora              3
    Melania              7
    Melanatria           4
    Paludomus            2
    Vivipara             1
    Bithynia             2
    Cleopatra            2
    Ampullaria           6
    Cyclophorus          2
    Cyclotus (?)         1
    Cyclostoma          54
    Otopoma              5
    Lithidion            1
    Acroptychia          3
    Hainesia             3
    Unio                 1
    Corbicula            2
    Sphaerium            1
    Pisidium             1

_The Comoro Islands._--This isolated group possesses about 100 species,
almost all of which are peculiar. The principal feature is the rich
development of _Ennea_ (30 sp.). On the whole the group shows more
relationship to Madagascar than to the mainland. Thus we have six
species of true _Cyclostoma_, and only one _Achatina_, while among the
fresh-water genera is _Septaria_, which is characteristic of the whole
Malagasy Sub-region, but is absent from the mainland. The Helicidae
are all of insignificant size. Peculiar to the group is the remarkable
genus _Cyclosurus_ (Fig. 152, p. 247).

(_b_) _The Mascarene Province_ (Mauritius, Bourbon, Rodriguez, and the
Seychelles).--The percentage of peculiar species, which is very high,
can only be paralleled in the case of some of the West Indian islands,
and sufficiently attests the extreme isolation of the group from
Madagascar. We have--

                                     Fresh-water            Peculiar to
                Total sp.  Land sp.      sp.      Peculiar     group

    Mauritius     113        104          9          78     102 (90 p.c.)
    Bourbon        45         40          5          19      38 (84 p.c.)
    Rodriguez      23         19          4          15      21 (95 p.c.)
    Seychelles     34         27          7          24      30 (90 p.c.)

The Mollusca of the group exhibit three distinct elements, the
Indigenous, the Madagascan, and the Indian and Australasian.

The genus _Pachystyla_ (Naninidae) is quite peculiar, forming the main
portion of the land snails proper. It attains its maximum in Mauritius
(17 sp.), with 5 sp. in Bourbon and one sub-fossil sp. in Rodriguez,
while in the Seychelles it is absent. But the principal feature of the
Mascarene group is the extraordinary development of the carnivorous
genus _Gibbus_, which has 27 sp. in Mauritius, 8 in Bourbon, 4 in
Rodriguez; in the Seychelles, it is replaced by _Edentulina_ and
_Streptostele_. The principal link with Madagascar is found in a part
of the operculate land fauna. _Cyclostoma_ is present (with _Otopoma_)
in several fine living forms, and the number of sub-fossil species is a
clear indication that this group was, not long ago, much more abundant,
for of the 16 _Cyclostoma_ known from Mauritius 10 are sub-fossil. The
operculates form a decided feature of the land fauna; thus in Mauritius
there are 32 species, or more than 28 per cent of the whole.

  [Illustration: FIG. 225.--Characteristic Mauritian land shells:
  =A=, _Gibbus palanga_ Fér.; =A´=, young of same; =B=, _Gibbus
  lyonetianus_ Pall.]

Indian and Australasian affinities are unmistakably present. Thus
_Omphalotropis_, a genus characteristic of _small_ islands, is
profusely represented, but it does not occur in Madagascar or Africa.
Two _Helicina_ (Mauritius and Seychelles) and a single _Leptopoma_
(possibly a _Leptopomoides_) are also of eastern relationship.
_Cyclotopsis_, _Cyathopoma_, and _Geostilbia_ are markedly Indian
genera. _Microcystis_, _Patula_, and _Tornatellina_ are Polynesian.
_Hyalimax_--and this is a very striking fact--occurs nowhere else but
in the Andamans and Nicobars, and on the Aracan coast. The nearest
relation to the Seychelles _Mariaella_ appears to be the Cingalese
_Tennentia_. Not a single representative of these eleven genera has
been found even in Madagascar.

The fresh-water Mollusca (omitting the Neritidae) are: Mauritius 9
species, Bourbon 5, Rodriguez 4, Seychelles 6, with only 15 species in
all. The one _Planorbis_ and the _Vivipara_, the _Paludomus_ and two of
the _Melania_ are of Indian types. The _Lantzia_ (peculiar to Bourbon)
is probably allied to the Indian _Camptonyx_. Owing to the paucity of
permanent streams, no fresh-water bivalves occur. Among the Neritidae
is a single _Septaria_, a genus which, though occurring in Madagascar,
is entirely strange to Africa, and is abundant in the Oriental and
Australasian regions.

It would seem probable that when the closer connexion which at one
time undoubtedly existed between India and Eastern Africa began to
be less continuous,[376] the Mascarene group was first severed from
what ultimately became Madagascar, while the Seychelles, and perhaps
the Comoros, still continued united to it. The Comoros, which lack
the great _Helices_, separated off from Madagascar first, while the
Seychelles continued in more or less direct union with that island
sufficiently long to receive the progenitors of _Stylodonta_ (a
peculiar group of _Helix_), but became disunited at an exceedingly
remote period.


                       =E. The Nearctic Region=

The southern boundary of this region may be regarded as roughly
corresponding to that of the United States, _i.e._ Lower California and
Mexico are excluded. The southern portion of Florida belongs to the
Antillean sub-region.

The principal characteristic of the Nearctic Region is the remarkable
poverty of its land Mollusca. No district in the world of equal extent
is so poor in genera, while those which occur are generally of small
size, with scarcely anything remarkable either in colouring or form.
The elongated land shells (_Clausilia_, _Buliminus_), so characteristic
of Europe, are entirely wanting, but a few _Bulimulus_, of Neotropical
origin, penetrate Texas, and from the same sources come a few species
of _Glandina_ (as far north as S. Carolina), _Holospira_ (Texas), and
_Helicina_.

The region falls into two well-marked sub-regions, the N. American
and the Californian, with the Rocky Mountain district as a sort of
debatable ground between them. The Californian sub-region consists of
the narrow strip of country between the Sierra Nevada, the Cascade
Mountains and the coast-line, from San Diego to Alaska; the N. American
sub-region consists of the remainder of the region.

(1) =The N. American Sub-region.=--The Carnivorous genera are
represented solely by the few _Glandina_ mentioned above, and by the
indigenous genus _Selenites_, a form midway between _Testacella_ and
_Limax_, whose metropolis is on the Pacific slope, but which spreads
eastward into the Antilles. Among the Limacidae, _Limax_ is common
to both sub-regions, but _Tebennophorus_ (4 sp., 3 of which belong
to the genus _Pallifera_), a genus found also in China and Siam,
and _Vitrinozonites_ do not occur in the Californian. _Hyalinia_
(_Zonites_) is fairly abundant, especially in the groups _Mesomphix_
and _Gastrodonta_ (peculiar to this sub-region), and _Hyalinia_ proper.
_Patula_ is well represented. The Helicidae belong principally to
the groups _Mesodon_, _Stenotrema_, _Triodopsis_, _Polygyra_, and
_Strobila_, only 6 of which, out of a total of 84, reach the Pacific
slope. Land operculates are conspicuous for their almost complete
absence (see map, _frontispiece_).

  [Illustration: FIG. 226.--Characteristic North American Mollusca.
  =A=, _Helix_ (_Mesodon_) _palliata_ Say, Ohio. =B=, _Helix_
  (_Polygyra_) _cereolus_ Mühlf., Texas. =C=, _Patula alternata_
  Say, Tennessee.]

The poverty of the land fauna is atoned for by the extraordinary
abundance and variety of the fresh-water genera. A family of
operculates, the Pleuroceridae, with 10 genera and about 450 species,
is quite peculiar, a few stragglers only reaching Central America and
the Antilles. The nucleus of their distribution is the Upper Tennessee
River with its branches, and the Coosa River. They appear to dislike
the neighbourhood of the sea, and are never found numerously within
100 miles of it. They adhere to stones in rapid water, and differ from
the Melaniidae of the Old World and of S. America in the absence of a
fringe to the mantle and in being oviparous. They do not occur north
of the St. Lawrence River, or north of U.S. territory in the west,
or in New England. Three-quarters of all the known species inhabit
the rough square formed by the Tennessee River, the Mississippi,
the Chattahoochee River, and the Gulf of Mexico. The Mississippi
is a formidable barrier to their extension, and a whole section
(_Trypanostoma_, with the four genera _Io_, _Pleurocera_, _Angitrema_,
and _Lithasia_) does not occur west of that river. The Viviparidae are
also very largely developed, the genera _Melantho_, _Lioplax_, and
_Tulotoma_ being peculiar. The Pulmonata are also abundant, while the
richness of the Unionidae may be gathered from the fact that Wetherby
states[377] that in 1874 no less than 832 species in all had been
described.

The entire Mississippi basin is inhabited by a common assemblage of
Unionidae, and a considerable number of the species are distributed
over the whole of this area, Texas, and parts of E. Mexico. Some
species have spread out of this area into Michigan, Canada, the Red
River, and Hudson’s Bay district, and even into streams in New York
which drain into the Atlantic. An entirely different set of forms
occupy the great majority of the rivers falling into the Atlantic, the
Appalachian Mountains acting as an effective barrier between the two
groups of species, which appear to mingle below the southern end of the
range. In many cases Unionidae seem to have no difficulty in migrating
from river to river, if the distance is not extreme; they probably are
carried across overflowed districts in time of flood.[378]

  [Illustration: FIG. 227.--_Helix_ (_Arionta_) _fidelis_ Gray,
  Oregon.]

(2) =The Californian Sub-region= is markedly distinct from the rest of
N. America. The characteristic sombre Helices of the Eastern States
are almost entirely wanting, and are replaced by _Arionta_ (20 sp.), a
larger and more varied group, which may have some affinity to Chinese
forms. _Glyptostoma_ (1 sp.) is also peculiar. _Selenites_ here has
its metropolis, and _Pristiolma_ is a remarkable group of small
_Hyalinia_ (_Zonites_), but the larger forms of the Eastern States are
wanting. Several remarkable and quite peculiar forms of slug occur,
namely, _Ariolimax_ (whose nearest relation is _Arion_), _Prophysaon_,
_Hemphillia_, and _Binneya_. There are no land operculates.

Not more than 15 to 20 species of the Pleuroceridae (sect.
_Goniobasis_) occur west of the Rocky Mountains, and only a single
_Unio_, 5 _Anodonta_, and 1 _Margaritana_, which is common to New
England. _Pompholyx_ is a very remarkable ultra-dextral form of
_Limnaea_, apparently akin to the _Choanomphalus_ of L. Baikal.
_Bithynia_, absent from the Eastern States, is represented by two
species. The general indications are in favour of the Californian fauna
having migrated from an Old World source after the upheaval of the
Sierras; the American fauna, on the other hand, is purely indigenous,
with no recent Old World influence at all.


                _Land Mollusca of the Nearctic Region_

    Glandina        4
    Selenites       6
    Limax           4
    Vitrina         4
    Vitrinozonites  1
    Mesomphix      15
    Hyalinia       22
    Conulus         1
    Gastrodonta     9
    Pristiloma      2
    Tebennophorus   4
    Ariolimax       6
    Prophysaon      2
    Hemphillia      1
    Binneya         1
    Patula         18
    Punctum         2
    Arionta        20
    Praticola       2
    Glyptostoma     1
    Mesodon        27
    Stenotrema     11
    Triodopsis     21
    Polygyra       23
    Polygyrella     2
    Gonostoma       1
    Vallonia        1
    Strobila        2
    Pupa           18
    Vertigo         8
    Holospira       2
    Cionella        1
    Bulimulus       6
    Macroceramus    1
    Succinea       21
    Vaginulus       1
    Helicina        2


=F. The Neotropical Region=

The land Mollusca of the Neotropical Region stand in complete contrast
to those of the Nearctic. Instead of being scanty, they are exceedingly
abundant; instead of being small and obscure, they are among the
largest in size, most brilliant in colour, and most singular in shape
that are known to exist. At the same time they are, as a whole,
isolated in type, and exhibit but little relation with the Mollusca of
any other region.

The most marked feature is the predominance of the peculiar genera
_Bulimus_ and _Bulimulus_, the centre of whose development appears to
lie in Peru, Ecuador, and Bolivia, but which diminish, both in numbers
and variety of form, in the eastern portion of the region. In the
forests of Central America, Venezuela, and Ecuador, and, to a lesser
degree, in those of Peru and Brazil, occurs the genus _Orthalicus_,
whose tree-climbing habits recall the _Cochlostyla_ of the Philippines.
These three groups of bulimoid forms constitute, as far as the
mainland is concerned, the preponderating mass of the land Mollusca.
_Helix_ proper is most strongly developed in the Greater Antilles,
which possess several peculiar groups of great beauty. In Central
America _Helix_ is comparatively scarce, but in the northern portions
of the continent several fine genera (_Labyrinthus_, _Isomeria_,
_Solaropsis_) occur, which disappear altogether towards the south.

Carnivorous land Mollusca are, so far as Central America is concerned,
more highly developed than in any other quarter of the world,
particularly in the genera _Glandina_ and _Streptostyla_. These genera
also penetrate the northern portions of the continent, _Glandina_
reaching as far as Ecuador, and _Streptostyla_ as far as Peru. The
Greater Antilles have also characteristic forms of these genera.
_Streptaxis_ is tolerably abundant all over tropical South America, and
is the one pulmonate genus which shows any affinity with the African
fauna.

The slugs are exceedingly scarce. _Vaginula_ occurs throughout, and is
the only genus in any sense characteristic.

_Clausilia_, in the sub-genus _Nenia_, occurs along the Andean chain
from the extreme north (but not in Central America) as far south as
Bolivia. It has in all probability made its way into S. America in
exceedingly remote ages from its headquarters in Eastern Asia. No
species survives in N. America, and a single straggler is found in
Porto Rico. The genera _Macroceramus_, _Cylindrella_, and _Strophia_,
are characteristic West Indian forms, which are only slightly
represented on the mainland. _Homalonyx_, a curious form akin to
_Succinea_, is peculiar to the region.

  [Illustration: FIG. 228.--_Homalonyx unguis_ Fér., Demerara.
  _sh_, Shell (shown also separate); _p.o_, pulmonary orifice.]

Land operculates attain a most extraordinary development in the Greater
Antilles, and constitute, in some cases, nearly one-half of the whole
Molluscan fauna. Several groups of the Cyclostomatidae find their
headquarters here, and some spread no farther. On the mainland this
prominence does not continue. West Indian influence is felt in Central
America and on the northern coast district, and some Antillean genera
make their way as far as Ecuador. The whole group entirely disappears
in Chili and Argentina, becoming scarce even in Brazil.

Among the fresh-water operculates, _Ampullaria_ is abundant, and
widely distributed. _Vivipara_, so characteristic of N. America, is
entirely absent. _Chilina_, a remarkable fresh-water pulmonate, akin
to _Limnaea_, is peculiar to Chili, Patagonia, and Southern Brazil,
but is not found in the tropical portion of the continent. Of the
fresh-water Pelecypoda _Mycetopus_, _Hyria_, _Castalia_, _Leila_, and
_Mülleria_ are peculiar forms, akin to the Unionidae.

(1) =The Antillean Sub-region= surpasses all other districts in the
world in respect of (1) extraordinary abundance of species, (2) sharp
definition of limits as a whole, (3) extreme localisation of the
fauna of the separate islands. The sub-region includes the whole of
the half-circle of islands from the Bahamas to Grenada, together with
the extreme southern end of the peninsula of Florida, which was once,
no doubt, a number of small islands like the Bahamas. Trinidad, and
probably Tobago, although containing an Antillean element, belong to
the mainland of S. America, from which they are only separated by very
shallow water.

The sub-region appears to fall into four provinces:--

(_a_) Cuba, the Bahamas, and S. Florida; (_b_) Jamaica; (_c_) San
Domingo (Haiti), Porto Rico, and the Virgin Is., with the Anguilla and
St. Bartholomew group; (_d_) the islands from Guadeloupe to Grenada.
The first three provinces contain the mass of the characteristic
Antillean fauna, the primary feature being the extraordinary
development of the land operculates, which here reaches a point
unsurpassed in any other quarter of the globe. The relative numbers are
as follows:--

                 Cuba  Jamaica  San Domingo  Porto Rico

    Inoperculate  362    221       152            75
    Operculate    252    242       100            23

It appears, then, that the proportion of operculate to inoperculate
species, while very high in Cuba (about 41 per cent of the whole),
reaches its maximum in Jamaica (where the operculates are actually in
a majority), begins to decline in San Domingo (about 40 per cent), and
continues to do so in Porto Rico, where they are not more than 24 per
cent of the whole. These operculates almost all belong to the families
Cyclostomatidae and Helicinidae, only two genera (_Aperostoma_ and
_Megalomastoma_) belonging to the _Cyclophorus_ group. Comparatively
few genera are absolutely peculiar to the islands, one or two species
of most of them occurring in Central or S. America, but of the several
hundreds of operculate species which occur on the islands, not two
score are common to the mainland.

  [Illustration:

    Map to illustrate the
    GEOGRAPHICAL DISTRIBUTION
    of the Land Mollusca of the
    WEST INDIES.

  _The red line marks the 100 fathom line._

  London: Macmillan and C^ọ. _London: Stanford’s Geog^ḷ Estab^ṭ._]

The next special feature of the sub-region is a remarkable development
of peculiar sub-genera of _Helix_. In this respect the Antilles
present a striking contrast to both Central and S. America, where the
prime feature of the land Pulmonata is the profusion of _Bulimus_
and _Bulimulus_, and _Helix_ is relatively obscured. No less than 14
sub-genera of _Helix_, some of which contain species of almost unique
beauty and size, are quite peculiar to the Greater Antilles, and some
are peculiar to individual islands.

Here, too, is the metropolis of _Cylindrella_ (of which there are 130
species in Cuba alone), a genus which just reaches S. America, and
has a few species along the eastern sea-board of the Gulf of Mexico.
_Macroceramus_ and _Strophia_ are quite peculiar; the former, a genus
allied to _Cylindrella_, which attains its maximum in Cuba and San
Domingo, is scarcely represented in Jamaica, and disappears south of
Anguilla; the latter, a singular form, resembling a large _Pupa_ in
shape, which also attains its maximum in Cuba, is entirely wanting
in Jamaica, and has its last representative in S. Croix. One species
irregularly occurs at Curaçao.

The carnivorous group of land Mollusca are represented by several
peculiar forms of _Glandina_, which attain their maximum in Jamaica and
Cuba, but entirely disappear in the Lesser Antilles.

A certain number of the characteristic N. American genera are found in
the Antillean Sub-region, indicating a former connexion, more or less
intimate, between the W. Indies and the mainland. The genera are all of
small size. The characteristic N. American _Hyalinia_ are represented
in Cuba, San Domingo, and Porto Rico; among the _Helicidae_, _Polygyra_
reaches Cuba, but no farther, and _Strobila_ Jamaica. The fresh-water
Pulmonata are of a N. American type, as far as the Greater Antilles
are concerned, but the occurrence of _Gundlachia_ (Tasmania and
Trinidad only) in Cuba is an unexplained problem at present. Unionidae
significantly occur only at the two ends of the chain of islands, not
reaching farther than Cuba (_Unio_ 3 sp.) at one end, and Trinidad
(which is S. American) at the other.

A small amount of S. American influence is perceptible throughout the
Antilles, chiefly in the occurrence of a few species of _Bulimulus_
and _Simpulopsis_. The S. American element may have strayed into
the sub-region by three distinct routes: (1) by way of Trinidad,
Tobago, and the islands northward; (2) by a north-easterly extension
of Honduras towards Jamaica, forming a series of islands of which the
Rosalind and Pedro banks are perhaps the remains; (3) by a similar
approximation of the peninsula of Yucatan and the western extremity
of Cuba. Central America is essentially S. American in its fauna, and
the characteristic genera of Antillean operculates which occur on its
eastern coasts are sufficient evidence of the previous existence of a
land connexion more or less intimate (see map).

(_a_) _Cuba_ is by far the richest of the Antilles in land Mollusca,
but it must be remembered that it is also much better explored than San
Domingo, the only island likely to rival it in point of numbers. It
contains in all 658 species, of which 620 are land and 38 fresh-water,
the land operculates alone amounting to 252.

Carnivorous genera form but a small proportion of the whole. There are
18 _Glandina_ (which belong to the sections _Varicella_ and _Boltenia_)
and 4 _Streptostyla_, the occurrence of this latter genus being
peculiar to Cuba and Haiti (1 sp.) among the Antilles, and associating
them closely with the mainland of Central America, where _Streptostyla_
is abundant. These two genera alone represent the Agnatha throughout
the sub-region.

There are no less than 84 species of _Helix_, belonging to 12
sub-genera. Only one of these (_Polymita_) is quite peculiar to Cuba,
but of 7 known species of _Jeanerettia_ and 8 of _Coryda_, 6 and 7
respectively are Cuban. _Thelidomus_ has 15 species (Jamaica 3, Porto
Rico 3); _Polydontes_ has 3, the only other being from Porto Rico;
_Hemitrochus_ has 12 (Jamaica 1, Bahamas 6); _Cysticopsis_ 9 (Jamaica
6); _Eurycampta_ 4 (Bahamas 1).

The Cylindrellidae find their maximum development in Cuba. As many as
34 _Macroceramus_ occur (two-thirds of the known species), and 130
_Cylindrella_, some of the latter being most remarkable in form (see
Fig. 151, B, p. 247).

The land operculates belong principally to the families Cyclostomatidae
and Helicinidae. Of the former, Cuba is the metropolis of _Ctenopoma_
and _Chondropoma_, the former of which includes 30 Cuban species, as
compared with 1 from San Domingo and 2 from Jamaica. _Megalomastoma_
(Cyclophoridae) is also Haitian and Porto Rican, but not Jamaican.
_Blaesospira_, _Xenopoma_, and _Diplopoma_ are peculiar. The
Helicinidae consist mainly of _Helicina_ proper (58 sp.), which here
attains by far its finest development in point of size and beauty,
and of _Eutrochatella_ (21 sp.), which is peculiar to the three great
islands (Jamaica 6 sp., San Domingo 6 sp.).

The _Bahamas_, consisting in all of more than 700 islands, are very
imperfectly known, but appear to be related partly to Cuba, partly to
San Domingo, from each of which they are separated by a narrow channel
of very deep water. They are certainly not rich in the characteristic
groups of the Greater Antilles. The principal forms of _Helix_ are
_Plagioptycha_ (6 sp.), common with San Domingo, and _Hemitrochus_
(6 sp.), common with Cuba. _Strophia_ is exceedingly abundant, but
_Cylindrella_, _Macroceramus_, and _Glandina_ have but few species.
There are a few species of _Ctenopoma_, _Chondropoma_, and _Cistula_,
while a single _Schasicheila_ (absent from the rest of the sub-region)
forms a link with Mexico.

  [Illustration: FIG. 229.--Characteristic Cuban Helices. =A=,
  _Polydontes imperator_ Montf. =B=, _Caracolus rostrata_ Pfr. =C=,
  _Polymita muscarum_ Lea.]

_Southern Florida_, with one or two species each of _Hemitrochus_,
_Cylindrella_, _Macroceramus_, _Strophia_, _Ctenopoma_, and
_Chondropoma_, belongs to this province.

(_b_) _Jamaica._--The land Mollusca of Jamaica are, in point of numbers
and variety, quite unequalled in the world. There are in all as many
as 56 genera and more than 440 species, the latter being nearly all
peculiar. The principal features are the Glandinae, the Helicidae,
and the land operculates. The Glandinae belong principally to the
sub-genera _Varicella_, _Melia_, and _Volutaxis_, _Streptostyla_ being
absent, although occurring in Cuba and San Domingo. There are 10 genera
of _Helix_, of which _Pleurodonta_ is quite peculiar, while _Sagda_
(13 sp.) is common only with S.W. San Domingo (2 sp.), and _Leptoloma_
(8 sp.) only with Cuba (1 sp.). The single _Strobila_ seems to be a
straggler from a N. American source. _Macroceramus_ has only 2 species
as against 34 in Cuba, and of _Cylindrella_, in which Cuba (130 sp.) is
so rich, only 36 species occur. The genus _Leia_, however (14 sp.), is
all but peculiar, occurring elsewhere only in the neighbouring angle
of San Domingo, which is so closely allied with Jamaica. The complete
absence of _Strophia_ is remarkable.

  [Illustration: FIG. 230.--Characteristic Jamaican and Haitian
  Mollusca: =A=, _Sagdae pistylium_ Müll., Jamaica; =B=,
  _Chondropoma salleanum_ Pfr., San Domingo; =C=, _Eutrochatella
  Tankervillei_ Gray, Jamaica; =D=, _Cylindrella agnesiana_ C. B.
  Ad., Jamaica.]

The land operculates form the bulk of the land fauna, there being
actually 242 species, as against 221 of land Pulmonata, a proportion
never again approached in any part of the world. As many as 80 of
these belong to the curious little genus _Stoastoma_, which is all but
peculiar to the island, one species having been found in San Domingo,
and one in Porto Rico. _Geomelania_ and _Chittya_, two singular inland
forms akin to _Truncatella_, are quite peculiar. _Alcadia_ reaches
its maximum of 14 species, as against 4 species in San Domingo and 9
species in Cuba, and _Lucidella_ is common to San Domingo only; but, if
_Stoastoma_ be omitted, the Helicinidae generally are not represented
by so many or by so striking forms as in Cuba, which has 90 species, as
against Jamaica 44, and San Domingo 35.

(_c_) _San Domingo_, although not characterised by the extraordinary
richness of Cuba and Jamaica, possesses many specially remarkable forms
of land Mollusca, to which a thorough exploration, when circumstances
permit, will no doubt make important additions. From its geographical
position, impinging as it does on all the islands of the Greater
Antilles, it would be expected that the fauna of San Domingo would not
exhibit equal signs of isolation, but would appear to be influenced
by them severally. This is exactly what occurs, and San Domingo is
consequently, although very rich in peculiar species, not equally so in
peculiar genera. The south-west district shows distinct relations with
Jamaica, the Jamaican genera _Leia_, _Stoastoma_, _Lucidella_, and the
_Thaumasia_ section of _Cylindrella_ occurring here only. The north and
north-west districts are related to Cuba, while the central district,
consisting of the long band of mountainous country which traverses the
island, contains the more characteristic Haitian forms.

The Helicidae are the most noteworthy of the San Domingo land
Mollusca. The group _Eurycratera_, which contains some of the finest
existing land snails, is quite peculiar, while _Parthena_, _Cepolis_,
_Plagioptycha_, and _Caracolus_ here reach their maximum. The
Cylindrellidae are very abundant, but no section is peculiar. Land
operculates do not bear quite the same proportion to the Pulmonata
as in Cuba and Jamaica, but they are well represented (100 to 152);
_Rolleia_ is the only peculiar genus.

The relations of San Domingo to the neighbouring islands are
considerably obscured by the fact that they are well known, while San
Domingo is comparatively little explored. To this may perhaps be due
the curious fact that there are actually more species common to Cuba
and Porto Rico (26) than to Porto Rico and San Domingo. Cuba shares
with San Domingo its small-sized _Caracolus_ and also _Liguus_, but
the great _Eurycratera_, _Parthena_, and _Plagioptycha_ are wholly
wanting in Cuba. The land operculates are partly related to Cuba,
partly to Jamaica, thus _Choanopoma_, _Ctenopoma_, _Cistula_, _Tudora_,
and many others, are represented on all these islands, while the
Jamaican _Stoastoma_ occurs on San Domingo and Porto Rico, but not
on Cuba, and _Lucidella_ is common to San Domingo and Jamaica alone.
An especial link between Jamaica and San Domingo is the occurrence
in the south-west district of the latter island of _Sagda_ (2 sp.).
The relative numbers of the genera _Strophia_, _Macroceramus_, and
_Helicina_, as given below (p. 351), are of interest in this connexion.

_Porto Rico_, with Vièque, is practically a fragment of San Domingo.
The points of close relationship are the occurrence of _Caracolus_,
_Cepolis_, and _Parthena_ among the Helicidae, and of _Simpulopsis_,
_Pseudobalea_, and _Stoastoma_. _Cylindrella_ and _Macroceramus_
are but poorly represented, but _Strophia_ still occurs. The land
operculates (see the Table) show equal signs of removal from the
headquarters of development. _Megalomastoma_, however, has some
striking forms. The appearance of a single _Clausilia_, whose nearest
relations are in the northern Andes, is very remarkable. _Gaeotis_,
which is allied to _Peltella_ (Ecuador only), is peculiar.

   [Illustration: FIG. 231.--Examples of West Indian Helices: =A=,
  _Helix_ (_Parthena_) _angulata_ Fér., Porto Rico; =B=, _Helix_
  (_Thelidomus_) _lima_ Fér., Vièque; =C=, _Helix_ (_Dentellaria_)
  _nux denticulata_ Chem., Martinique.]


                _Land Mollusca of the Greater Antilles_

                  Cuba.  Jamaica.  S. Domingo.  Porto Rico.

    Glandina       18       24         15            8
    Streptostyla    4      ...          2          ...
    Volutaxis     ...       11 (?)      1          ...
    Selenites       1      ...        ...          ...
    Hyalinia        4       11          5            6
    Patula          5        1        ...          ...
    Sagda         ...       13          2          ...
    Microphysa      7       18          8            3
    Cysticopsis     9        6        ...          ...
    Hygromia (?)  ...      ...          3          ...
    Leptaxis (?)  ...      ...          1          ...
    Polygyra        2      ...        ...          ...
    Jeanerettia     6      ...        ...            1
    Euclasta      ...      ...        ...            4
    Plagioptycha  ...      ...         14            2
    Strobila      ...        1        ...          ...
    Dialeuca      ...        1        ...          ...
    Leptoloma       1        8        ...          ...
    Eurycampta      4      ...        ...          ...
    Coryda          7      ...        ...          ...
    Thelidomus     15        3        ...            3
    Eurycratera   ...      ...          7          ...
    Parthena      ...      ...          2            2
    Cepolis       ...      ...          3            1
    Caracolus       8      ...          6            2
    Polydontes      3      ...        ...            1
    Hemitrochus    12        1        ...          ...
    Polymita        5      ...        ...          ...
    Pleurodonta   ...       34        ...          ...
    Inc. sed.       5      ...        ...          ...
    Simpulopsis   ...      ...          1            1
    Bulimulus       3        3          6            7
    Orthalicus      1        1        ...          ...
    Liguus          3      ...          1          ...
    Gaeotis       ...      ...        ...            3
    Pineria         2      ...        ...            1
    Macroceramus   34        2         14            3
    Leia          ...       14          2          ...
    Cylindrella   130       36         35            3
    Pseudobalea     2      ...          1            1
    Stenogyra       6        7        (?)          ...
    Opeas           8      (?)          4            6
    Subulima        6       14          2            2
    Glandinella     1      ...        ...          ...
    Spiraxis        2      (?)          2            1
    Melaniella      7      ...        ...          ...
    Geostilbia      1      ...          1          ...
    Cionella        2      ...        ...          ...
    Leptinaria    ...        1        ...            3
    Obeliscus     ...      ...          1            2
    Pupa            2        7          3            2
    Vertigo         4      ...        ...          ...
    Strophia       19      ...          3            2
    Clausilia     ...      ...        ...            1
    Succinea       11        2          5            3
    Vaginula        2        2          2            1
    Megalomastoma  13      ...          1            3
    Neocyclotus     1       33(?)     ...          ...
    Licina          1      ...          3          ...
    Jamaicia      ...        2        ...          ...
    Crocidopoma   ...        1          3          ...
    Rolleia       ...      ...          1          ...
    Choanopoma     25       12         19            3
    Ctenopoma      30        2          1          ...
    Cistula        15        3          3            3
    Chondropoma    57      (?)         19            4
    Tudora          7       17          5          ...
    Adamsiella      1       12        ...          ...
    Blaesospira     1      ...        ...          ...
    Xenopoma        1      ...        ...          ...
    Cistula        15        3          3          ...
    Colobostylus    4       13          5          ...
    Diplopoma       1      ...        ...          ...
    Geomelania    ...       21        ...          ...
    Chittya       ...        1        ...          ...
    Blandiella    ...      ...          1          ...
    Stoastoma     ...       80          1            1
    Eutrochatella  21        6          6          ...
    Lucidella     ...        4          1          ...
    Alcadia         9       14          4          ...
    Helicina       58       16         24            9
    Proserpina      2        4        ...          ...

The Virgin Is., with St. Croix, Anguilla, and the St. Bartholomew group
(all of which are non-volcanic islands), are related to Porto Rico,
while Guadeloupe and all the islands to the south, up to Grenada (all
of which are volcanic), show marked traces of S. American influence.
St. Kitt’s, Antigua, and Montserrat may be regarded as intermediate
between the two groups. St. Thomas, St. John, and Tortola have each
one _Plagioptycha_ and one _Thelidomus_, while St. Croix has two
sub-fossil _Caracolus_ which are now living in Porto Rico, together
with one _Plagioptycha_ and one _Thelidomus_ (sub-fossil). The gradual
disappearance of some of the characteristic greater Antillean forms,
and the appearance of S. American forms in the Lesser Antilles, is
shown by the following table:--

    +-----------------------+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    |                       |   |   |   |   |   |   |   |   |   |   |   |   |   | S |   |   |
    |                       | P | S |   |   |   |   | S |   | G |   | M |   |   | t |   |   |
    |                       | o | t |   | S |   |   | t |   | u |   | a | S |   | . |   |   |
    |                       | r | . |   | t |   | A | . |   | a | D | r | t | B |   |   | T |
    |                       | t |   | S | . | T | n |   | A | d | o | t | . | a | V | G | r |
    |                       | o | T | t |   | o | g | K | n | e | m | i |   | r | i | r | i |
    |                       |   | h | . | C | r | u | i | t | l | i | n | L | b | n | e | n |
    |                       | R | o |   | r | t | i | t | i | o | n | i | u | a | c | n | i |
    |                       | i | m | J | o | o | l | t | g | u | i | q | c | d | e | a | d |
    |                       | c | a | a | i | l | l | ’ | u | p | c | u | i | o | n | d | a |
    |                       | o | s | n | x | a | a | s | a | e | a | e | a | s | t | a | d |
    |                       | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . |
    +-----------------------+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | Bulimulus             | 7 | 4 | 2 | 4 | 1 | 2 | 2 | 3 | 8 | 9 | 5 | 3 | 3 | 6 | 2 | 4 |
    | Cylindrella           | 3 | 2 | 1 | 1 | 1 | . | . | . | 1 | . | 1 | 1 | 1 | . | . | 1 |
    | Macroceramus          | 3 | 1 | 1 | . | 2 | 1 | . | . | . | . | . | . | . | . | . |   |
    | Cyclostomatidae, etc. |23 | 4 | 1 | 5 | 1 | 1 | 1 | . | 4 | . | . | . | . | . | . | 1 |
    | Dentellaria           | . | . | . | . | . | . | 1 | 1 | 8 | 5 |11 | 2 | 2 | . | 1 | 1 |
    | Cyclophorus           | . | . | . | . | . | . | . | . | 1 | 2 | 2 | . | . | . | . | . |
    | Amphibulimus          | . | . | . | . | . | . | . | . | 2 | 3 | 1 | . | . | . | . | . |
    | Homalonyx             | . | . | . | . | . | . | . | . | 1 | 1 | . | . | . | . | . | . |
    +---------------------------------------------------------------------------------------+

(_d_) In Guadeloupe we find _Cyclophorus_, _Amphibulimus_, _Homalonyx_,
and _Pellicula_, which are characteristic of S. America, and nearly
all recur in Dominica and Martinique. These islands are the metropolis
of _Dentellaria_, a group of _Helix_, evidently related to some of the
forms developed in the Greater Antilles. Stragglers occur as far north
as St. Kitt’s and Antigua, and there are several on the mainland as
far south as Cayenne. Traces of the great _Bulimus_, so characteristic
of South America, occur as far north as S. Lucia, where also is found
a _Parthena_ (San Domingo and Porto Rico). Trinidad is markedly S.
American; 55 species in all are known, of which 22 are peculiar, 28
are common to S. America (8 of these reach no farther north along the
islands), and only 5 are common to the Antilles, but not to S. America.
The occurrence of _Gundlachia_ in Trinidad has already been mentioned.

_The Bermudas_ show no very marked relationship either to the N.
American or to the West Indian fauna. In common with the former they
possess a _Polygyra_, with the latter (introduced species being
excluded) one species each of _Hyalosagda_, _Subulina_, _Vaginula_, and
_Helicina_, so that, on the whole, they may be called West Indian. The
only peculiar group is _Poecilozonites_, a rather large and depressed
shell of the _Hyalinia_ type.

(2) =The Central American Sub-region= may be regarded as extending
from the political boundary of Mexico in the north to the isthmus of
Panama in the south. It thus impinges on three important districts--the
N. American, West Indian, and S. American; and it appears, as we
should perhaps expect, that the two latter of these regions have
considerably more influence upon its fauna than the former. Of the
N. American Helicidae, _Polygyra_ is abundant in Mexico only, and
two species of _Strobila_ reach N. Guatemala, while the Californian
_Arionta_ occurs in Mexico. S. American Helicidae, in the sub-genera
_Solaropsis_ and _Labyrinthus_, occur no farther north than Costa Rica.
Not a single representative of any of the characteristic West Indian
Helicidae occurs. _Bulimulus_ and _Otostomus_, which form so large a
proportion of the Mollusca of Venezuela, Colombia, Ecuador, and Peru,
together with _Orthalicus_, are abundant all over the region. Again,
_Cylindrella_, _Macroceramus_, and some of the characteristic Antillean
operculates, are represented, their occurrence being in most cases
limited to the eastern coast-line and eastern slope of the central
range.

Besides these external elements, the region is rich in indigenous
genera. Central America is remarkable for an immense number of large
carnivorous Mollusca possessing shells. There are 49 species of
_Glandina_, the bulk of which occur in eastern and southern Mexico; 36
of _Streptostyla_ (S.E. Mexico and Guatemala, only 1 species reaching
Venezuela and another Peru); 5 of _Salasiella_, 2 of _Petenia_, and 1
of _Strebelia_; the last three genera being peculiar. _Streptaxis_,
fairly common in S. America, does not occur. _Velifera_ and
_Cryptostracon_, two remarkable slug-like forms, each with a single
species, are peculiar to Costa Rica. Among the especial peculiarities
of the region are the giant forms belonging to the Cylindrellidae,
which are known as _Holospira_, _Eucalodium_, and _Coelocentrum_ (Fig.
232). They are almost entirely peculiar to Mexico, only 7 out of a
total of 33 reaching south of that district, and only 1 not occurring
in it at all.

  [Illustration: FIG. 232.--Examples of characteristic Mexican
  Mollusca: =A=, _Coelocentrum turris_ Pfr.; =B=, _Streptostyla
  Delattrei_ Pfr.]

The land operculates are but scanty. _Tomocyclus_ and _Amphicyclotus_
are peculiar, and _Schasicheila_, a form of _Helicina_, occurs
elsewhere only in the Bahamas. _Ceres_ (see Fig. 18, C, p. 21) and
_Proserpinella_, two remarkable forms of non-operculate Helicinidae
(compare the Chinese _Heudeia_), are quite peculiar. _Pachychilus_, one
of the characteristic fresh-water genera, belongs to the S. American
(Melaniidae) type, not to the N. American (Pleuroceridae). Among the
fresh-water Pulmonata, the _Aplecta_ are remarkable for their great
size and beauty. In the accompanying table “Mexico” is to be taken as
including the region from the United States border up to and including
the isthmus of Tehuantepec, and “Central America” as the whole region
south of that point.


                  _Land Mollusca of Central America_

                    Mexico only.  Central   Common to
                                  America   both.
                                  only.

    Strebelia            1          ...        ...
    Glandina            33           13          3
    Salasiella           4          ...          1
    Streptostyla        18           12          6
    Petenia            ...            1          1
    Limax              ...            1        ...
    Velifera           ...            1        ...
    Omphalina           10            1          1
    Hyalinia             2            5          3
    Guppya             ...            8          3
    Pseudohyalina        2          ...          2
    Tebennophorus        1          ...        ...
    Cryptostracon      ...            1        ...
    Xanthonyx            4          ...        ...
    Patula               3          ...          4
    Acanthinula          1            2          2
    Vallonia           ...            1        ...
    Trichodiscus         2            2          3
    Praticolella         1          ...          1
    Arionta              3          ...        ...
    Lysinoe              1            1          1
    Oxychona             2            5        ...
    Solaropsis         ...            2        ...
    Polygyra            14            1          2
    Strobila             1            1        ...
    Labyrinthus        ...            5        ...
    Otostomus           23           20          7
    Bulimulus            6            5          2
    Berendtia            1          ...        ...
    Orthalicus           6            3          3
    Pupa                 1            1          1
    Vertigo              1          ...        ...
    Holospira           12          ...        ...
    Coelocentrum         6            1          1
    Eucalodium          15          ...          5
    Cylindrella          6            4        ...
    Macroceramus         2            1        ...
    Simpulopsis          2            1        ...
    Caecilianella        1          ...        ...
    Opeas                1            2          3
    Spiraxis             8            2          1
    Leptinaria         ...            2        ...
    Subulina             2            3          4
    Succinea            11            3          1
    Vaginula             1          ...        ...
    Aperostoma         ...            4          1
    Amphicyclotus        2            1          2
    Cystopoma            2          ...        ...
    Tomocyclus         ...            1          2
    Choanopoma           2            2        ...
    Chondropoma          2           11        ...
    Helicina            13           10          6
    Schasicheila         2          ...          1
    Ceres                2          ...        ...
    Proserpinella        1          ...        ...

(3) =The Colombian Sub-region= includes Colombia, New Grenada,
Venezuela, Guiana, Ecuador, Peru, and Bolivia. It has been usual to
separate off the two latter countries as forming a distinct “Peruvian”
sub-region; but there is, as will be seen, absolutely no line to be
drawn between the Mollusca of Peru and those of Ecuador; nor would
one, on geographical considerations, expect to be able to draw such
a line. A better method of subdivision, so far as the species of the
whole eastern portion of the region are concerned, would be to group
the Mollusca according to the altitude at which they occur, were it not
that the evidence on this point is at present but fragmentary. We know,
however, that all along the line of the Andes certain species, more
particularly of _Bulimulus_, occupy their own zones of elevation, some
ascending as high as 10,000 feet above the sea, and never occurring on
the plains.

In the northern portions of this sub-region, Central American and West
Indian influence is felt to a certain extent. Thus there are eight
_Glandina_ and one _Streptostyla_ in Venezuela and Colombia together
with one or two species of _Cistula_, _Chondropoma_, _Proserpina_, and
_Cylindrella_, while a single _Strophia_ (decidedly a straggler) occurs
at Curaçao. In Demerara and Cayenne there are three or four species
of _Dentellaria_. In Ecuador, however, _Glandina_ diminishes to three
species, and in Peru disappears altogether, although one _Streptostyla_
occurs. Similarly the West Indian operculates are reduced to one
_Chondropoma_ (Ecuador) and disappear entirely in Peru.

  [Illustration: FIG. 233.--=A=, _Orthalicus Deburghiae_ Reeve,
  Ecuador; =B=, _Bulimus_ (_Pachyotus_) _egregius_ Jay, Brazil.]

The Helicidae are most abundant in the north and west, and are
represented by several very striking sub-genera, some of which possess
remarkably toothed apertures, and perhaps betray an ancestry common to
some of the West Indian genera. Of these, _Labyrinthus_ has 12 species
in Venezuela and Colombia, 5 in Ecuador, and 3 in Peru and Bolivia;
_Isomeria_ 12 in Venezuela and Colombia, 20 in Ecuador, and 2 in Peru
and Bolivia; _Salaropsis_ is represented in these countries by 6, 3,
and 7 species, and _Systrophia_ by 4, 5, and 8 species respectively.

_Clausilia_--in the group _Nenia_--appears in some numbers along the
Andes chain, the only other representative in the New World being the
solitary species occurring at Porto Rico. There have been described,
from Venezuela and Colombia 10 species, from Ecuador 5, and from Peru
and Bolivia 12.

Another marked feature of the region is the occurrence of the
Orthalicidae, in the two genera _Orthalicus_ and _Porphyrobaphe_.
The latter of these magnificent forms is peculiar, while the former
reaches Mexico, the West Indies, and Brazil. Ecuador, which contains 23
species, seems the metropolis of the group.

_Bulimus_ and _Bulimulus_, the former genus being peculiar to S.
America and the adjacent islands, are largely represented, the former
in the three groups _Borus_, _Dryptus_, and _Orphnus_. These attain
their maximum in Peru, with 25 species, but Venezuela and Colombia have
as many as 17. _Bulimulus_ has been subdivided into a number of groups,
_e.g._ _Drymaeus_, _Mesembrinus_, _Thaumastus_, _Mormus_, _Scutalus_,
with many others,--the exact scientific limits of which are not easily
discernible. It must suffice here to state that Peru seems to be the
headquarters of the group with about 190 species (which probably may
well be reduced), Ecuador having about 70, and Venezuela and Colombia
between 80 and 90.

  [Illustration: FIG. 234.--_Rhodea gigantea_ Mouss., New Grenada.]

Two very remarkable forms belonging to the Pupidae, _Anostoma_ (Fig.
154, p. 248) and _Tomigerus_, occur in Venezuela, the metropolis.
_Rhodea_, another very peculiar shell (Fig. 234), whose exact family
position is uncertain, is peculiar to New Grenada. The land operculates
are few in number, and in Bolivia almost disappear. They belong
principally to _Neocyclotus_ (of which 11 species occur in Venezuela
and Colombia) and _Helicina_ (10 species in the same district),
besides the stragglers already mentioned from West Indian sources, and
a few _Cyclophorus_. _Bourcieria_ is a form of _Helicina_ peculiar
to Ecuador. _Ampullaria_, with _Ceratodes_, a peculiar planorbiform
sub-genus, and _Hemisinus_, form the bulk of the fresh-water
operculates.

_The Galapagos._--Thirty-four species of land Mollusca, all peculiar,
are known from these islands; 25 of these are forms of _Bulimulus_.
There are no Helicidae, one each of _Hyalinia_, _Leptinaria_, and
_Helicina_, and two _Pupa_. The _Bulimulus_ are mostly of the group
_Nesiotis_, and in their brown colour bear some outward resemblance to
the dark _Achatinella_ of the Sandwich Is., living as they do mostly
under scoriae on the ground, and not on trees. In type, however, they
appear to be derived from Chili and Peru, rather than from the parts
of S. America immediately contiguous. Another section (_Pleuropyrgus_
2 sp.) closely resembles a marine _Chemnitzia_. The islands are all
volcanic, and are probably not the result of subsidence; thus the
existing species are not to be regarded as the relics of a more
widespread fauna, but as a new set of inhabitants.

(4) =The Brazilian Sub-region.=--This immense district is very
little known, except in the south, and it is consequently impossible
to give any satisfactory account of its Mollusca. It is possible
that eventually it will be found that it falls into provinces
which correspond more or less to (_a_) the Amazon basin; (_b_) the
mountainous district in the east, drained by the Tocantins and the San
Francisco; (_c_) the Parana basin in the south central district; and
(_d_) the Argentine or Pampas district in the extreme south. But at
present the data are insufficient to establish any such subdivisions,
whose existence, if proved, would have an important bearing on the
problem of the coalescence of S. America into its present form.[379]

The Agnatha are represented by _Streptaxis_ alone (17 sp.). _Helix_ is
rare, but includes the peculiar _Polygyratia_ (Fig. 150 A, p. 246),
while _Labyrinthus_ (2 sp.), _Solaropsis_ (5 sp.), and _Systrophia_
are common with the Colombian Sub-region, and _Oxychona_ (4 sp.) with
the Central American. _Bulimus_ has in all 36 species, the sub-genera
_Pachyotus_ (Fig. 233) and _Strophochilus_ being peculiar. _Bulimulus_,
though not so abundant as in Peru and Ecuador, has about 60 species, of
which _Navicula_ (Fig. 235) is the most remarkable group. _Megaspira_
is peculiar. _Orthalicus_ has only 4 species, while _Tomigerus_ (4 sp.)
and _Anostoma_ (3 sp.) are common with Venezuela. Land operculates are
scarce, and appear to include only _Neocyclotus_, _Cyclophorus_, and
_Helicina_.

In Argentina, which may probably rank as a separate province, the
tropical forms greatly decrease, _Streptaxis_ being reduced to
2 species, and _Bulimus_ and _Bulimulus_ together to 40, while
_Orthalicus_, the great Helices, and the land operculates disappear
altogether. _Odontostomus_ (Fig. 236), a genus of the Pupidae, is
abundant in the northern part of the province. Two or three species of
_Chilina_ occur.

  [Illustration: FIG. 235.--_Bulimulus_ (_Navicula_) _navicula_
  Wagn., Brazil.]

  [Illustration: FIG. 236.--_Odontostomus pantagruelinus_ Moric.,
  S. Brazil. × ½.]

(5) =The Chilian Sub-region.=--The greater part of Chili, from its
arid and rainless climate, is unfavourable to the existence of land
Mollusca. _Bulimus_ (_Borus_) still has 3 or 4 species, and _Bulimulus_
(_Plectostylus_ 11, _Scutalus_ 9, _Peronaeus_ 7) is fairly abundant,
but the profusion of the tropics is wanting. There are no carnivorous
genera, and only two land operculates. A remarkable form of _Helix_
(_Macrocyclis_, Fig. 237) is quite peculiar, but the majority of
the species belong to two rather obscure groups, _Stepsanoda_ and
_Amphidoxa_. _Chilina_, a singularly solid form of _Limnaea_ (of which
8 sp., with a sub-genus _Pseudochilina_, occur in Chili), is peculiar
to Chili, S. Brazil, and Patagonia. From the two islands of _Juan
Fernandez_ and _Masafuera_, are known several _Helix_, of Chilian
affinity, several curious _Succinea_, a _Homalonyx_, _Leptinaria_,
and _Nothus_, and three species of _Tornatellina_, with the almost
universal _Limax gagates_.

The question of the existence at some remote period of a Neantarctic
continent, which formed a communication between the three great
southern peninsulas of the world, is one on which the Mollusca may
offer evidence. Von Ihering holds that an essential difference can be
observed between certain of the Unionidae which inhabit S. America,
Africa, and Australia with New Zealand, and those which inhabit
Europe, Asia, and N. America, but the point can hardly be regarded as
definitely established at present. Something perhaps may be made of
the distribution of _Bulimus_ and _Bulimulus_. It seems difficult to
explain the occurrence of sub-fossil _Bulimus_ on St. Helena except
on some such lines as have been recently adduced to account for the
presence of struthious birds in the Mascarenes, and possibly the
form _Livinhacea_ may be a trace of the same element in S. Africa.
Again, the _Liparus_ of S. and W. Australia, with the _Caryodes_ of
Tasmania, and the _Leucotaenia_ and _Clavator_ of Madagascar (which
all may be related to _Bulimus_), together with the _Placostylus_ of
New Caledonia and the adjacent islands, reaching even to New Zealand,
and perhaps even the _Amphidromus_ of Malaysia (which are more akin to
_Bulimulus_), may be thought to exhibit, in some remote degree, traces
of a common ancestry.

The land operculates give no help, and, of the carnivorous genera,
_Rhytida_ is a marked link between Africa and Australia, while
_Streptaxis_ is equally so between S. America and Africa. As regards
fresh-water Gasteropoda, _Ampullaria_ is common to S. America and
Africa, while _Isidora_ is common to Africa, Australia, and New
Zealand, but is altogether absent from S. America. _Gundlachia_ occurs
in Florida, Trinidad, and Tasmania, but has not been detected in
Africa. It must be concluded, therefore, that the present state of
the evidence which the Mollusca can afford, while exhibiting certain
curious points of relationship between the three regions in question,
is insufficient to warrant any decided conclusion.

  [Illustration: FIG. 237.--_Macrocyclis laxata_ Fér., Chili.]




                              CHAPTER XII

          DISTRIBUTION OF MARINE MOLLUSCA--DEEP-SEA MOLLUSCA
                       AND THEIR CHARACTERISTICS


Marine Mollusca may be divided roughly into Pelagic and non-Pelagic
genera. To the former division belong all Pteropoda and Heteropoda, and
a large number of Cephalopoda, together with a very few specialised
forms of Gasteropoda (_Ianthina_, _Litiopa_, _Phyllirrhoe_, etc.).
Pelagic Mollusca appear, as a rule, to live at varying depths below
the surface during the day, and to rise to the top only at night. The
majority inhabit warm or tropical seas, though some are exceedingly
abundant in the Arctic regions; _Clione_ and _Limacina_ have been
noticed as far north as 72°.[380]

The vertical range of Pelagic Mollusca has received attention from
Dr. Murray of the _Challenger_, Professor Agassiz of the _Blake_ and
_Albatross_, and others. Agassiz appears to have established the fact
that the surface fauna of the sea is limited to a comparatively narrow
belt of depth, and that there is no intermediate belt of animal life
between creatures which live on or near the bottom and the surface
fauna. Pelagic forms sink to avoid disturbances of various kinds,
to depths not much exceeding 150 to 200 fathoms, except in closed
seas like the Gulf of California and the Mediterranean, where the
bathymetrical range appears to be much greater.[381]

Non-Pelagic Mollusca are, from one point of view, conveniently
classified according to the different _zones of depth_ at which
they occur. Thus we are enabled to distinguish Mollusca of (_a_)
the _littoral_, (_b_) the _laminarian_, (_c_) the _nullipore_, or
_coralline_, and (_d_) the _abyssal_ zones. It must be borne in mind,
however, that these zones cannot be exactly defined, and that while the
littoral zone may be understood to imply the area between tide-marks,
and the abyssal zone a depth of 500 fathoms and upwards, the limits
between the laminarian and the coralline, and between the coralline and
abyssal zones can only be fixed approximately.

The difficulty of assigning special genera or species to special
‘zones of depth’ is increased by two important facts in the phenomena
of distribution. In the first place, it is found that species which
occur in shallow water in northern seas often extend to very deep
water in much lower latitudes. This interesting fact, which shows
the importance of temperature in determining distribution, was first
established by the dredgings of the _Lightning_ and _Porcupine_ off
the western coasts of Europe. In the second place, a certain number
of species seem equally at home in shallow and in abyssal waters, in
cases where a great difference of latitude does not occur to equalise
the temperature. Thus the _Challenger_ found _Venus mesodesma_ living
on the beach (New Zealand) and at 1000 fath. (Tristan da Cunha); _Lima
multicostata_ in ‘shallow water’ (Tonga and Port Jackson) and at 1075
fath. (Bermuda); _Scalaria acus_ from 49 to 1254 fath. (N. Atlantic);
and _S. hellenica_ from 40 to 1260 fath. (Canaries). The _Lightning_
and _Porcupine_ found, or record as found,[382] _Anomia ephippium_
at 0 to 1450 fath., _Pecten groenlandicus_ at 5 to 1785 fath., _Lima
subauriculata_ at 10 to 1785 fath., _Modiolaria discors_ at 0 to 1785
fath., _Crenella decussata_ at 0 to 1750 fath., _Dacrydium vitreum_
at 30 to 2750 fath., _Arca glacialis_ at 25 to 1620 fath., _Astarte
compressa_ at 3 to 2000 fath., and _Scrobicularia longicallus_ at 20 to
2435 fath. _Puncturella noachina_ has been found at 20 to 1095 fath.,
_Natica groenlandica_ at 2 to 1290 fath., _Rissoa tenuisculpta_ at 25
to 1095 fath. In many of these cases we are assured that no appreciable
difference can be detected between specimens from the two extremes of
depth.

In spite, however, of these remarkable vagaries on the part of certain
species, we are enabled roughly to distinguish a large number of
genera as ‘shallow-water’ and ‘deep-water’ respectively, while a still
larger number occupy an intermediate position. Among shallow-water
genera may be named _Patella_, _Littorina_, _Nassa_, _Purpura_,
_Strombus_, _Haliotis_, _Mytilus_, _Cardium_, _Solen_; while among
deep-water genera are _Pleurotoma_, _Scissurella_, _Seguenzia_,
_Dentalium_, _Cadulus_, _Limopsis_, _Nucula_, _Leda_, _Lima_, and
_Axinus_.

Theories on the geographical distribution of marine Mollusca have been
revolutionised by the discoveries of recent exploring expeditions.
The principal have been those of Torell (Swedish) (1859–61) on the
coasts of Greenland and Spitzbergen; of the _Lightning_ and _Porcupine_
(British) in 1868–70, in the N.E. Atlantic, off the Scotch, Irish,
French, and Portuguese coasts, and in the Mediterranean; of the
_Challenger_ (British), under Sir C. Wyville Thomson, in 1873–76,
in which all the great ocean basins were dredged or sounded; of the
_Blake_ (American), under Alexander Agassiz, in 1877–80, in the West
Atlantic, Gulf of Mexico, and Caribbean Seas; of the _Travailleur_
(French) in 1880–83, off the west coasts of France, Portugal, and
Morocco, Madeira, the Canaries, and the Golfe du Lion; of the
_Talisman_ (French) in 1882, off the west coast of Africa from Tangier
to Senegal, the Atlantic Islands, and the Sargasso Sea; of the
_Albatross_ (American) in 1891, off the west coast of tropical America;
of several other vessels belonging to the U.S. Fish Commission and
Coast Survey, off east American shores; and of the Prince of Monaco in
the _Hirondelle_ and _Princesse Alice_ at the present time, in the N.
Atlantic and Mediterranean.

The general result of these explorations has been to show that the
marine fauna of very deep water is much the same all the world
over, and that identical species occur at points as far removed as
possible from one another. The ocean floor, in fact, with its uniform
similarity of temperature, food, station, and general conditions of
life, contains no effectual barrier to the almost indefinite spread
of species.[383] To give a few instances. The _Challenger_ dredged
_Silenia Sarsii_ in 1950 fath., 1100 miles south-west of Australia,
and also in 2650 fath. off the mouth of the Rio de la Plata; _Semele
profundorum_ in 1125 fath. near the Canaries, and in 2900 fath. mid
N. Pacific; _Verticordia deshayesiana_ in 155 fath. near Cape York,
and in 350 fath. off Pernambuco; _Arca pteroessa_ in 2050 fath. mid N.
Pacific, in 1000–1675 fath. west of the Azores, and in 390 fath. off
the West Indies; _Arca corpulenta_ in 1400 fath. off N.E. Australia, in
2425 fath. mid-Pacific, and in 1375 fath. near Juan Fernandez; _Lima
goliath_ in 775 fath. off S. Japan, and in 245 fath. off S. Patagonia;
_Pleurotoma engonia_ in 700 fath. north-east of New Zealand, and in
345 fath. off Inoshima. A surprising range was occasionally found even
in shallow-water species; thus _Petricola lapicida_ was discovered
by the same expedition in the West Indies and N. Australia, _Cardita
calyculata_ off Teneriffe and in Bass Strait, _Arca imbricata_ off Cape
York and in the West Indies, _Modiolaria cuneata_ at Port Jackson and
Cape of Good Hope, _Lima squamosa_ at Teneriffe and the Philippines. In
these latter cases it is not improbable that the species lives in deep
water as well, from which it has not yet been dredged.

It follows from these considerations that any attempt to classify
marine Mollusca under Regions and Provinces can only apply to Mollusca
which occur at moderate depths. The most important factor in the
environment, as determining distribution, is the _temperature of the
water_, which is probably to be regarded as affecting not so much the
adult Mollusca as their ova; for the adult might possibly support
life under conditions in which the ova would perish. It appears that
a sudden change of temperature is the most effective barrier to
distribution,[384] and may bring the range of a species to an almost
instantaneous stop, while a very gradual change will allow it to extend
its range very widely.

It has been usual to classify marine Mollusca from moderate depths
under the following regions and sub-regions:--

         Regions         Sub-regions

                        {1. Arctic.
                        {2. Boreal.
    =A.= =Atlantic= and {3. Celtic.
       =Circumpolar=    {4. Lusitanian.
                        {5. West African.
                        {6. South African.

    =B.= =Indo-Pacific= {1. Indo-Pacific.
                        {2. Japanese.

    =C.= =Australian=   {1. Australian.
                        {2. Neozealanian.

                        {1. Aleutian.
                        {2. Californian.
                        {3. Panamic.
    =D.= =American=     {4. Peruvian.
                        {5. Magellanic.
                        {6. Argentinian.
                        {7. Caribbean.
                        {8. Transatlantic.


=A. The Atlantic Region=

includes the whole to the eastern shores of the Atlantic, from the
extreme north of the Cape of Good Hope, together with the circumpolar
seas, which may be regarded as roughly bounded by the Aleutian Islands
and the coast of Newfoundland.

(1) _The Arctic Sub-region_ includes the circumpolar seas, and is
bounded in the N. Pacific by a line drawn between Cape Avinoff in
Alaska, and Cape Lopatka in Kamchatka, so as to exclude the Aleutian
Islands. On the western shores of the Atlantic the cold Labrador
current brings it as far south as the coast of Newfoundland, but on
the eastern shores the influence of the Gulf Stream has the contrary
effect, so that the North Cape may be taken as its southern limit.

The principal genera (many species of which are common to the whole
sub-region) are _Volutomitra_, _Buccinum_, _Buccinopsis_, _Neptunea_,
_Trophon_, _Bela_, _Admete_, _Velutina_, _Trichotropis_, _Lacuna_,
_Margarita_, _Philine_, _Pecten_, _Leda_, _Yoldia_, _Astarte_, and
_Mya_. The shells are generally unicoloured, and of a dead white or
rather sombre tint.

(2) _The Boreal Sub-region_ may be subdivided into two provinces, the
European and the American. The former includes the entire coast-line
of Norway, the Färoe Islands, and Iceland (except perhaps the northern
coast), and possibly the Shetland Islands; the latter the American
coasts from the Gulf of St. Lawrence to Cape Cod (lat. 42°). Thus the
Boreal American province does not extend nearly so far south as the
Boreal European, the reason being that on the American coasts the
cold Labrador current, which hugs the land, bars back the advance of
southern genera, but allows Boreal genera to spread southwards, while
on the European side the warmer conditions produced by the Gulf Stream
keep the Boreal species back, and allow more southern forms to spread
northwards.

Many of the Boreal species occur on both sides of the Atlantic, and
thus support the theory of a more continuous fringe of continental land
once existing along the north of the Atlantic. Among the prominent
genera, besides several of those mentioned under the Arctic Sub-region,
are _Purpura_, _Chenopus_, _Littorina_, _Gibbula_, _Natica_, _Patella_,
_Tectura_, _Chiton_, _Doris_, _Aeolis_, _Tellina_, _Thracia_.

(3) _The Celtic Sub-region_ includes the British Islands (excepting
perhaps the Shetland Islands), the coasts of the North Sea and the
Baltic, with N. France to Cape Ushant. The absence of any cold or
warm current exerting direct influence upon the coast-line of this
sub-region causes a very gradual change in the conditions of life
as we move either southward or northward. The fauna of the British
seas contains a decided mixture of northern and southern forms. The
following are among the common Boreal species which attain their
southward range on our coasts: _Tectura testudinalis_ Müll. (to Dublin
Bay and Scarborough), _Trichotropis borealis_ Brod. (to the Dogger
Bank), _Margarita helicina_ Fabr. (to Yorkshire and Dublin Bay), _M.
groenlandica_ Chem. (western Scotland), _Natica montacuti_ Forb. (to
Cornwall), _Trophon truncatus_ Str. (to Tenby), _Chiton marmoreus_
Fabr. (to Dublin Bay and Scarborough). _Buccinum undatum_ and
_Littorina littorea_ become very scarce on our extreme south-western
coasts. Among Lusitanian species which reach our coasts are _Gibbula
magus_ L. (to Orkney and Shetland Islands), _Phasianella pullus_ L.
(to Caithness), _Galerus chinensis_ L. (to Milford Haven), _Galeomma
Turtoni_ Turt. (to Weymouth), _Cardium aculeatum_ L. (to Isle of Man),
_Solen vagina_ L. (to north Ireland).

It appears from the Mollusca of our Crag formations that at the
time of their deposition the temperature of our seas must have been
considerably warmer than it is now. Thus in the Crag we find many
species and even genera (_e.g._ _Mitra_, _Fossarus_, _Triton_,
_Vermetus_, _Ringicula_, _Chama_) which now occur no farther north
than the southern coasts of the Channel, the west of France, and the
Mediterranean.

The Baltic, a sea specially liable to violent changes of temperature,
with a large admixture of fresh water at its eastern end, appears to
possess only about 65 species in all. More than 50 genera occurring on
the western coasts of Denmark do not enter the Sound. In the eastern
portion of the Baltic marine and fresh-water species live together (p.
12).

(4) _The Lusitanian Sub-region_ extends from Cape Ushant in the north
to Cape Juby (lat. 28°) in the south, and includes the whole of the
Mediterranean, as well as the Azores, Canaries, and Madeira groups.

The English Channel acts as an effectual barrier to the northward
extension of many species; as many as 81 species which occur in
western France do not reach British coasts (P. Fischer). At the same
time, the western coasts of France are rather intermediate between
the two sub-regions than distinctly Lusitanian, for between 50 and 60
Mediterranean genera do not occur on those coasts.

The Mediterranean itself is exceedingly rich in species, about 1200
in all (including deep-water species) being known. A certain number
of these belong to tropical genera which here find their northern
limit, _e.g._ _Fasciolaria_, _Cancellaria_, _Sigaretus_, _Siliquaria_,
_Chama_, _Spondylus_. Here too occur _Carinaria_, _Lobiger_, _Oxynoe_,
_Pedicularia_, _Cypraea_, _Marginella_, _Mitra_, _Dolium_, _Cassis_,
_Cassidaria_, _Pisania_, _Euthria_, _Vermetus_, _Argonauta_, and many
others. A few Celtic and even Boreal species, which occur on the
western coasts of Morocco, do not enter the Mediterranean. Among these
are _Purpura lapillus_, _Helcion pellucidum_, and _Tellina balthica_.
_Halia_, a rare West African genus akin to _Pleurotoma_, is found in
Cadiz Bay, and the West African _Cymbium_ occurs on the Spanish coasts
as far as Malaga.

The Black Sea, whose northern and western coasts are exceedingly
cold, is comparatively poor in species. The Sea of Azof is chiefly
characterised by forms of _Cardium_.

(5) _The West African Sub-region_ extends from Cape Juby to a point
probably not very far south of lat. 30° S., the cold current which
sweeps up from the Pole probably limiting the southward extension of
tropical species on this side of Africa, while the warm Mozambique
current on the eastern side permits the spread of many Indo-Pacific
species almost as far south as the Cape. Owing to its extreme
unhealthiness, and the absence of harbours, the sub-region is very
little known.

The principal genera are _Cymbium_, _Pleurotoma_, _Marginella_,
_Terebra_, _Mitra_, _Agaronia_, _Murex_, _Cancellaria_, _Purpura_,
_Pseudoliva_, _Natica_, _Tellina_, _Lucina_, _Tugonia_, _Schizodesma_,
and _Arca_. Studer has enumerated as many as 55 species common to West
Africa and the opposite American shores. The north and south equatorial
currents, which circulate in this part of the Atlantic, probably
transport the larvae from one coast to the other. _Purpura coronata_
Lam., a characteristic West African species, is represented by a
well-marked variety in Demerara.

The Mollusca of St. Helena (178 known species) most resemble those of
the West Indies, 50 per cent being common, while 30 per cent are common
to the Mediterranean. From Ascension Island only 33 species are known,
which in their general relations resemble those of St. Helena.[385]

(6) _The South African Sub-region_ extends along the coast from about
lat. 30° on the west, to about East London on the east. Mr. G. B.
Sowerby enumerates 740 species from ‘South Africa,’ but includes in
this list Natal species, which more properly belong to the Indo-Pacific
fauna. Of these 740, 323 are peculiar, while 67 also occur in European
seas, some being familiar on our own shores. It is remarkable to
find in a sub-region separated from ourselves by the whole width
of the tropics, such well-known forms as _Mangilia costata_ Don.,
_M. septangularis_ Mont., _Cylichna cylindracea_ Penn., _Pholas
dactylus_ L., _Solen marginatus_ Pult., _Cultellus pellucidus_ Penn.,
_Ceratisolen legumen_ L., _Lutraria oblonga_ Chem., _Tellina fabula_
Gmel., _T. tenuis_ Da C., _Modiolaria discors_ L., and many others.

The leading genera are _Euthria_, _Triton_, _Cominella_, _Bullia_,
_Nassa_, _Cypraeovula_, _Oxystele_, _Fissurella_, _Fissurellidaea_,
_Patella_, and _Chiton_.

The Mollusca of Kerguelen Island and the Marion and Crozets groups
show relationship partly with South America, partly with the Cape, and
partly with South Australia and New Zealand, thus showing some trace
of a circumpolar antarctic fauna corresponding to, but not nearly so
well marked as that of the circumpolar arctic sub-region. Among the
remarkable forms discovered off Kerguelen are _Neobuccinum_ and a
sub-genus of _Struthiolaria_ (_Perissodonta_).


                     =B. The Indo-Pacific Region=

includes the whole of the coast-line of the Indian and western Pacific
oceans, from about East London in South Africa to the north of Niphon
(lat. 42°) in Japan, with the Red Sea and Persian Gulf, the whole of
the Indo-Malay Archipelago, Polynesia to the Sandwich Islands in the
north-east, and Easter Island in the south-east, and Australia to Swan
River in the west, and to Sandy Cape and Lord Howe’s Island in the
east. It is especially the region of coral reefs, which furnish so
favourite a home of the Mollusca, and which are entirely absent from
the Atlantic Region.

(1) _The Indo-Pacific Sub-region proper_ (which includes the whole of
this region except that part defined below as the Japanese Sub-region)
is by far the richest in the world. The marine Mollusca of the
Philippines alone (in some respects the nucleus of the whole region)
have been estimated at between 5000 and 6000 species, and Jousseaume
estimates Red Sea species at about 1000. Some prominent genera are
very rich in species. Garrett enumerates from Polynesia 81 species of
_Conus_, 60 of which occur on the Viti Is., 21 on the Sandwich Is.,
and only 14 on the Marquesas, where coral reefs are almost absent; 82
species of _Cypraea_, Viti Is. 44, Sandwich Is. 31, Marquesas only 13;
167 species of _Mitra_ (besides 29 recorded by others), Viti Is. 120,
Sandwich Is. 36, Marquesas 7. Of 50 existing species of _Strombus_, 39
occur in this region, and 10 out of 11 _Eburna_.

The following important genera are quite peculiar to the region:
_Nautilus_, several forms of Purpuridae, _e.g._ _Rapana_, _Magilus_,
_Rapa_, _Melapium_, and _Ricinula_; _Tudicla_, several forms of
Strombidae, _e.g._ _Rostellaria_, _Terebellum_, _Pteroceras_, and
_Rimella_; _Cithara_, _Melo_, _Neritopsis_, _Stomatia_, _Malleus_,
_Vulsella_, _Cucullaea_, _Tridacna_, _Hippopus_, _Libitina_,
_Glaucomya_, _Anatina_, _Aspergillum_, and many others.

The number of species common to the Red Sea and Mediterranean is
exceedingly small, some authorities even denying the existence of a
single common species. The present author, from an examination of the
shells dredged by MacAndrew at Suez, regarded 17 species as common, and
Mr. E. A. Smith has confirmed this view with regard to 8 of the species
in question.[386] The Mollusca occurring in Post-pliocene beds at Suez
show that Mediterranean species lived there in comparatively recent
geological times.

The opening of the Suez Canal appears to have already induced several
species to start on their travels from the Mediterranean to the Red Sea
and _vice versâ_. Two Red Sea species (_Mactra olorina_ Phil., _Mytilus
variabilis_ Kr.) had in 1882 established themselves at Port Said, while
two Mediterranean species (_Pholas dactylus_ L., _Solen vagina_ L.) had
reached Ismailia.[387]

(2) _The Japanese Sub-region_ consists of the Japanese Islands to
Niphon, together with Corea and a stretch of adjacent mainland coast of
unknown extent. The warm Kuro Siwo current, sweeping up between Luzon
and Formosa, permits tropical species to extend much farther north than
on the opposite shores of America, where a cold polar current keeps
them back. A certain number of species, however, are common to the
two shores of the Pacific, and a few circumpolar species occurring on
our own coasts reach Japan, _e.g.__Trophon clathratus_, _Puncturella
noachina_, _Mya arenaria_, _Modiola modiolus_, _Lasaea rubra_, and
_Nucula tenuis_.

Among the characteristic genera are _Fusus_, _Siphonalia_,
_Columbarium_, _Hemifusus_, _Rapana_, _Chlorostoma_, _Pleurotomaria_,
_Haliotis_, and _Cyclina_.


                      =C. The Australian Region=

includes the Australian coast-line from about Swan R.[388] (lat.
32° S.) to Sandy Cape (lat. 25° S.), Tasmania, New Zealand, and the
adjacent islands (except Lord Howe’s I.).

(1) _The Australian Sub-region proper_ (which consists of the whole
of the region excepting New Zealand and the adjacent islands) is
determined by the influence of the Antarctic Drift, which washes the
whole of the southern coasts of Australia, and runs strongly northward
between Australia and New Zealand. The E. Australian warm current from
the north is checked at Sandy Cape by this cold current, and flows off
to New Zealand, the western shores of which island are consequently
much warmer than the eastern. On the western coast of Australia the
Antarctic Drift has less force, and tropical genera accordingly range
some 7 degrees farther south on the western than on the eastern coasts.

The characteristic genera are _Voluta_ (of which half the known
species occur on Australian coasts[389]), _Cominella_, _Siphonalia_,
_Struthiolaria_, _Risella_, _Phasianella_, a number of genera belonging
to the Trochidae, _e.g._ _Liotia_, _Clanculus_, _Euchelus_, _Thalotia_,
_Elenchus_, _Trochocochlea_, _Zizyphinus_, _Bankivia_, _Trigonia_,
_Myodora_, _Myochama_, _Solenomya_, _Ephippodonta_, _Anapa_, _Mylitta_,
_Mesodesma_, and _Chamostrea_. _Trigonia_, originally discovered as
a recent form in Sydney Harbour (p. 65), is not peculiar to that
locality, occurring also off Cape York, West Australia, and Tasmania.

(2) _The Neozealanian Sub-region_ includes New Zealand, with the
outlying islands (Chatham, Auckland, and Campbell Is.).

As many as 455 species (Cephalopoda, 8; Gasteropoda, 311; Scaphopoda,
2; Pelecypoda, 134) have been enumerated by Professor F. W. Hutton as
occurring within the sub-region, of which only 64 are found elsewhere,
the proportion of peculiar species being thus nearly 86 per cent. New
Zealand therefore is, in its marine, no less than its land Mollusca,
greatly isolated.

The characteristic genera are _Anthora_, _Cryptoconchus_, and
_Vanganella_, which appear to be quite peculiar, _Trophon_,
_Cominella_, _Euthria_, most of the Trochidae also characteristic of
S. Australia, _Haliotis_, _Patella_, _Taria_, _Mesodesma_, _Mylitta_,
_Zenatia_, _Standella_, and _Myodora_.


                       =D. The American Region=

includes the entire coasts of North and South America with the adjacent
islands, south of Cape Avinoff on the western, and south of Cape Cod
on the eastern coast, the portions north of these points belonging to
the Arctic Sub-region.

(1) _The Aleutian Sub-region_ consists of the islands of Yesso and
Saghalien, with the adjacent shores of the Sea of Okhotsk to Cape
Lopatka, the Aleutian Is., and the west American coast from about Cape
Avinoff (lat. 60° N.) to St. Jean de Fuca Straits.

A certain number of species, probably of arctic origin, are common
with British and also with East American shores, the former being the
more numerous. Species as familiar to us as _Lacuna divaricata_ Fabr.,
_Trichotropis borealis_ Brod., _Pholas crispata_ L., _Mya truncata_ L.,
_M. arenaria_ L., _Mytilus edulis_ L., and _Modiolaria nigra_ Gray,
occur. The more characteristic genera are _Chrysodomus_, _Volutharpa_,
_Buccinum_, _Tectura_, _Scurria_, _Chiton_, _Cryptochiton_ (_Cr.
Stelleri_ Midd. is by far the largest known of the Chitonidae, 6 inches
long), _Tellina_ and _Pecten_.

(2) _The Californian Sub-region_ extends from St. Jean de Fuca Straits
(lat. 48° N.) to Cape St. Lucas, the Gulf of California belonging to
the Panamic sub-region. The northern polar current, which washes the
shores of this sub-region throughout their whole extent, prolongs the
southward range of the more northern genera, and keeps back those more
markedly tropical, the latter, however, creeping northward in the
warmer waters of the Gulf of California. Some authorities subdivide
this immense stretch of coast-line, as characterised by sub-temperate,
temperate, and sub-tropical genera, into the Oregonian, Californian,
and Lower Californian provinces.

The characteristic genera are--in the north, _Argobuccinum_,
_Zizyphinus_, _Chlorostoma_, _Tectura_, _Scurria_, _Chiton_
(_Katharina_, _Mopalia_, _Tonicia_), _Cryptochiton_, _Placunanomia_,
and _Mytilimeria_; in the centre, _Purpura_, _Monoceros_, _Amphissa_,
_Norrisia_, _Platyodon_, _Tapes_, and _Macoma_; and, towards the south,
_Olivella_, _Chorus_, _Macron_, _Pseudoliva_, _Trivia_, and _Haliotis_.

(3) _The Panamic Sub-region_ extends from the head of the Gulf of
California to Payta in Peru (lat. 5° S.). It is exceedingly rich in
species, about 1500 having been described. The Mollusca are entirely
distinct from those of the Indo-Pacific Region, which, although
extending from Natal to the Sandwich Islands, are unable to pass the
enormous extent of sea which separates the nearest Polynesian island
from the American coast.

On the two sides of the isthmus of Panama there occur certain pairs
of species, which, while specifically distant, are evidently closely
related to one another. Amongst these are, on the Panamic side,
_Purpura speciosa_, _Cypraea cervinetta_, _Cassis abbreviata_, _Natica
Chemnitzii_, and _Strombus gracilior_, corresponding to _Purpura
deltoidea_, _Cypraea exanthema_, _Cassis inflata_, _Natica maroccana_,
and _Strombus pugilis_, on the Caribbean. It is reasonable to conclude
that these “analogous species” are descendants of a stock which was
common to both seas when the isthmus was open (probably not later than
Miocene times), and which have, since the closing of the isthmus,
become modified, some species considerably more than others.

Among the characteristic genera (compare p. 3) are _Conus_,
_Pleurotoma_, _Terebra_, _Murex_, _Purpura_, _Oliva_, _Northia_,
_Cantharus_, _Columbella_, _Anachis_, _Cypraea_, _Strombus_,
_Cerithium_, _Coecum_, _Crepidula_, _Crucibulum_, _Vitrinella_;
_Tellina_, _Semele_, _Tellidora_; and _Arca_.

(4) _The Peruvian Sub-region_ extends from Payta in Peru to about the
latitude of Conception in S. Chili (37° S.), being checked from further
extension southward by the cold Humboldt current, whose force is
distinctly felt as far north as Callao. This cold current thus produces
the same results as the similar current which impinges on S. Africa,
but has even more effect in decisively separating the fauna on the two
sides of the great peninsula, scarcely a single species being common
to the western and eastern coasts of S. America. The characteristics
of the coast-lines themselves contribute to this result. The Chilian
coast is rocky, and descends abruptly to a great depth, while that of
Patagonia and Argentina is sandy and very shallow to a great distance
from land.

The characteristic genera are _Cancellaria_, _Columbella_, _Monoceros_,
_Concholepas_, _Trochita_, _Fissurella_, _Chiton_; _Ceronia_,
_Malletia_, and _Cumingia_. Some of the Californian genera, absent or
poorly represented in the Panamic Sub-region, reappear in Chili, _e.g._
_Scurria_, _Tectura_, and _Chlorostoma_.

(5) _The Magellanic Sub-region_ includes the coast-line and adjacent
islands (with the Falklands) from Conception in S. Chili to about Port
Melo in Eastern Patagonia (lat. 45° S.).

The principal genera (many of which find a habitat on the gigantic
Macrocystis which grows on every rock at low water) are _Euthria_,
_Voluta_ (6 species, one, _V. magellanica_, the largest known)
_Monoceros_, _Photinula_, _Patella_, _Chiton_; _Modiolarca_,
_Malletia_, and _Mulinia_. Several genera characteristic of the Boreal
and Arctic sub-regions recur, _e.g._ _Trophon_, _Admete_, _Margarita_,
_Puncturella_, _Cyamium_, and _Astarte_.

(6) _The Argentinian[390] Sub-region_ extends from about Cape Melo in
Patagonia to the neighbourhood of S. Caterina I. in South Brazil (lat.
28° S.). The sub-region stands in the same relation to the Magellanic,
on the east coast, as the Peruvian sub-region on the west, but, owing
to the influence of the warm Brazil current, which overpowers the
colder water of the Falkland branch of the Cape Horn current, it
reaches a point much farther south.

The Mollusca are not well known. The prevailing genera appear to
be _Oliva_, _Olivancillaria_, _Voluta_, _Bullia_, _Crepidula_;
_Periploma_, and _Lyonsia_.

(7) _The Caribbean Sub-region_ extends from S. Caterina I. in the south
to Florida in the north, and includes the shores of the Gulf of Mexico
and the whole of the West Indies. The influence of the warm Brazil
current (a branch of the South Equatorial) carries the range of the
purely tropical species to a point much farther south than is reached
by the tropical species on the west coast.

The sub-region is very rich in species, especially on the coral reefs
of the Bahamas and N. Cuba, but the exceedingly small tide-fall makes
shore collecting somewhat difficult beyond a certain point. The leading
genera are _Murex_, _Purpura_, _Melongena_, _Latirus_, _Marginella_,
_Strombus_, _Triton_, _Cerithium_, _Littorina_, _Nerita_, _Scalaria_;
_Tellina_, _Strigilla_, _Lucina_, and _Venus_. _Pleurotomaria_, a genus
long regarded as extinct, has been dredged alive off Tobago.

As compared with the tropical fauna of the Old World, that of the New
World is poor in peculiar genera (compare p. 368). The relations of
this sub-region to the West African and the Panamic have been already
dealt with (pp. 367 and 372).

(8) _The Transatlantic Sub-region_ extends from Florida to Cape Cod
(see p. 364). In the north the limits of the sub-region are distinctly
marked, in the south Caribbean species intermingle. Gould and Binney,
in their _Invertebrata of Massachusetts_, enumerate 275 species
(Cephalopoda, 6; Gasteropoda, 159; Scaphopoda, 2; Pelecypoda, 108), of
which 59 (Gasteropoda, 37; Pelecypoda, 22) are British.

Among the characteristic genera are _Urosalpinx_, _Eupleura_, _Fulgur_,
_Ptychatractus_, _Nassa_, _Crepidula_; _Solenomya_, _Mactra_,
_Cypricia_, _Raëta_, _Astarte_, and _Yoldia_. Our common _Littorina
littorea_ appears to have been introduced into Nova Scotian waters in
about 1857, no previous trace of it occurring either in literature or
shell-heaps. Since then it has spread rapidly into the Gulf of St.
Lawrence, and also as far south as Newhaven, and is said to be driving
out the indigenous _L. palliata_ from New England shores.[391] The debt
has been repaid by the introduction into British waters of the American
clam (_Venus mercenaria_ L.), which, according to the _Manchester City
News_ of 23rd March 1889, was first observed in the Humber in 1864, and
has steadily increased up to the present time, when it bids fair to
compete, in those waters, with the familiar _Cardium edule_.

=Characteristics of Abyssal Mollusca.=--Large shells appear to be
rare in the great ocean depths, and are usually very fragile; even
moderately-sized specimens are far from common. The only group in which
species occur larger than the usual size is the Nudibranchs, which are
represented by at least one form larger than an orange.

It would seem that abyssal molluscs are much less active and energetic
than their brethren on the shores. This view is favoured by the
looseness of their tissues, which seem ill adapted for prompt and
vigorous action. The tenacious character of the mud on the ocean floor
must make rapid motion very difficult. The shell itself is usually
fragile and delicate, the upper layers of arragonite being thin as
compared with shallow-water species, which makes the nacreous layer,
when present, appear unusually conspicuous; in many cases the surface
is characterised by a peculiar iridescence or sheen. The colour in
the shell of deep-sea Mollusca is never very pronounced, and is often
absent altogether. Light pink and salmon, pale yellow and brown, are
not uncommon. If the colour is in pattern, it is usually in the form
of necklaces of spots, which sometimes coalesce into bands. With
regard to sculpture, stout knobs and powerfully buttressed varices,
such as occur in the tropical _Murex_ and _Purpura_, are not found in
deep-sea species. But the ornamentation is frequently elaborate, and
the sculpture rich and varied. There is an especial tendency towards
strings of bead-like knobs, revolving striae, and delicate transverse
waves, the sculpture being in many cases of a character which tends to
strengthen the structure of the shell, like the ridges in corrugated
iron.

A remarkable feature in some deep-sea Mollusca is their singular
resemblance, in shape, and particularly in the possession of a strong
green periostracum, to some of our common fresh-water species.
According to Dr. Dall, the cause of this phenomenon is the same in both
cases. The fresh-water Mollusca secrete a strong periostracum, in order
to protect the shell against the corrosive influence of the carbonic
acid gas with which the water is surcharged. The shells of deep-sea
Mollusca, living, as they do, in water probably undisturbed by currents
of any kind, have to protect themselves against the same eroding
influence, and do so in the same way.[392]

Mollusca which live exclusively on algae and other forms of plant life
are almost entirely wanting in the great depths, where vegetation is
probably unknown. The struggle for existence must be much less keen
than in the thickly populated shallows, where vicissitudes of every
kind occur. The absence of rapid motion of water must obliterate many
of those mechanical effects which tend to produce modifying influences
upon the animals affected. In the absence of circumstances tending
to cause variation, in the unbroken monotony of their surroundings,
species must, one would think, preserve a marked uniformity over an
exceedingly wide area of range.

Vegetable food being wanting, those genera which in shallower waters
never taste flesh, are compelled to become carnivorous. Characteristic
of the great depths are very remarkable forms of Trochidae, in whose
stomachs have been found the remains of Corallines and Foraminifera.
According to Dr. Dall, the results of this diet show themselves in the
greatly increased space occupied by the intestine, in the diminution,
as regards size, of the masticatory organs, the teeth and jaws, and
also in the prolongation of the anal end of the intestine into a free
tube, which carries away the excreta in such a way that they do not
foul the water taken into the gills. The amount of nutriment contained
in the bodies of dead Foraminifera is so small that a comparatively
large quantity must be swallowed to keep the vital energies active,
and therefore the amount evacuated must be proportionately larger
also. The abyssal Trochidae, then, and many other genera, sustain
themselves by feeding on the ‘rain’ of dead animal matter which falls
upon the ocean floor, not so much hunting their prey as opening their
mouths and eating whatever happens to fall into them. Genera which are
normally carnivorous would appear to do the same. The Pleurotomidae,
for instance, are a family markedly characteristic of very deep water.
Representatives of the genus which occur in shallower water are known
to secure their prey while in the living state. But, according to
Dr. Dall, a singularly small proportion of deep-sea Mollusca, as
compared with those from the littoral region, show signs of having
been drilled or attacked by other Mollusca. This could hardly be the
case if the Pleurotomidae retained their predatory habits, since they
are more numerous in the great depths than any six other families
taken together. It has already been mentioned (p. 186) that a large
proportion of deep-sea Mollusca are perfectly blind.

Amongst other remarkable forms from the great depths may be mentioned
_Pleurotomaria_, with its singular anal slit (Fig. 269, p. 407)
extending in some cases half way round the last whorl. Three or four
species of this genus, so characteristic of almost all fossiliferous
strata down to the Cambrian, have been obtained in very limited
numbers off the West Indies and Japan. Dentaliidae, especially the
sub-genus _Cadulus_, find a congenial home in the slimy ocean mud.
One of the greatest molluscan treasures procured by the _Challenger_
was _Guivillea alabastrina_ Wats., a magnificent Volute as white as
alabaster, 6½ inches long, which was dredged from 1600 fath. in the
South Atlantic, between Marion Island and the Crozets. Another very
curious form, belonging to the same family, is _Provocator pulcher_
Wats., a shell about half the size of _Guivillea_, of stouter
proportions, and with an angulated and patulous mouth. This shell was
dredged by the _Challenger_ in comparatively shallow water (105–150
fath.) off Kerguelen Island. Among the Trochidae are the fine new
genera _Basilissa_, _Bembix_, and _Gaza_. The exploring voyages of
the American surveying steamer _Blake_, in the Gulf of Mexico and the
Caribbean Sea, have given us the remarkable new forms _Benthobia_
(possibly akin to _Admete_), _Mesorhytis_ (a sub-genus of _Fasciolaria_
hitherto only known from the Cretaceous of North America), and
_Benthodolium_ (possibly = _Oocorys_), a genus akin to _Cassis_.

In his report on the Pelecypoda obtained by the _Challenger_, Mr. E.
A. Smith remarks that as a rule “very deep-water ‘benthal’ species
certainly have a tendency to be without colour and of thin structure,
facts no doubt resulting from the absence of light, the difficulty of
secreting lime, the scarcity of food, and other unfavourable conditions
of existence.” At the same time, he notices that most of the species
obtained belong to genera which, even when occurring in shallow water,
are thin and colourless, _e.g._ _Neaera_, _Lima_, _Cryptodon_, _Abra_,
_Verticordia_, etc. Deep-water species of such genera as have a decided
periostracum (_Malletia_, _Limopsis_, _Leda_, _Nucula_, _Arca_) retain
it with little if any modification. The deep-water Pelecypoda of the
Atlantic and Pacific Oceans present no special features of interest.
The species are few in number, and the genera are not remarkable either
for novelty or peculiarity of form.

The greatest depth at which Pelecypoda have been obtained is 2900 fath.
mid North Pacific (_Callocardia pacifica_ Sm., _Abra profundorum_
Sm.); the greatest depth at which Gasteropoda have been obtained is
2650 fath. South Atlantic (_Stylifer brychius_ Wats.), both by the
_Challenger_. The deepest _Challenger_ Nudibranch came from 2425 fath.,
and the deepest _Chiton_ from 2300 fath. The greatest depth ever
dredged is 4575 fath. off the east coast of Japan.




                             CHAPTER XIII

                           CLASS CEPHALOPODA


The Cephalopoda present a complete contrast to the majority of the
Mollusca in habits and in many points of organisation. In their power
of rapid movement and their means of progression, their extreme
ferocity and carnivorous habits, their loss, in so many cases, of a
shell, and in its constitution when present, in the general symmetry of
their parts, in their reproductive and nervous system, they stand in
a position of extreme isolation with nothing to connect them with the
rest of the phylum.

Professor A. E. Verrill has collected many interesting details with
regard to gigantic Cephalopoda occurring on the north-eastern coasts
of America. From these it appears that the tentacular arms of some
species of _Architeuthis_ measure as much as 32, 33, 35, and 42 feet in
length, while the total length, including the body, sometimes exceeds
50 feet. Even off the Irish coast a specimen was once captured whose
tentacular arms were 30 feet long, the mandibles 4 inches across, and
the eyes about 15 inches in diameter.[393] The strength of these giant
Cephalopods, aided as they are by formidable rows of suckers and other
means of securing a grip, is almost incredible. Cases are not uncommon,
in which persons diving or bathing have been attacked, and have with
difficulty made their escape.

Great damage is frequently inflicted by Cephalopoda upon shoals of fish
on British coasts. Off Lybster (Caithness) _Loligo_ and _Ommastrephes_
devour the herring, large numbers of which are cut up and bitten on
the back of the neck by these creatures. On the American coasts the
mackerel fisheries are sometimes entirely spoiled by the immense
schools of squid which infest the Bay of St. Lawrence.[394] When
excited in the pursuit of fish Cephalopoda leap high out of the sea.
Dr. W. H. Rush[395] relates that when about 300 miles off the coast of
Brazil, a swarm of hundreds of decapods flew from the water and landed
on the deck of the ship, which was 12 feet above the surface level, and
they had to go over the hammock nettings to reach it.

  [Illustration: FIG. 238.--_Octopus vulgaris_ Lam., Naples: =A=,
  At rest; =B=, in motion; _f_, funnel, the arrow showing the
  direction of the propelling current of water. (After Merculiano.)]

The common _Octopus vulgaris_ Lam., of British and south European
coasts, inhabits some rocky hole, the approaches to which, like the
den of a fabled giant, are strewn with the bones of his victims. Homer
himself knew how hard it is to drag the polypus out of his hole,
and how the stones cling fast to his suckers. The colour-changes,
which flit across the skin of the _Octopus_, appear, to some extent,
expressive of the different emotions of the animal. They are also
undoubtedly protective, enabling it to assimilate itself in colour to
its environment. Mr. J. Hornell[396] has noticed an _Octopus_, while
crawling over the rock-work in his tank, suddenly change the colour of
the whole right or left side of its body, and of the four arms on the
same side, to a snowy whiteness. They have also been seen to change
colour, as if involuntarily, according to the material on which they
crawl. The nerve-centres which control the chromatophores or pigment
cells, causing them to expand or contract, are found to connect with
the optic ganglia; hence the changes of colour may be regarded as a
reflex result of the creature’s visual perception of its surroundings.


                         =Order Dibranchiata.=

Cephalopoda with two symmetrical branchiae, funnel completely tubular,
mouth surrounded by 8 or 10 arms furnished with suckers or hooks,
ink-sac and fins usually present, eyes with a lens; shell internal or
absent.

The Dibranchiata are not known from Palaeozoic strata, and first
appear (_Belemnites_, _Belemnoteuthis_) in the Trias. Whether they
are to be regarded as derived from some form of Tetrabranchiata,
_e.g._ _Orthoceras_, or as possessing an independent origin from some
common stock, cannot at present be decided. They attain their highest
development at the present time. The earliest representatives of the
Order (the _Phragmophora_) possessed a shell chambered like that of the
Tetrabranchiata. These chambered Dibranchiates rapidly reached their
maximum in the upper Lias and as rapidly declined, until at the close
of the Cretaceous epoch they were comparatively scarce, only a few
genera (_Beloptera_, _Spirulirostra_) surviving into Tertiary times.

The ordinary Dibranchiate Cephalopod may be regarded as consisting of
two parts--(_a_) the head, in which are situated the organs of sense,
and to which are appended the prehensile organs and the principal
organs of locomotion; (_b_) a trunk or visceral sac, enclosed in
a muscular mantle and containing the respiratory, generative, and
digestive organs. The visceral sac is often strengthened, and the
viscera protected, by an internal non-spiral shell. The ‘arms’ which
surround the mouth are modifications of the molluscan foot (p. 200),
and are either eight or ten in number. In the former case (Octopoda)
the arms, which are termed ‘sessile,’ are all of similar formation, in
the latter (Decapoda), besides the eight sessile arms there are two
much longer ‘tentacular’ arms, which widen at their tips into ‘clubs’
covered with suckers.

Remarks have already been made on the generative organs of Cephalopoda
(p. 136 f.), the branchiae (p. 170), the nervous system (p. 206), the
eye (p. 182), the radula (p. 236), and the ink-sac (p. 241).

One of the most characteristic features of the Dibranchiata are the
_acetabula_, or suckers, with which the arms are furnished. They
are usually disposed on the sessile arms in rows (of which there
are four in most _Sepia_, two in _Octopus_, and one in _Eledone_),
and become more numerous and smaller at the tip of the arm. They
are massed together in large numbers of unequal size on the ‘clubs’
in the Decapoda, particularly in _Loligo_. In most Octopoda their
base is flush with the surface of the arm, but in Decapoda the
acetabula are pedunculate, or raised on short stalks. In Octopoda
again, the acetabula are fleshy throughout, but in the Decapoda they
are strengthened by a corneous rim with a smooth or denticulate
edge (_Ommastrephes_, _Architeuthis_). Many of the acetabula on the
tentacular and sometimes on the sessile arms of the Onychoteuthidae
enclose a powerful hook, which is retractile like the claws of a cat.

In mechanical structure the acetabula consist of a disc with a slightly
swollen margin, from which a series of muscular folds converge
towards the centre of the disc, where a round aperture leads to a
gradually widening cavity. Within this cavity is a sort of button, the
_caruncle_, which can be elevated or depressed like the piston of a
syringe; thus when the sucker is applied the piston is withdrawn and a
vacuum created (Owen).

  [Illustration: FIG. 239.--‘Club’ of _Loligo vulgaris_ L., showing
  the crowded pedunculate acetabula, × ½.]

  [Illustration: FIG. 240.--One of the suckers of _Architeuthis
  dux_ Stp., showing the denticulate margin and corneous ring; _p_,
  peduncle.]

In many Octopoda the arms are connected by a web (the _umbrella_),
which sometimes extends up the greater part of the arms
(_Cirrhoteuthis_, some _Eledone_), at others occurs only at the
base. The use of the umbrella is perhaps to assist in locomotion, by
alternate contraction and expansion.

A cartilaginous skeleton is well developed, especially in the Decapoda.
In _Sepia_ a cephalic cartilage forms a complete ring round the
oesophagus, the eyes being situated in lateral prolongations of
the same. In front of the cephalic cartilage occurs a piece like an
inverted T, which supports the base of the anterior arms. The
Decapoda have also a ‘nuchal’ cartilage, connecting the head with the
anterior dorsal portion of the mantle, while cartilaginous knobs on the
ventral mantle button into corresponding sockets on the funnel.

=Sub-order I.=--_Octopoda._--Body round or bag-like, generally without
fins, arms eight, suckers fleshy, usually sessile, oviducts paired, no
nidamental glands, shell absent.

  [Illustration: FIG. 241.--_Cirrhoteuthis magna_ Hoyle, S.
  Atlantic. Two of the left arms and their web have been removed:
  _f_, funnel; _fi_, _fi_, fins; _m_, mouth. (After Hoyle, × 1/12.)]

FAM. 1. _Cirrhoteuthidae._--Body with two prominent fins; arms in great
part united by a web; one row of small suckers, with cirrhi on each
side.--Atlantic and Pacific Oceans, deep water (Fig. 241).

FAM. 2. _Amphitretidae._--Body gelatinous, mantle fused with the
funnel in the median line, forming two openings into the branchial
cavity; arms with one row of suckers; umbrella extending more than
two-thirds up the arms.--South Pacific (Fig. 242).

The two pocket-like openings into the branchial cavity are unique among
Cephalopoda (Hoyle).

  [Illustration: FIG. 242.--_Amphitretus pelagicus_ Hoyle, off
  Kermadec Is.: _e_, eyes; _f_, funnel; _p_, right mantle-pocket.
  (After Hoyle.)]

FAM. 3. _Argonautidae._--Female furnished with a symmetrical,
unilocular shell, spiral in one plane, secreted by thin terminal
expansions (the _vela_) of the two dorsal arms, no attachment muscle;
suckers in two rows, pedunculate; male very small, without veligerous
arms or shell.--All warm seas (Fig. 243). Pliocene----.

The shell consists of three layers, the two external being prismatic,
the middle fibrous. Its secretion by the arms and not by the mantle
edge is unique, and shows that it is not homologous with the ordinary
molluscan shell.

  [Illustration: FIG. 243.--_Argonauta argo_ L., the position
  assumed by a specimen kept in captivity, the arrow showing
  the direction of movement: _f_, funnel; _m_, mouth, with jaws
  projecting; _sh_, shell, with arms as seen through it; _wa_,
  webbed arm clasping shell. (After Lacaze-Duthiers.)]

The great controversy on the _Argonauta_, which once raged with so
much fierceness, is now matter of ancient history. It seems scarcely
credible that between fifty and sixty years ago, two of the leading
zoologists of the day, Mr. Gray and M. de Blainville maintained that
the animal which inhabits the Argonaut shell is a parasite, without any
means of depositing or forming a shell of its own, but which possesses
itself of the Argonaut shell, either by expelling or succeeding the
original inhabitant, a supposed nucleo-branchiate (Heteropod) mollusc
akin to _Carinaria_. The final blow to this strange hypothesis--which
was urged by the most ingenious series of arguments--was given by
Professor Owen, who in 1839 brought before the Zoological Society
of London the admirable observations of Madame Jeannette Power, who
made a continuous study of a number of specimens of _Argonauta_ in
her vivarium at Messina. The result of these observations tended to
show that the young _Argonauta_ when first excluded from the egg is
naked, but that in ten or twelve days the shell begins to form, that
the principal agents in the deposition of shell are the two velated
or web-like arms; and that portions of the shell, if broken away, are
repaired by a deposition of calcareous matter.[397]

FAM. 4. _Philonexidae._--Mantle supported by two ridges placed on
the funnel; large ‘aquiferous’ pores (supposed to introduce water
into the tissues) near the head or funnel; suckers in two rows,
pedunculate.--Atlantic and Mediterranean.

Genera: _Ocythoe_, arms of unequal size, no intervening membrane, third
arm on the right hectocotylised (see Fig. 51, p. 138), two aquiferous
pores at the base of the siphon; male very small; _Tremoctopus_, two
aquiferous pores between the eyes, two on the ventral side of the head.

FAM. 5. _Alloposidae._--Mantle edge united to the head by three
commissures; arms extensively webbed, acetabula sessile. Hectocotylised
arm developed in a cavity in front of the right eye.--N. Atlantic.

FAM. 6. _Octopodidae._--Head very large, arms elongated, similar,
more or less webbed, acetabula usually in two rows, sessile; mantle
supported by fleshy bands, no cephalic aquiferous pores.

In _Octopus_ proper the web is usually confined to the lower part of
the arms; Fischer separates off as _Pteroctopus_ a form in which it
reaches almost to their extremity. The third right arm (Fig. 52, p.
140) is hectocotylised, the modified extremity being, according to
Hoyle, sometimes minute, sometimes spoon-shaped, with a tendency to
transverse ridges, rarely slender and very long. The relative length
of the pairs of arms varies in different species. Two cartilaginous
stylets, imbedded in the dorsal mantle, are said by Owen to represent
the shell.

Other genera; _Pinnoctopus_, body furnished with broad lateral wings
which meet at the posterior end; _Cistopus_, a large web prolonged
along the sides of the arms, fitted with oval aquiferous pouches, with
pores at their base, between each pair of arms; _Eledone_ (Fig. 244),
one row of acetabula; _Tritaxeopus_, _Iapetella_.

=Sub-order II.=--_Decapoda._--Body oblong, mouth surrounded by
four pairs of sessile and one pair of tentacular arms, the latter
terminated by a ‘club’; acetabula pedunculate and furnished with a
corneous margin; mantle margin locked to the base of the funnel by a
cartilaginous apparatus; head and anterior part of body furnished with
aquiferous pores; fins present; mandibles corneous; oviduct single,
large nidamental glands in the female; shell internal.

  [Illustration: FIG. 244.--_Eledone Aldrovandi_ Delle Chiaje,
  Naples, from ventral side, × ½.]

The tentacular arms, which are the principal external feature of the
Decapoda, are not derived from the same muscular ring as the sessile
arms, but arise from the cephalic cartilage, and emerge between the
third and fourth arm on each side. In _Sepia_ they can be entirely
retracted into a kind of pocket behind the eyes, while in _Loligo_ they
are simply folded over one another. In _Chiroteuthis_ the arms are six
times as long as the body, and the clubs have four rows of denticulate
suckers.

The anterior ventral[398] portion of the mantle is furnished with a
singular contrivance for locking it to the funnel, and so rendering the
whole animal more capable of resisting the impact of any force. This
contrivance generally consists of a series of ridges or buttons which
fit into grooves or button-holes, the ridges being on the interior face
of the mantle and the corresponding grooves on the funnel, or _vice
versâ_. The ‘resisting apparatus’ is most elaborate in the pelagic
genera, and least so in the more sluggish littoral forms. A similar,
but not so complex, arrangement occurs also in the Octopoda.

The different forms of the shell appear to indicate successive stages
in a regular course of development. We have in _Spirula_ (Fig. 247)
a chambered shell of the Tetrabranchiate type, but of considerably
diminished size, which has ceased to contain the animal in its last
chamber, and has become almost entirely enveloped in reflected folds
of the mantle. These folds gradually concresce to form a definite
shell-sac, by the walls of which are secreted additional laminae of
calcareous shell-substance. These laminae invest the original shell,
which gradually (_Spirulirostra_, _Belosepia_) loses the spiral form
and becomes straight, eventually disappearing, while the calcareous
laminae alone remain (_Sepia_). These in their turn disappear, leaving
only the plate or ‘pen’ upon which they were deposited (_Loligo_),
which itself also, with the shell-sac, finally disappears, surviving
only in the early stages of _Octopus_ (Lankester).

The Decapoda are divided, according to the character of the shell, into
_Phragmophora_, _Sepiophora_, and _Chondrophora_.[399]

  [Illustration: FIG. 245.--‘Club’ of _Onychoteuthis_ sp., showing
  the hooks and clusters of fixing cushions and acetabula below
  them, × ½.]

A. PHRAGMOPHORA.--Arms furnished with hooks or acetabula; shell
consisting of a _phragmocone_ or chambered sac enclosed in a thin wall
(the _conotheca_), septa pierced by a siphuncle near the ventral margin
(in _Spirula_ alone this chambered sac forms the whole of the shell).
The apex of the cone lies towards the posterior end of the body, and
is usually enveloped in a calcareous _guard_ or _rostrum_. Beyond the
anterior end of the rostrum the conotheca is extended forward dorsally
by a _pro-ostracum_ or anterior shell, which may be shelly or horny,
and corresponds to the _gladius_ of the Chondrophora. The rostrum
consists of calcareous fibres arranged perpendicularly to the planes
of the laminae of growth, and radiating from an axis, the so-called
_apical line_, which extends from the extremity of the phragmocone to
that of the rostrum. _Distribution_, see p. 380.

FAM. 1. _Spirulidae._--Arms with acetabula, shell a loose spiral,
without rostrum or pro-ostracum, partially external, enclosed in two
lobes of the mantle (Figs. 247 and 248).

The single species of the single genus (_S. Peronii_ Lam. = _laevis_
Gray) has not yet been thoroughly investigated, although the shell
occurs in thousands on many tropical beaches, and is sometimes drifted
on our own shores. The animal appears to have the power of adhesion
to the rocks by means of a terminal sucker or pore. The protoconch
is present, and contains a prosiphon, which does not connect with
the siphuncle. The septal necks are continuous, not broken as in
_Nautilus_. The siphuncle is on the ventral margin of the shell, the
last whorl of which projects slightly on the dorsal and ventral sides,
but is even there covered by a thin fold of the mantle. The retractor
muscles of the funnel and of the head find their _point d’appui_ on the
shell, the last chamber of which contains the posterior part of the
liver, with which the membranous siphuncle is connected.

FAM. 2. _Belemnitidae._--Arms hooked as in _Onychoteuthis_, fins large;
phragmocone straight, initial chamber globular, larger than the second,
rostrum often very long, investing the phragmocone, pro-ostracum
sword- or leaf-shaped, rounded in front, seldom preserved, ink-sac
present.--Lower Lias to Cretaceous.

  [Illustration: FIG. 246.--_Sepia officinalis_ L., with mantle cut
  away to show position of internal shell, × ½. (The ends of the
  tentacular arms are cut off.)]

The Belemnitidae are believed to have been gregarious, and to have
lived in shallow water on a muddy bottom. Specimens are sometimes found
in which even the ink-sac can be recognised _in situ_. The relative
proportions of rostrum and phragmocone vary greatly in different
groups, the rostrum being in some cases two feet long, in others only
just enclosing the phragmocone. As a rule the rostrum is the only
portion which has been preserved.

FAM. 3. _Belosepiidae._--Phragmocone short, slightly curved, chambers
small, placed at the posterior end of a sepion, rostrum solid,
obtuse.--Eocene (Paris, Bracklesham, etc.).

_Fam. 4._ _Belopteridae._--Sepion not known; phragmocone curved,
siphuncle on the ventral margin, rostrum well developed, pointed.
Principal genus, _Spirulirostra_.--Miocene of Turin.

These two families, with their small, curved phragmocone and (in the
case of the Belosepiidae) large sepion, are clearly intermediate
between the Phragmophora and Sepiophora. Some authorities place them
with the latter group.

  [Illustration: FIG. 247.--Shell of _Spirula Peronii_ Lam.
  =A=, Cutside view; =B=, showing last chamber and position of
  siphuncle; =C=, in section, showing the septa and course of
  siphuncle; =D=, shell broken to show the convexity of the inner
  side of the septa; =E=, portion of a septal neck.]

  [Illustration: FIG. 248.--_Spirula Peronii_ Lam.: _d_, terminal
  sucker; _f_, funnel; _s_{1}_, _s_{2}_, projecting portions of
  shell, the internal part of which is dotted in. (From Owen and A.
  Adams combined.)]

B. SEPIOPHORA.--Shell internal, consisting usually of (_a_) an anterior
cancellated portion, (_b_) a posterior laminated portion, the laminae
enclosing air. It terminates in a very rudimentary phragmocone and a
rostrum, but there is no siphuncle.

FAM. _Sepiidae._--Eyes with cornea complete, body oval, fins narrow,
lateral, as long as the body, generally united behind; sessile arms
short, tentacular arms long, acetabula generally in four rows,
fourth left arm in the male hectocotylised near the base (Fig.
249).--World-wide.

The sepion or ‘cuttle-bone’ runs the whole length and width of the
body. In _Sepia_ it is very thick in front, while the posterior ventral
end is concave and terminated by a prominent spine, the _rostrum_ or
_mucro_ which points downwards. The whole shell is surrounded by a thin
chitinous margin, which forms a lateral expansion. Other genera are
_Sepiella_, _Hemisepius_, and _Trachyteuthis_ (fossil only).

C. CHONDROPHORA.--Shell (_gladius_ or _pen_) long, chitinous.

(_a_) _Myopsidae:[400] cornea entire, species mostly sub-littoral._

FAM. 1. _Sepiolidae._--Fins large, dorso-lateral; tentacular arms
retractile; two first dorsal arms in the male hectocotylised; gladius
narrow, half as long as the body.--World-wide.

Principal genera: _Sepiola_, dorsal mantle connected with the head by a
broad cervical band, ventral mantle with the funnel by a ridge fitting
into a groove; _Rossia_, dorsal mantle supported by a ridge, arms with
never more than four rows of acetabula; _Inioteuthis_, _Stoloteuthis_,
_Nectoteuthis_, and _Promachoteuthis_.

FAM. 2. _Sepiadariidae._--Fins not as long as the body, mantle
united to the head on the dorsal side, fourth left arm in the
male hectocotylised; no gladius. Principal genera, _Sepiadarium_,
_Sepioloidea_.--Chiefly Pacific Ocean.

  [Illustration: FIG. 249.--Hectocotylised arm (_h.a._) of _Sepia
  officinalis_ L., shown in contrast to one of the ordinary sessile
  arms; _m_, mouth; _p_, pocket into which the tentacular arm is
  retracted.]

FAM. 3. _Idiosepiidae._--Fins very small, terminal; fourth pair of arms
in the male hectocotylised, bare of suckers.

The only genus, _Idiosepion_, with a single species (_I. pygmaeum_
Stp.) is from the Indian Ocean, and is the smallest known Cephalopod,
measuring only about 15 mm. in length.

FAM. 4. _Loliginidae._--Body rather long, fins varying in size,
tentacular arms partially retractile, gladius as long as the back,
pointed in front, shaft keeled on the ventral side.--World-wide.

_Loligo_ proper has a pointed body with triangular posterior fins
united behind; sessile arms with two rows of acetabula, tentacular
arms with four; fourth left arm hectocotylised at the tip; funnel
attached to the head. Other genera are _Loliguncula_, _Sepioteuthis_,
and _Loliolus_. _Belemnosepia_, _Beloteuthis_, _Leptoteuthis_, and
_Phylloteuthis_ are fossil genera only, differing in the shape of the
gladius.

(_b_) _Oigopsidae: cornea more or less open; species pelagic._

FAM. 5. _Ommastrephidae._--Body cylindrical, fins generally terminal,
united together, regularly rhomboidal, sessile arms with varying number
of rows of acetabula, mantle connexions elaborate; gladius horny,
narrow lanceolate, with a hollow cone at the posterior end.--World-wide.

  [Illustration: FIG. 250.--_Architeuthis princeps_, Verr., E.
  America: _f_, Right fin; _fu_, funnel; _f.c_, fixing cushions and
  acetabula on the tentacular arms (_t_, _t_). (After Verrill, ×
  1/60.)]

_Ommastrephes_ proper has a natatory web on the sessile arms; the wrist
of each club has a series of acetabula with corresponding cushions on
the other wrist. In _Thysanoteuthis_ (often made a separate family)
the sessile arms have two rows of cirrhi, with lateral expansions of
the skin; fins as long as the body. In _Architeuthis_, to which belong
the largest Cephalopoda known, the fins together are shaped like a
broad arrow-head; acetabula of sessile arms strongly denticulate;
tentacular arms very long, with equidistant pairs of acetabula and
fixing cushions throughout their entire length, and a group of the same
at the base of the club. The acetabula and cushions correspond on the
opposing tentacles, and enable them to pull together. Other genera are
_Dosidicus_, _Todarodes_, _Illex_, _Bathyteuthis_ and _Mastigoteuthis_.

FAM. 6. _Onychoteuthidae._--Body cylindrical, fins terminal or lateral,
mantle-locking apparatus elaborate, tentacular arms very long, sessile
or tentacular arms furnished with retractile hooks, gladius lanceolate,
with a terminal cone.--World-wide.

The prehensile apparatus of Cephalopoda reaches its maximum of power
and singularity in this family. In _Onychia_, _Onychoteuthis_
and _Ancistroteuthis_, the sessile arms have acetabula only, in
_Gonatus_ and _Abralia_ they have hooks as well, while in _Verania_,
_Ancistrochirus_ and _Enoploteuthis_, the sessile arms have hooks only.
The number of rows of hooks or acetabula varies with the different
genera.

FAM. 7. _Chiroteuthidae._--Head nearly as large as the body; fins
terminal, tentacular arms very long, sessile arms slightly webbed,
acetabula denticulated; mantle-supports consisting of cartilaginous
ridges on the mantle, which fit into corresponding depressions on the
funnel, gladius expanded at each end.--Atlantic Ocean.

The six dorsal arms in _Histioteuthis_ are united by a broad web,
while in _Histiopsis_ the web only reaches half way up the arm. In
_Chiroteuthis_ the tentacular arms have scattered sessile suckers
throughout their whole length, and four rows of very long pedunculate
suckers on the clubs.

FAM. 8. _Cranchiidae._--Head small, body rounded, barrel-shaped, fins
terminal, eyes often very large, sessile arms short, tentacular arms
long, thread-like.--World-wide.

_Cranchia_ proper has the tentacular clubs finned, with eight rows of
suckers, body sometimes covered with warty tubercles. _Loligopsis_ has
a very attenuated body, with fins terminally united; some species are
spotted with colour, or have rows of tubercles on the ventral side.
_Taonius_ (Fig. 251) is doubtfully distinct from _Loligopsis_.

  [Illustration: FIG. 251.--_Taonius hyperboreus_ Stp., N.
  Atlantic: _e_, _e_, eyes; _f_, _f_, fins; _t_, _t_, tentacular
  arms. (After Hoyle, × ¼.)]


                        =Order Tetrabranchiata=

Cephalopoda with four branchiae and four kidneys; animal inhabiting the
last chamber of an external multilocular shell; funnel consisting of
two separate lobes; tentacles numerous, without suckers or hooks; no
ink-sac.

The shell consists of two layers, the outer being porcellanous, and
the inner, as well as the walls of the chambers or _septa_, nacreous.
The septa vary greatly in shape. In most of the Nautiloidea they are
regularly curved, as in _Nautilus_, or straight, as in _Orthoceras_,
but in the Ammonoidea they are often exceedingly complex. The edge of
the septum, where it unites with the shell-wall, is called the suture,
and the sutural line, which is not seen until the porcellanous layer is
removed, varies in shape with the septum.

  [Illustration: FIG. 252.--_Nautilus pompilius_ L., in section,
  showing the septa (_s_, _s_), the septal necks (_s.n_, _s.n_),
  the siphuncle dotted in (_si_), and the large body chamber
  (_ch_).]

The septa are traversed by a membranous tube known as the _siphuncle_,
which in _Nautilus_ is said by Owen to connect ultimately with the
pericardium. The _septal necks_, or short tubular prolongations of the
septa where they are perforated by the siphuncle, are in the great
majority of the Nautiloidea directed backwards (Fig. 252), _i.e._,
they project from the front wall of each chamber, while in nearly all
Ammonoidea they are directed forwards. When the siphuncle is narrow,
as in the Ammonoidea, it is simple, but when wide, as in many of the
Nautiloidea, its walls are often thickened by the deposition of
masses of calcareous matter, or by rings and radiating lamellae of
the same material. In position, the siphuncle is sometimes central,
sometimes sub-central, sometimes (Ammonoidea) marginal. In some cases
its position is believed to change during the growth of the individual.
The precise object served by the siphuncle is at present unknown. Some
hold that it preserves the vitality of the unoccupied chambers, by
connecting them with the soft parts of the animal; others have regarded
it as a means for lightening the shell by the passage of some gas into
the chambers.

  [Illustration: FIG. 253.--_Ammonites_ (_Cadoceras_) _sublaevis_
  Sowb., Kellaway’s Rock, showing the marginal position of the
  siphuncle (_si_).]

The initial chamber in Nautiloidea consists of an obtuse incurved cone,
marked on the outer surface of its posterior wall by a small scar known
as the _cicatrix_, which may be slit-like, round, oval, or cruciform
in shape. It has been held that the cicatrix originally communicated
with the protoconch or larval shell, which probably dropped off as
development proceeded. In the Ammonoidea, on the other hand, there
is no cicatrix, and the initial chamber probably represents the
protoconch, as seen in the nucleus of many Gasteropoda.

=Sub-order 1.= _Nautiloidea._--Shell straight, bent, or coiled,
aperture simple or contracted; siphuncle often narrowed by internal
deposits, position variable; septal necks short, usually directed
backwards; septa concave towards the aperture; initial chamber conical,
with a cicatrix on the posterior wall.

The Nautiloidea, of which _Nautilus_ is the sole living representative,
date back to the Cambrian epoch, and attain their maximum in the
Silurian and Devonian. At the close of the Palaeozoic era, every
family, with the sole exceptions of the _Orthoceratidae_ and
_Nautilidae_, appears to have become extinct. The former disappear with
the Trias, and after the lapse of the whole Secondary era, _Aturia_, a
form closely related to _Nautilus_, makes its appearance.

(_a_) _Retrosiphonata_: septal necks directed _backwards_.

FAM. 1. _Orthoceratidae._[401]--Shell straight or slightly curved,
aperture simple, body-chamber large; siphuncle cylindrical, position
variable. Single genus, _Orthoceras_ (Fig. 254). Cambrian to Trias.

FAM. 2. _Endoceratidae._--Shell straight, siphuncle wide, marginal,
septal necks produced into tubes fitting into one another. Principal
genera: _Endoceras_ (specimens of which occur six feet long), and
_Piloceras_--Ordovician.

FAM. 3. _Actinoceratidae._--Shell straight or slightly curved,
siphuncle wide, contracted at the septa by obstruction-rings.
Principal genera: _Actinoceras_, _Discosorus_, _Huronia_,
_Sactoceras_.--Ordovician to Carboniferous.

FAM. 4. _Gomphoceratidae._--Shell globular, straight or considerably
curved, aperture narrowed, T-shaped, body-chamber large, siphuncle
variable in position. The aperture is in some cases so narrow
that probably only the arms could be protruded. Principal genus,
_Gomphoceras_ (Fig. 255).--Silurian.

FAM. 5. _Ascoceratidae._--Shell sac-like or flask-shaped, apex
truncated, unknown, body-chamber occupying nearly the whole of
the shell on the ventral side, contracting at the aperture, last
few septa coalescing on the dorsal side and encroaching upon the
body-chamber. The young form has a symmetrical shell like _Orthoceras_,
attached to the sac-like shell above described; as growth proceeds
the former portion is thrown off. Principal genera: _Ascoceras_,
_Glossoceras_.--Ordovician and Silurian.

  [Illustration: FIG. 254.--=A=, Section of _Orthoceras_, showing
  the septa (_s_, _s_), and siphuncle (_si_, _si_); =B=, portion of
  the exterior of _Orthoceras annulatum_ Sowb., × ½. (Woodwardian
  Museum, Cambridge.)]

FAM. 6. _Poterioceratidae._--Shell fusiform, contracted at both ends,
aperture simple, siphuncle variable in position, inflated between the
septa. The form generally resembles _Gomphoceras_, except for the
simple aperture and fusiform shape.--Ordovician to Carboniferous.

FAM. 7. _Cyrtoceratidae._--Shell conical or sub-cylindrical, slightly
curved, body-chamber large, siphuncle variable in position. Single
genus, _Cyrtoceras_.--Cambrian to Carboniferous.

FAM. 8. _Lituitidae._--Shell coiled in a flat, sometimes loose spiral,
last whorl straight, containing the body-chamber, often greatly
prolonged. Principal genera: _Lituites_, _Ophidioceras_.--Ordovician
and Silurian.

FAM. 9. _Trochoceratidae._--Shell helicoid, with seldom more than two
whorls, dextral or sinistral, last whorl sometimes partly uncoiled.
Principal genera: _Trochoceras_, _Adelphoceras_.--Ordovician to
Devonian.

FAM. 10. _Nautilidae._--Shell with few whorls, more or less
overlapping, septa simple, siphuncle central or sub-central, aperture
not contracted.

The ‘tentacles’ are about 90 in number, and consist of four groups each
of 12 or 13 labial tentacles surrounding the mouth, two groups each of
17 larger (brachial) tentacles on each side of the head, two thicker
tentacles which combine to form the ‘hood,’ and two small tentacles
on each side of the eye. When the animal swims, the tentacles are
extended radially from the head, somewhat like those of a sea-anemone.
The direction of the many pairs of tentacles at constant but different
angles from the head, is the most striking feature in the living
_Nautilus_, and accounts for its being described, when seen on the
surface, as ‘a shell with something like a cauliflower sticking out
of it.’[402] The funnel is not a complete tube, but is formed by the
overlapping of the margins of two thin fleshy lobes (which are probably
morphologically epipodia), so that when the two lobes are parted, a
broad canal appears, leading to the branchial cavity. The head is
conical, and the mouth and its appendages can be retracted into a sort
of sheath, over which fits the ‘hood.’

  [Illustration: FIG. 255.--=A=, _Gomphoceras ellipticum_ M’Coy,
  Silurian: =B=, aperture (_ap_) of same; _s_, _s_, septa; _si_,
  position of siphuncle. (After Blake.)]

Other genera are _Trocholites_, _Gyroceras_, _Hercoceras_, _Discites_,
_Aturia_.--Ordovician to present time.

FAM. 11. _Bactritidae._--Shell straight, conical, siphuncle small,
marginal, septal necks long, funnel-shaped, sutures undulating, with
a sinus corresponding to the siphuncle. This family, from the form of
its sutures, appears to constitute a passage to the Ammonoidea. Single
genus, _Bactrites_.--Silurian and Devonian.

(_b_) _Prosiphonata._--Septal necks directed _forwards_.

The two genera are _Bathmoceras_ (Ordovician), shell straight, conical
always truncated, siphon marginal; and _Nothoceras_ (Silurian), shell
nautiloid with simple sutures.

=Sub-order 2.= _Ammonoidea._--Shell multiform, straight, curved, flat
spiral, or turreted, sutural line more or less complex, siphuncle
simple.

Some authorities hold that the members of this great sub-order, now
totally extinct, belong to the Dibranchiata, on the ground that
the protoconch resembles that of _Spirula_ rather than that of the
Nautiloidea. Others again regard the Ammonoidea as a third, and
distinct Order of Cephalopoda. Their distribution extends from the
Silurian to (possibly) the early Tertiary. No trace has ever been
found of an ink-sac, mandible, or hooks on the arms; the shell was
undoubtedly external.

  [Illustration: FIG. 256.--Diagram of the sutures of Ammonites:
  =A=, an elaborate suture (_Phylloceras_); =B=, a simple suture
  (_Ceratites_); _s.s_, siphonal, _s.v_, ventral, _s.l_, first
  lateral, _s.l´_, second lateral saddles; _s.a_, _s.a_, auxiliary
  saddles; _l.v_, ventral, _l_, first lateral, _l´_, second lateral
  lobe; _l.a_, _l.a_, auxiliary lobes. The arrow points _towards_
  the aperture. (From Woodward.) Compare Fig. 258.]

The sutural line, which indicates the septa, and is generally
concealed beneath the outer layer of shell, consists of a number of
_lobes_ or depressions, the concave part of which is directed towards
the aperture. Between these lobes lie corresponding elevations, or
_saddles_, the convex part of which is directed towards the aperture.
There are six principal lobes (Fig. 256): the _siphonal_ or _ventral_,
which is traversed by the siphuncle, the _dorsal_, and a superior and
inferior lateral on each side; smaller auxiliary lobes may succeed
these latter. The adjacent saddles have received corresponding
names. As a rule the sutural line is very complex, but in some cases
(_Goniatites_, _Lobites_) it is simple (Fig. 258, A). The first saddle
of a large number of genera serves as a means of classification,
according as it is broad or narrow. Some authorities reverse the terms
ventral and dorsal, as applied above. It is probable, however, that
the position of the animal of _Ammonites_ in its shell resembled that
of _Nautilus_. The siphuncle is dorsal (internal) in _Clymenia_ only,
ventral (external) in all other genera.

The _aptychus_ of Ammonoidea is a corneous or calcareous valve-like
body, generally formed of two symmetrical parts (Fig. 257). It has been
regarded by some as the covering of the nidamental gland, and hence as
occurring only in the female, by others, with more probability, as an
operculum, covering or imbedded in a hood formed, as in _Nautilus_,
of modified arms. Sometimes the Aptychus is in a single piece
(_Anaptychus_), sometimes the two pieces are united on the median line
(_Synaptychus_).

  [Illustration: FIG. 257.--Aptychus of Ammonite (_Trigonellites
  latus_). Kimmeridge Clay, Ely. × ½.]

The Ammonoidea are thus classified by Dr. P. Fischer:--

 (_a_) Retrosiphonata                                      _Goniatitidae_.

                                        { First saddle,  { _Arcestidae_, _Tropitidae_,
                      { No Aptychus or  { wide           { _Ceratitidae_, _Clydonitidae_.
                      { Anaptychus      {
                      { corneous,       {                { _Pinacoceratidae_,  _Amaltheidae_,
 (_b_) Prosiphonata   { single          { First saddle,  { _Ammonitidae_,
                      {                 { narrow         { _Lytoceratidae_.
                      {
                      { Aptychus calcareous, valves      { _Harpoceratidae_, _Stephanoceratidae_.
                      { double or united

(_a_) _Retrosiphonata._ FAM. 1. _Goniatitidae._--Shell nautiloid,
whorls sometimes disjoined, siphuncle ventral or dorsal, sutures
simple. Principal genera: _Clymenia_, _Goniatites_ (Fig. 258,
A).--Devonian to Carboniferous.

(_b_) _Prosiphonata._ FAM. 2. _Arcestidae._--Shell globular, smooth
or striated and rayed, body-chamber very long, aperture often with
a projecting hood, umbilicus closed by a callosity, lobes numerous,
foliaceous, aptychus present. Principal genera: _Arcestes_,
_Lobites_.--Principally Trias.

FAM. 3. _Tropitidae._--Differs from Arcestidae mainly in the more
highly ornamented surface, which is decorated with ribs which become
granular at the periphery. Principal genus, _Tropites_.--Trias and Lias.

FAM. 4. _Ceratitidae._--Shell ribbed and tuberculated, body-chamber
short, lobes denticulated, saddles simple. Principal genera:
_Ceratites_ (Fig. 258, B), _Trachyceras_.--Principally Trias.

FAM. 5. _Clydonitidae._--Shell variable in form, body-chamber short,
sutural line undulated, simple. Principal genera: _Clydonites_,
_Choristoceras_, _Rhabdoceras_, _Cochloceras_.--Trias.

FAM. 6. _Pinacoceratidae._--Shell discoidal, usually smooth,
body-chamber short, sutural line very complex, lobes numerous.
Principal genera: _Pinacoceras_, _Sageceras_.--Carboniferous to Trias.

FAM. 7. _Amaltheidae._--Shell broad, keeled, last whorl concealing most
of the spire, sutures with auxiliary lobes, incised.--Principal genera:
_Amaltheus_, _Schloenbacia_, _Sphenodiscus_.--Trias, Cretaceous.

  [Illustration: FIG. 258.--Various forms of Ammonoidea: =A=,
  _Goniatites crenistria_ J. Phil., Carb. Limestone; =B=,
  _Ceratites nodosus_ de Hann., Muschelkalk; =C=, _Ammonites_
  (_Parkinsonia_) _Parkinsoni_ Sowb., Inf. Oolite; =D=,
  _Phylloceras helerophyllum_ Sowb., Upper Lias; _s_, _s_, sutural
  lines.]

FAM. 8. _Ammonitidae._--Body-chamber long, whorls narrow,
uncovered, more or less ribbed, aperture simple, sutural line
normal, aptychus single, corneous. Principal genera: _Ammonites_,
_Aegoceras_.--Principally Lias.

FAM. 9. _Lytoceratidae._--Shell discoidal, body-chamber short, aperture
simple, no aptychus. Principal genera: _Lytoceras_, _Phylloceras_ (Fig.
258, D).--Trias to Cretaceous.

FAM. 10. _Harpoceratidae._--Shell discoidal, compressed, margin
keeled, surface with straight or arched ribs, aperture with lateral
projections, suture with accessory lobes, aptychus in two pieces.
Principal genera: _Harpoceras_, _Oppelia_, _Lissoceras_.--Jurassic to
Cretaceous.

  [Illustration: FIG. 259.--=A=, _Turrilites catenulatus_ d’Orb,
  Gault; =B=, _Macroscaphites Iranii_ d’Orb, Upper Neocomian. (From
  Zittel.)]

FAM. 11. _Stephanoceratidae._--Shell discoidal, helicoid or straight,
whorls sometimes disunited, surface often with bifurcating ribs, which
are tubercled, aperture often with lateral projections, sutural line
incised, aptychus in two pieces, sometimes united.

In the discoidal group, _Stephanoceras_ is strongly ribbed,
tubercled at the point of bifurcation, _Cosmoceras_ has long lateral
projections of the aperture when young, _Perisphinctes_ has a large
body-chamber and numerous smooth ribs. Other genera are _Acanthoceras_,
_Peltoceras_, _Aspidoceras_, and _Hoplites_. Among the loosely
whorled genera, _Scaphites_ (Fig. 260, A) has the last whorl produced
and bent back again in horse-shoe form, while the early whorls are
concealed; _Hamites_, _Hamulina_, and _Ptychoceras_ have a shell
shaped like a single or double hook, the sides of which may or may not
be united; _Crioceras_ (Fig. 260, B) in form of whorls resembles a
_Spirula_, _Ancyloceras_ a _Scaphites_ with the first whorls disunited.
_Macroscaphites_ (Fig. 259, B) is similar, but with the first whorls
united and not concealed. _Turrilites_ (Fig. 259, A) is turreted
and sinistral, while _Baculites_ is quite straight, with a long
body-chamber.

  [Illustration: FIG. 260.--=A=, _Scaphites aequalis_ Sowb.,
  Cretaceous; =B=, _Crioceras bifurcatum_ Quenst., Cretaceous.
  (From Zittel.)]




                              CHAPTER XIV

           CLASS GASTEROPODA--AMPHINEURA AND PROSOBRANCHIATA


                         =Order I. Amphineura=

Bilaterally symmetrical Mollusca, anus at the terminal end of the body,
dorsal tegument more or less furnished with spicules.

=Sub-order 1.= =Polyplacophora= (Chitons).--Foot co-extensive with
ventral surface of the body, dorsum with eight transverse plates,
articulated (except in _Chitonellus_), a row of ctenidia on each side
between the mantle and the foot. Silurian ----.

The Chitons are found in all parts of the world, ranging in size from
a length of about half an inch to six inches or more in the giant
_Cryptochiton_. Although in the main sub-littoral, they occur at very
great depths; the _Challenger_ dredged _Leptochiton benthus_ Hadd. at
2300 fathoms. _Chiton Polii_ exceptionally occurs at Malta--_teste_
MacAndrew--above sea margin, but within reach of the ripple. As a
rule, the Chitons live in concealment, on the under surface of stones
or in deep and narrow fissures in the rocks. When the stone to which
they are attached is turned over, they crawl slowly to the side which
is not exposed, as if disliking the light. An undescribed species,
however, which I took at Panama, crawled quite as fast as an ordinary
snail. _Chiton fulvus_ Wood, apparently is accustomed to crawl with
some rapidity. MacAndrew took it in abundance on his anchor chain in
Vigo Bay every time his yacht was got under weigh. He also found it
crawling in sand on the shore, to which habit is no doubt due its
extreme cleanness and freedom from the foreign growths which are
so characteristic of many of the species. When detached a _Chiton_
contracts the muscles of the whole body, and rolls up into a ball like
a wood-louse.

  [Illustration: FIG. 261.--Valves of a _Chiton_ separated to
  show the various parts (anterior valve uppermost): _a_, _a_,
  articulamentum; _b_, beak; _j_, jugum; _pl_, _pl_, pleura; _t_,
  _t_, tegmentum.]

  [Illustration: FIG. 262.--Valves of _Chitonellus_ separated out
  (anterior valve uppermost): _a_, _a_, articulamentum; _t_, _t_,
  tegmentum. × 2.]

The _Polyplacophora_ are characterised, _externally_, by their usually
articulated shell of eight _plates_ or _valves_, which is surrounded
and partly kept in position by a muscular _girdle_. These plates
overlap like tiles on a roof in such a way that the posterior edge of
the first, cephalic, or anterior valve projects over the anterior edge
of the succeeding valve, which in its turn overlaps the next, and so on
throughout. Seven-valved monstrosities very rarely occur.

A certain portion of each valve is covered either by the girdle or
by the valve next anterior to it. This portion, which is whitish in
colour and non-porous in structure, forms part of an inner layer which
underlies the rest of the substance of the valve, and is called the
_articulamentum_. The external portion of the valves, or _tegmentum_,
is generally more or less sculptured, and is largely composed of
chitin, impregnated with salts of lime, thus answering more to a
cuticle than to a shell proper. It is very porous, being pierced by
a quantity of minute holes of two sizes, known as _megalopores_ and
_micropores_, which are connected together by minute canals containing
what is probably fibrous or nerve tissue, the mouths of the pores being
occupied by sense organs connected with these nerves. The tegmentum of
the six intermediate valves is generally divided into three triangular
areas by two more or less prominent ribs, which diverge from the
neighbourhood of the median _beak_ or _umbo_. The space enclosed
between these ribs is known as the _median area_ or _jugum_, the other
two spaces as the _lateral areas_ or _pleura_. The ribs terminate with
the edge of the tegmentum, and are not found on the articulamentum.
In certain genera these areas are either non-existent, or are not
distinctly marked. The sculpture of the lateral areas (which is, as
a rule, much stronger than that of the median area) will generally
be found to resemble that of the anterior valve, which has no proper
median area. In the posterior valve the median area is very small,
while the sculpture of the rest of the valve corresponds to that of the
lateral areas generally (see Fig. 261).

  [Illustration: FIG. 263.--First, fourth, and eighth valves of a
  _Chiton_, showing _l.i_, laminae of insertion; _n_, _n_, notches;
  _s.l_, _s.l_, sutural laminae. × 2.]

The articulamentum of the intermediate valves is divided into two equal
parts in the middle of the anterior edge, opposite to the beak, by a
_sinus_. Each of the portions thus formed is again divided by a _notch_
or suture into two unequal parts, the anterior of which is known as
the _sutural lamina_, and is more or less concealed by the valve in
front of it, while the lateral part, or _lamina of insertion_, is
entirely concealed by the girdle. The articulamenta of the anterior and
posterior valves are either simple or pierced by a series of notches
(Fig. 263).

The girdle of the _Chitonidae_ varies considerably in character.
Sometimes its upper surface is simply corneous or cartilaginoid, with
no other sculpture than fine striae, at others it is densely beset with
spines or bristles, or tufted at intervals with bunches of deciduous
hairs; again it is marbled like shagreen or mossy down, or covered with
serpent-like scales. The width of the girdle varies greatly, being
sometimes very narrow, sometimes entirely covering all the valves
(_Cryptochiton_). As a rule, its outer edge is continuous, but in
_Schizochiton_ it is sharply notched over the anus.

A description has already been given of the dorsal eyes in Chiton (p.
187), the nervous system (p. 202), the branchiae (p. 154), the radula
(p. 228), and the generative system (p. 126).

  [Illustration: FIG. 264.--Girdles of various Chitonidae. =A=,
  _Radsia sulcata_ Wood, × 2; =B=, _Maugeria granulata_ Gmel., ×
  3; =C=, _Enoplochiton niger_ Barnes, × 3; =D=, _Acanthochiton
  fascicularis_ L., × 4; =E=, _Tonicia fastigiata_ Sowb., × 4.]

The recent Chitons are thus classified by Dr. W. H. Dall:--

   SECTION I. CHITONES REGULARES.--_Anterior and posterior valves
   of similar character._

   A. _Leptoidea._--Insertion plates obsolete, or, if present,
   unslit; _Leptochiton_, _Hanleyia_, _Hemiarthrum_, _Microplax_.

   B. _Ischnoidea._--Insertion plates sharp, smooth, fissured; with
   eaves; _Trachydermon_, _Callochiton_, _Tonicella_, _Schizoplax_,
   _Leptoplax_, _Chaetopleura_, _Spongiochiton_, _Ischnochiton_,
   _Callistochiton_.

   C. _Lophyroidea._--Insertion plates broad, pectinated,
   projecting backward; _Chiton_, _Tonicia_, _Eudoxochiton_,
   _Craspedochiton_.

   D. _Acanthoidea._--Insertion plates thrown forward;
   _Sclerochiton_, _Acanthopleura_, _Dinoplax_, _Middendorffia_,
   _Nuttallina_, _Arthuria_, _Phacellopleura_.

   SECTION II. CHITONES IRREGULARES.--_Posterior valve abnormal, or
   with a sinus behind._

   E. _Schizoidea._--Posterior valve fissured; _Lorica_,
   _Schizochiton_.

   F. _Placiphoroidea._--Posterior valve unslit, internally
   ridged, umbo nearly terminal; _Enoplochiton_, _Ornithochiton_,
   _Plaxiphora_.

   G. _Mopaloidea._--Posterior valve with posterior sinus and one
   slit on each side; _Mopalia_, _Katherina_, _Acanthochiton_,
   _Notoplax_.

   H. _Cryptoidea._--With double sutural laminae; _Cryptoconchus_,
   _Amicula_, _Cryptochiton_.

   I. _Chitonelloidea._--Posterior valve funnel shaped; laminae
   thrown forward; _Chitonellus_, _Choneplax_.

  [Illustration: FIG. 265.--_Chitonellus fasciatus_ Quoy; _ant_,
  anterior end.]

=Sub-order 2.= =Aplacophora.=--Animal vermiform, foot absent,
or a mere groove, cuticle more or less covered with spicules.

According to Marion, one of the principal authorities on the group, the
Aplacophora are perhaps Amphineura whose development has been arrested
at an early stage, their worm-like exterior being due to adaptation
to surroundings. They have hitherto been found chiefly in the N.
Atlantic and Mediterranean, generally at considerable depths, and
often associated with certain polyps in a way which suggests a kind of
commensalism.

FAM. 1. _Neomeniidae._--Foot a narrow groove, intestinal tube without
differentiated liver, kidneys with common exterior orifice, sexes
united, ctenidia present or absent. Genera: _Neomenia_ (Fig. 266),
_Paramenia_, _Proneomenia_, _Ismenia_, _Lepidomenia_, _Dondersia_.

  [Illustration: FIG. 266.--_Neomenia carinata_ Tullb.: _a_, anus;
  _gr_, ventral groove; _m_, mouth.]

  [Illustration: FIG. 267.--_Chaetoderma nitidulum_ Lov.: _a_,
  anus; _m_, mouth. × 3.]

FAM. 2. _Chaetodermatidae._--Body cylindrical, no ventral groove, liver
a single sac, kidneys with separate orifices into the branchial cloaca,
two bipectinate ctenidia. Single genus, _Chaetoderma_ (Fig. 267).


                      =Order II. Prosobranchiata=

Visceral loop twisted into a figure of 8 (streptoneurous), right half
supra-intestinal, left half infra-intestinal; heart usually in front
of the branchia (ctenidium), which is generally single; head with a
single pair of tentacles; animal dioecious, usually marine, more or
less contained within a shell, operculum generally present. Cambrian to
present time.

=Sub-order 1.= =Diotocardia.=--Heart with two auricles (except in the
_Docoglossa_ and _Helicinidae_), branchiae bipectinate, front end
free; two kidneys, the genital gland opening into the right (except in
Neritidae); nervous system not concentrated; no proboscis or siphon,
penis usually absent.

(_a_) DOCOGLOSSA (p. 227).--Heart with a single auricle, ventricle not
traversed by the rectum, visceral sac not spiral, shell widely conical,
non-spiral, no operculum; radula very long, with few hooked teeth in
each row.

FAM. 1. _Acmaeidae._--Left ctenidium alone occurring, free on a long
stalk. Cretaceous----. Principal genera: _Pectinodonta_, front part
of head much produced, radula 0 (1. 0. 1.) 0; _Acmaea_ (= _Tectura_),
with sub-genera _Collisella_ and _Collisellina_, no accessory branchial
ring, shell closely resembling that of _Patella_, but generally with a
distinct internal border; _Scurria_, accessory branchial ring on the
mantle.

FAM. 2. _Lepetidae._--No ctenidia or accessory branchiae, animal
generally blind. Pliocene----. Principal genera: _Lepeta_;
_Propilidium_, apex with internal septum; _Lepetella_.

FAM. 3. _Patellidae._--No ctenidia, the osphradial patch at the base
of each alone surviving, a circlet of secondary branchiae between the
mantle and sides of the foot. Ordovician----. (i.) _Patellinae._--Three
lateral teeth on each side, two of them anterior. Principal genera:
_Patella_, branchial circlet complete; chief sections _Patella_ proper,
_Scutellastra_, _Ancistromesus_ (_A. mexicana_ Brod., measures 8–14 in.
long); _Helcion_, branchial circlet interrupted in front; _Tryblidium_
(Ordovician).--(ii.) _Nacellinae._--Two developed laterals on each
side, one anterior. Genera: _Nacella_, branchial circlet complete;
_Helcioniscus_, branchial circlet interrupted in front.

(_b_) RHIPIDOGLOSSA (p. 225).--Ventricle of the heart traversed by the
rectum (except in Helicinidae), one or two ctenidia; jaw in two pieces,
radula long, marginals multiplied, rows curved.

Of all the Gasteropoda, this section of the Diotocardia approach
nearest to the Pelecypoda, particularly in the least specialised
forms. The auricle, the branchiae, and the kidneys are in many cases
paired, and more or less symmetrical. The ventricle is generally
traversed by the rectum, there is a long labial commissure between the
cerebral ganglia, special copulative organs are usually absent, while
the shell is often nacreous, like those of Pelecypoda of a primitive
type.

SECTION I. ZYGOBRANCHIATA.--Two ctenidia, shell with apical or marginal
slit or holes, corresponding to an anal tube in the mantle (p. 265).

FAM. 1. _Fissurellidae._--Two symmetrical ctenidia and kidneys,
visceral mass conical, shell conical, elevated or depressed, with
a single anterior or apical slit or impression; no operculum.
Jurassic----. (i.) _Fissurellinae_. Shell wholly external, apex
entirely removed by perforation, apical callus not truncated
posteriorly; central tooth narrow. Genera: _Fissurella_ (Figs. 171, p.
261; 178, p. 265), _Fissuridea_, _Clypidella_. (ii.) _Fissurellidinae_.
Shell partly internal, otherwise as in (i.); central tooth broad,
mantle more or less reflected over the shell, apical hole very wide.
Genera: _Fissurellidaea_, _Pupillaea_, _Lucapina_, _Megatebennus_,
_Macroschisma_, _Lucapinella_. (iii.) _Emarginulinae_. Shell usually
wholly external, apex usually not removed by perforation, sometimes
with internal septum, anal tube in a narrow slit or sinus. Genera:
_Glyphis_, externals of _Fissurella_, but holecallus truncated behind;
_Puncturella_ (sub-genera _Cranopsis_ and _Fissurisepta_), slit just
anterior to the apex, a small internal septum; _Zeidora_, large
internal septum as in _Crepidula_: _Emarginula_, shell elevated,
slit very narrow, on the anterior margin (in subg. _Rimula_, it is
between the apex and the margin), radula bilaterally asymmetrical;
_Subemarginula_, margin indented by a shallow groove; _Scutus_ (=
_Parmophorus_) shell oblong, depressed, nicked in front, largely
covered by the mantle.

  [Illustration: FIG. 268.--_Scutus australis_ Lam., Australia:
  _m_, _m_, mantle; _sh_, shell, × ⅔.]

FAM. 2. _Haliotidae_.--Right ctenidium the smaller, epipodial line
broad, profusely lobed; shell rather flattened, spire short, last
whorl very large, with a row of perforations on the left side, which
become successively obliterated; through these holes, the posterior of
which is anal, pass tentacular appendages of the mantle; no operculum.
Cretaceous----. Single genus, _Haliotis_; principal sub-genera
_Padollus_, _Teinotis_.

FAM. 3. _Pleurotomariidae_.--Central tooth single, narrow, about 26
laterals, 60 to 70 uncini. Shell generally variously trochiform,
nacreous, operculate, with a rather broad marginal sinus in the last
whorl; as this sinus closes up it forms an “anal fasciole” or “sinus
band.” Cambrian----. Principal genera: _Scissurella_, epipodial line
with several long ciliated appendages at each side, shell very small,
slit open, sinus band extending nearly to apex; _Schismope_, anal
slit closed in the adult into an oblong perforation; _Murchisonia_
(Palaeozoic only), shell long, turreted, whorls angulate or keeled
with a sinus band; _Odontomaria_ (Palaeozoic only), shell tubular,
curved; _Polytremaria_ (Carboniferous), shell turbinate, slit a series
of small holes connected by a passage; _Trochotoma_, shell trochiform,
perforation consisting of two narrow holes united by a slit;
_Pleurotomaria_, branchiae almost symmetrical, radula as above, shell
variously spiral.

In _Pleurotomaria_ we have the case of a genus long supposed to be
extinct. More than 1100 fossil species have been described, and within
the last 38 years about 20 specimens, belonging to 5 species, have been
discovered in a living state.

  [Illustration: FIG. 269.--_Pleurotomaria adansoniana_ Cr. and F.,
  Tobago. × ½.]

FAM. 4. _Bellerophontidae_.--Shell nautiloid, spire generally
concealed, aperture large, sinus or perforations central (Fig. 179,
p. 266). Ordovician--Trias. Genera: _Bellerophon_, _Trematonotus_,
_Cyrtolites_.

SECTION II. AZYGOBRANCHIATA.--One ctenidium (the left) present.

FAM. 1. _Cocculinidae._--A single cervical ctenidium, foot broad,
no eyes, shell patelliform, with caducous spire. Single genus,
_Cocculina_. Deep water.

FAM. 2. _Stomatellidae._--A single (left) ctenidium, front third
free, shell nacreous, spiral or patelliform, depressed, last whorl
large. Jurassic----. Genera: _Stomatella_ (subg. _Synaptocochlea_,
_Niphonia_), shell depressed, spirally ribbed, spire short, operculum
present; _Phaneta_, fluviatile only, shell trochiform, imperforate,
last whorl keeled, sinuate in front; _Stomatia_, spire short, surface
tubercled or keeled, no operculum; _Gena_, shell haliotis-shaped,
surface smooth, aperture very large; _Broderipia_, shell patelliform,
spiral apex often lost.

FAM. 3. _Cyclostrematidae._--Tentacles ciliated, thread-like, snout
bilobed, foot truncated in front, angles produced into a filament,
shell depressed, umbilicated, not nacreous. Eocene----. Principal
genera: _Cyclostrema_, _Teinostoma_, _Vitrinella_.

FAM. 4. _Liotiidae._--Epipodial line with a lobe behind each
eye-peduncle, shell solid, trochiform, longitudinally ribbed or
trellised, aperture round, operculum multispiral, hispid, corneous,
with a calcareous layer. Silurian----. Principal genera: _Liotia_,
_Craspedostoma_ (Silurian), _Crossostoma_ (Jurassic).

  [Illustration: FIG. 270.--_Monodonta canalifera_ Lam., New
  Ireland. (After Quoy and Gaimard.)]

FAM. 5. _Trochidae._--Snout short, broad, frontal lobes often present,
epipodial line furnished with cirrhi; shell nacreous, variously spiral,
operculum corneous, multispiral, nucleus central (Fig. 182, p. 268).
Silurian----. (i.) _Trochinae._--Frontal lobes present, lateral teeth
(= side centrals) 5 only, no jaws, peristome incomplete. Principal
genera: _Trochus_ (subg. _Cardinalia_, _Tectus_, _Infundibulum_,
_Clanculus_), _Monodonta_ (subg. _Diloma_), _Cantharidus_ (subg.
_Bankivia_, _Thalotia_), _Gaza_ (subg. _Microgaza_), _Callogaza_,
_Bembix_, _Chlorostoma_. (ii.) _Gibbulinae._--Frontal lobes and
jaws present, laterals often more than 5, peristome incomplete.
Principal genera: _Gibbula_ (subg. _Monilia_, _Aphanotrochus_,
_Enida_), _Minolia_, _Circulus_, _Trochiscus_, _Livona_, _Photinula_,
_Margarita_, _Solariella_, _Calliostoma_, _Turcica_, _Basilissa_,
_Euchelus_ (subg. _Olivia_, _Perrinia_). (iii.) _Delphinulinae._--No
frontal lobes, jaws present; shell solid, surface spirally lirate,
scaly, spinose, umbilicate, peristome continuous. Single genus,
_Delphinula_. (iv.) _Umboniinae._--Eyes pedunculate, left tentacle
attached to a frontal appendage, mantle reflected over edge of
aperture, lateral teeth 6 on each side; shell polished, peristome
incomplete, umbilicus generally closed by a callosity. Principal
genera: _Umbonium_, _Ethalia_, _Isanda_, _Camitia_, _Umbonella_,
_Chrysostoma_.

  [Illustration: FIG. 271.--_Phasianella australis_ Gmel.,
  Australia.]

FAM. 6. _Turbinidae._--Epipodial line with slender cirrhi, snout
broad, short, eyes pedunculate at outer base of tentacles, a frontal
veil between tentacles; shell turbinate, solid, aperture continuous,
operculum solid, calcareous, usually paucispiral, convex exteriorly
(Fig. 182, p. 268). Silurian----. (i.) _Phasianellinae._--Shell
bulimoid, polished, not nacreous, coloured in patterns, aperture oval.
Single genus, _Phasianella_ (Fig. 271). (ii.) _Turbininae._--Shell
very solid, nacreous within, aperture circular or long oval. Principal
genera, _Turbo_, whorls rounded above and below, spines, if present,
becoming more prominent with age, operculum smooth or granulose,
nucleus sub-central; subg. _Callopoma_, _Ninella_, _Marmorostoma_,
_Sarmaticus_, _Prisogaster_; _Astralium_, whorls flattened above and
below, spines, if present, becoming less prominent with age, operculum
oblong, often excavated at centre, last whorl large, nucleus marginal
or sub-marginal; subg. _Lithopoma_, _Imperator_, _Guildfordia_,
_Bolma_, _Cyclocantha_, _Uvanilla_, _Cookia_, _Pomaulax_,
_Pachypoma_. (iii.) _Cyclonematinae._--Shell nacreous, umbilicate,
operculum conical outside, whorls scalariform. Principal genera:
_Cyclonema_, _Horiostoma_ (?), _Amberleya_ (Silurian to Lias). (iv.)
_Leptothyrinae._--Shell small, solid, depressed, operculum nearly flat,
nucleus sub-central. Genera: _Leptothyra_, Collonia (?).

FAM. 7. _Neritopsidae._--Tentacles wide apart, long, eyes on short
peduncles at the outer base; shell solid, neritiform or naticoid,
aperture semi-lunar or oval; operculum (Fig. 183, p. 269) thick,
calcareous, non-spiral, exterior face smooth, interior face divided
into two unequal parts, with a broad median appendage. Devonian----.
Principal genera: _Neritopsis_ (one recent species), _Naticopsis_
(Devonian to Miocene).

FAM. 8. _Macluritidae._--Shell discoidal, whorls few, longitudinally
grooved behind, right side convex, deeply umbilicated, left side flat;
operculum very thick, nucleus excentrical, internal face with two
apophyses, one very large. The general appearance is more that of an
inequivalve bivalve, such as _Requienia_, than of a spiral gasteropod.
Palaeozoic----. Single genus, _Maclurea_.

FAM. 9. _Neritidae._--Snout short, tentacles long, eyes pedunculate
at their outer base, branchia triangular, free at the front end,
epipodium without cirrhi, penis near the right tentacle; shell
solid, imperforate, turbinate to almost patelliform, spire short,
internal partitions absorbed (p. 168), columellar region broad, edge
simple or dentate, operculum calcareous, spiral or non-spiral, with
prominent apophyses on the interior face, one of which locks behind
the columellar lip. Jurassic----. Principal genera: _Nerita_ (Fig. 13,
p. 17); _Neritina_ (chiefly brackish water and fluviatile), sub-genus
_Clithon_, usually coronated with spines; _Velates_ (Tertiary),
_Neritoma_ (Jurassic), _Deianira_ (Cretaceous), _Septaria_ (=
_Navicella_), shell more or less narrowly patelliform, with terminal
apex, aperture very large, with a broad columellar septum, operculum
too small for the aperture, more or less covered by the integument of
the foot; fluviatile only; _Pileolus_ (Jurassic to Cretaceous).

FAM. 10. _Hydrocenidae._--Branchia replaced by a pulmonary chamber,
eyes at the outer base of the tentacles, marginals of the radula very
oblique, centrals often wanting; shell small, conical, whorls convex,
operculum calcareous, with a prominent apophysis. Recent. Principal
genera: _Hydrocena_, _Georissa_.

FAM. 11. _Helicinidae._--Branchia replaced by a pulmonary chamber,
heart with one auricle; shell globular, with a short spire, internal
partitions absorbed; operculum without apophysis. Carboniferous----.
Principal genera: _Helicina_ (Fig. 18B, p. 21; subg. _Alcadia_,
_Schasicheila_, _Heudeia_, _Calybium_), _Eutrochatella_ (subg.
_Lucidella_), _Stoastoma_, _Bourcieria_, _Dawsonella_ (Carboniferous).

FAM. 12. _Proserpinidae._--Branchia replaced by a pulmonary chamber,
mantle partly reflected over the shell, eyes sessile; shell depressed,
discoidal, columella folded or truncated at the base, whorls with one
or more internal plicae, internal partitions absorbed, no operculum.
Eocene----. Single genus; _Proserpina_, subg. _Proserpinella_, _Cyane_,
_Dimorphoptychia_ (Eocene), and _Ceres_ (Fig. 18C, p. 21).

=Sub-order II.= =Monotocardia.=--Heart with one auricle, one ctenidium
(the left), monopectinate, fused with the mantle (except in _Valvata_),
one kidney, not receiving the genital products, nervous system somewhat
concentrated, proboscis and penis usually present.

(_a_) PTENOGLOSSA.--Radula with formula ∞. ᴑ. ∞, teeth similar
throughout, outermost largest (p. 224).

FAM. 1. _Ianthinidae._--Snout prominent, blunt, no eyes, shell
helicoid, fragile, bluish, no operculum; eggs carried on a raft of
vesicles attached to the foot (Fig. 42, p. 126). Pelagic only. Pliocene
----. Genera: _Ianthina_, _Recluzia_.

FAM. 2. _Scalariidae._--Shell long, turriculate, whorls often partly
uncoiled, with longitudinal ribs and prominent lamellae, aperture
circular, operculum spiral, corneous, animal carnivorous. Ordovician
----. Principal genera: _Scalaria_, _Eglisia_, _Elasmoneura_
(Silurian), _Holopella_ (Silurian to Trias), _Aclis_.

(_b_) TAENIOGLOSSA.--Radula with normal formula 2.1.1.1.2, marginals
sometimes multiplied (p. 223).

SECTION I. PLATYPODA.--Foot more or less flattened ventrally.

FAM. 1. _Naticidae._--Foot very large, produced before and behind,
propodium reflected upon the head, eyes absent or buried in the
integument, central and lateral tooth of the radula tricuspid,
middle cusp strong; shell globular or auriform, outer lip simple,
operculum corneous or calcareous, nucleus excentrical. Carboniferous
----. Principal genera: _Natica_, with many sub-genera; _Ampullina_
(Tertiary); _Amaura_; _Deshayesia_ (Tertiary); _Sigaretus_ (Fig. 91, p.
186), shell auriform, last whorl very large, operculum much too small
for the aperture.

FAM. 2. _Lamellariidae_.--Mantle reflected over more or less of the
shell, shell delicate, no operculum. Eocene----. Principal genera:
_Lamellaria_, shell completely internal, transparent, auriform; some
species deposit their eggs on compound Ascidians (p. 74); _Velutina_,
shell almost entirely external, paucispiral, with a thick periostracum;
_Marsenina_, shell auriform, partly internal; _Onchidiopsis_, shell a
membranous plate, internal.

FAM. 3. _Trichotropidae._--Branchial siphon short, eyes on the outer
side of the tentacles; radula closely allied to that of _Velutina_;
shell conical, last whorl rather large, periostracum thick and hairy,
operculum blunt claw-shaped, nucleus terminal. Cretaceous----. Genera:
_Trichotropis_, _Torellia_.

FAM. 4. _Naricidae._--Tentacles broad in the middle, with sessile eyes
at the exterior base, propodium narrow, quadrangular, a large epipodial
veil on each side of the foot; shell naticoid, cancellated, with
velvety periostracum. Jurassic----. Single genus: _Narica_.

FAM. 5. _Xenophoridae._--Foot divided by a groove, anterior portion
the larger; central tooth heart-shaped, with blunt cusps, lateral
large, roughly triangular, marginals long, falciform; shell trochiform,
somewhat flattened, attaching various fragments externally.
Devonian----. Single genus, _Xenophora_ (Figs. 25, 26, p. 64).

FAM. 6. _Capulidae._--Ctenidium deeply and finely pectinate, visceral
sac scarcely spiral, penis long, behind the right tentacle; shell
roughly patelliform, with scarcely any spire, interior polished,
usually with a septum or internal plate of variable form, no operculum.
Devonian----. Principal genera (Fig. 155, p. 248); _Capulus_, shell
cap-shaped, no internal plate; _Platyceras_ (Palaeozoic, see p. 76),
_Diaphorostoma_ (Palaeozoic), _Addisonia_ (?); _Crucibulum_, internal
appendage funnel-shaped; _Crepidula_ (including _Crepipatella_ and
_Ergaea_), shell slipper-shaped, with a large septum; _Calyptraea_
(including _Galerus_ and _Trochita_), internal lamina semi-spiral.

FAM. 7. _Hipponycidae._--Foot aborted, animal sedentary,
adductor-muscle shaped like a horse’s hoof, fastened on the ventral
side to the region of attachment, or to a thin calcareous plate which
closes the aperture like a valve; ventral side of the body surrounded
by a mantle with papillose border, which corresponds morphologically
to the epipodia, head emerging between the dorsal and ventral mantles.
Shell thick, bluntly conical, surface rugose. Eocene----. Genera:
_Hipponyx_; _Mitrularia_, a narrow half funnel-shaped appendage within
the shell.

  [Illustration: FIG. 272.--Two specimens of _Crepidula_ (marked
  _a_ and _b_) on an old shell of _Murex radix_ Gmel.]

FAM. 8. _Solariidae._--Foot large, eyes sessile, near the outer
base of the tentacles, radula abnormal (p. 224); shell more or less
depressed, lip simple, umbilicus wide, margins often crenulated,
operculum variable. The proper position of the family is quite
uncertain. Ordovician----. (i.) _Solariinae_. Genera: _Solarium_,
shell depressed, highly finished, angular at periphery, operculum
corneous, central tooth absent, laterals and marginals numerous, long,
and narrow; _Platyschisma_ (Silurian). (ii.) _Toriniinae._ Genera:
_Torinia_, whorls usually rounded, operculum (Fig. 183) conically
elevated, spiral externally, central tooth present, marginals few, edge
pectinated; _Omalaxis_. (iii.) _Euomphalinae_, shell planorbiform,
whorls rounded. Genera: _Euomphalus_, _Ophileta_, _Schizostoma_,
_Eccyliomphalus_ (all Palaeozoic).

FAM. 9. _Homalogyridae._--Tentacles absent, eyes sessile, central tooth
unicuspid on a quadrangular base, laterals and marginals replaced by
an oblong plate; shell very small, planorbiform. Recent. Single genus:
_Homalogyra_, whose true position is uncertain.

  [Illustration: FIG. 273.--_Solarium perspectivum_ Lam., Eastern
  Seas.]

FAM. 10. _Littorinidae._--Proboscis short, broad, tentacles long, eyes
at their outer bases, penis behind the right tentacle; reproduction
oviparous or ovoviviparous, radula very long; shell turbinate,
solid, columella thickened, lip simple, operculum corneous, nucleus
excentrical. Jurassic----. Principal genera: _Littorina_ (radula, Fig.
16, p. 20), _Cremnoconchus_ (p. 16), _Fossarina_; _Tectarius_, shell
tubercled or spinose; _Risella_, base slightly concave; _Lacuna_, shell
thin, grooved behind the columellar lip.

FAM. 11. _Fossaridae._--Shell turbinate, solid, small, white, spirally
ribbed, outer lip simple. Miocene----. Principal genus, _Fossarus_.

FAM. 12. _Cyclophoridae._--Ctenidium replaced by a pulmonary
sac, tentacles long, thread-like (radula, Fig. 17, p. 21); shell
variously spiral, peristome round, often reflected, operculum
circular. Terrestrial only. Cretaceous----. (i.) _Pomatiasinae_,
shell high, conical, longitudinally striated, operculum consisting
of two laminae united together. Single genus, _Pomatias_. (ii.)
_Diplommatininae_, shell more or less pupiform, peristome thickened
or reflected, often double. Genera: _Diplommatina_ (subg., _Nicida_,
_Palaina_, _Paxillus_, _Arinia_), shell dextral or sinistral, small,
columella often denticulated; _Opisthostoma_ (Fig. 208, p. 309),
last whorl disconnected, often reflected back upon the spire. (iii.)
_Pupininae_, shell more or less lustrous, bluntly conical, lip with
a channel above or below. Genera: _Pupina_ (subg., _Registoma_,
_Callia_, _Streptaulus_, _Pupinella_, _Anaulus_), _Hybocystis_ (Fig.
205, p. 305), _Cataulus_, _Coptochilus_, _Megalomastoma_. (iv.)
_Cyclophorinae_, shell turbinate or depressed, operculum corneous
or calcareous. Genera: _Alycaeus_, _Craspedopoma_, _Leptopoma_,
_Lagochilus_, _Cyclophorus_ (Fig. 206, p. 306); including _Diadema_,
_Aulopoma_, _Ditropis_, and others), _Aperostoma_ (including
_Cyrtotoma_ and others), _Cyathopoma_, _Pterocyclus_ (subg.,
_Myxostoma_, _Spiraculum_, _Opisthoporus_, and _Rhiostoma_ (Fig. 180,
p. 266), _Cyclotus_, _Cyclosurus_, and _Strophostoma_.

FAM. 13. _Cyclostomatidae._--Ctenidium replaced by a pulmonary sac,
tentacles obtuse, foot with a deep longitudinal median groove; central
tooth, lateral, and first marginal more or less bluntly cusped, second
marginal large, edge pectinate; shell variously spiral, spire usually
elevated, aperture not quite circular; operculum generally with an
external calcareous and an internal cartilaginoid lamina, rarely
corneous. Terrestrial only. Cretaceous----. Genera: _Cyclostoma_
(subg., _Leonia_, _Tropidophora_, _Rochebrunia_, _Georgia_, _Otopoma_,
_Lithidion_, _Revoilia_), _Cyclotopsis_, _Choanopoma_ (subg., _Licina_,
_Jamaicia_, _Ctenopoma_, _Diplopoma_, _Adamsiella_), _Cistula_
(subg., _Chondropoma_, _Tudora_), _Omphalotropis_ (subg., _Realia_,
_Cyclomorpha_), _Hainesia_, _Acroptychia_.

  [Illustration: FIG. 274.--_Cyclostoma campanulatum_ Pfr.,
  Madagascar.]

FAM. 14. _Aciculidae._--Ctenidium replaced by a pulmonary sac,
tentacles cylindrical, pointed at the end, eyes behind their base, foot
long and narrow; central tooth and lateral very similar, pinched in at
the sides, external marginal broad, edge finely pectinate; shell small,
acuminate, with a blunt spire, operculum corneous. Terrestrial only.
Tertiary----. Genus, _Acicula_ (= _Acme_).

FAM. 15. _Truncatellidae._--Ctenidium replaced by a pulmonary sac,
proboscis very long, eyes sessile, behind the base of the tentacles,
shell small, evenly cylindrical, apex truncated in the adult.
Eocene----. Genera: _Truncatella_ (subg., _Taheitia_, _Blanfordia_, and
_Tomichia_), _Geomelania_ (subg., _Chittya_ and _Blandiella_), _Cecina_
(?).

FAM. 16. _Rissoidae._--Eyes at the external base of the tentacles,
epipodium with filaments, operculigerous lobe with appendages;
central tooth pleated at the basal angles, lateral large, bluntly
multicuspid, marginals long, narrow, denticulate at the edge; shell
small, acuminate, often elaborately sculptured, mouth entire or
with a shallow canal, operculum corneous. Marine or brackish water.
Jurassic----. Principal genera: _Rissoa_ (subg., _Folinia_, _Onoba_,
_Alvania_, _Cingula_, _Nodulus_, _Anabathron_, _Fenella_, _Iravadia_,
and others), _Scaliola_ (shell agglutinating fragments of sand, etc.),
_Rissoina_ (lip thickened, operculum with an apophysis as in _Nerita_),
_Barleeia_, _Paryphostoma_ (Eocene).

FAM. 17. _Hydrobiidae._--Eyes at the outer base of the tentacles,
penis behind the right tentacle, prominent, operculigerous lobe
without filaments; radula rissoidan, central tooth often with basal
denticulations; shell more or less acuminate, small, aperture
entire, operculum corneous or calcareous. Brackish or fresh water.
Jurassic----. Principal genera: _Baicalia_, with its various sub-genera
(p. 290); _Pomatiopsis_, _Hydrobia_, _Bithynella_, _Micropyrgus_
(Tertiary), _Pyrgula_, _Emmericia_, _Benedictia_, _Lithoglyphus_,
_Tanganyicia_, _Limnotrochus_ (?), _Jullienia_, _Pachydrobia_,
_Potamopyrgus_, _Littorinida_, _Amnicola_, _Fluminicola_ (subg.,
_Gillia_, _Somatogyrus_), _Bithynia_, _Fossarulus_ (Tertiary),
_Stenothyra_.

FAM. 18. _Assimineidae._--Ctenidium replaced by a pulmonary sac,
no true tentacles, eye-peduncles long, retractile; radula that of
_Hydrobia_; shell small, conoidal, operculum corneous, nucleus
sub-lateral. Eocene----. Genera: _Assiminea_, _Acmella_.

FAM. 19. _Skeneidae._--Radula resembling that of _Hydrobia_; shell
very small, depressed, widely umbilicated, operculum corneous.
Pleistocene----. Single genus, _Skenea_.

FAM. 20. _Jeffreysiidae._--Mantle with two pointed ciliated appendages
in front, tentacles ciliated, eyes sessile, far behind the base of
the tentacles; marginal teeth sometimes absent; shell small, thin,
pellucid, whorls rather swollen, operculum with marginal nucleus,
divided by a rib on the inner face. Recent. Genera: _Jeffreysia_,
_Dardania_. Marine, living on algae.

FAM. 21. _Litiopidae._--Epipodium with cirrhi on each side,
operculigerous lobe with appendages; radula rissoidan; shell small,
conical, columella truncated, operculum corneous. Eocene----. Genera:
_Litiopa_, living on the Sargasso weed, suspended by a long filament;
_Alaba_, _Diala_.

FAM. 22. _Adeorbidae._--Radula essentially rissoidan; shell depressed,
circular or auriform, widely umbilicated, operculum corneous,
paucispiral, nucleus excentrical. Pliocene ----. Principal genera:
_Adeorbis_, _Stenotis_, _Megalomphalus_.

FAM. 23. _Viviparidae._--Snout blunt, tentacles long, right tentacle in
the male deformed, pierced with a hole corresponding to the aperture
of the penis, two cervical lobes, the right being siphonal, foot
with an anterior transverse groove; teeth broad, shallowly pectinate
at the ends; shell turbinate, whorls more or less rounded, aperture
continuous, operculum corneous, nucleus sub-lateral, with a false
sub-central nucleus on the external face. Animal ovoviviparous.
Fresh-water. Cretaceous ----. Genera: _Vivipara_ (= _Paludina_),
subg., _Cleopatra_, _Melantho_, _Tulotoma_; _Tylopoma_ (Tertiary), and
_Lioplax_.

FAM. 24. _Valvatidae._--Branchia exserted, bipectinate, carried on the
back of the neck, a filiform appendage (Fig. 66, p. 159) on the right
of the neck, penis under the right tentacle, prominent, eyes sessile,
behind the tentacles; radula like that of _Vivipara_; shell small,
turbinate or flattened, operculum corneous, nucleus central. Fresh
water. Jurassic ----. Single genus, _Valvata_.

FAM. 25. _Ampullariidae._--Snout with two tentacles, tentacles proper
very long, tapering, eyes prominently pedunculate, two cervical lobes,
the left siphonal, respiratory cavity divided by a partition, a large
branchia in the right chamber, the left functioning as a pulmonary
sac (Fig. 65, p. 158); radula large, central tooth multicuspid,
base broad, lateral and marginals falciform, simple or bicuspid;
shell large, turbinate or flattened, spire small, whorls rounded;
operculum generally corneous, nucleus sub-lateral, false nucleus as in
_Vivipara_. Fresh water. Cretaceous ----. Single genus, _Ampullaria_
(subg., _Ceratodes_, _Pachylabra_, _Asolene_, _Lanistes_, and
_Meladomus_).

FAM. 26. _Cerithiidae._--Branchial siphon present, short, eyes variable
in position; central tooth small, evenly cusped, lateral hollowed at
base, multicuspid, marginals narrow; shell long, turriculate, whorls
many, generally tuberculate, varicose or spiny, aperture sometimes
strongly channelled; operculum corneous, subcircular, nucleus
nearly central. Marine or brackish water. Trias ----. Principal
genera: _Triforis_, shell small, generally sinistral; _Fastigiella_,
_Cerithium_ (Fig. 12, p. 16), _Bittium_, _Potamides_ (subg.,
_Tympanotomus_, _Pyrazus_, _Pirenella_, _Telescopium_, _Cerithidea_,
_Lampania_, all brackish water), _Diastoma_ (Eocene), _Cerithiopsis_;
_Ceritella_ (Jurassic), _Brachytrema_ (Jurassic), and _Planaxis_
(subg., _Quoyia_ and _Holcostoma_).

FAM. 27. _Modulidae._--No siphon, radula of _Cerithium_; shell with
short spire, columella strongly toothed at the base, aperture nearly
circular. Recent. Single genus, _Modulus_.

FAM. 28. _Nerineidae_.--Shell solid, long, sub-cylindrical, aperture
channelled, columella and interior of whorls with continuous ridges,
extending up the spire. Genera: _Nerinea_ (Trias to Cretaceous),
_Aptyxiella_ (Jurassic).

FAM. 29. _Melaniidae._--Border of mantle festooned, foot broad, with
an anterior groove, penis present; radula closely resembling that of
_Cerithium_; shell long, spiral, with a thick periostracum, surface
with tubercles, ribs, or striae, suture shallow; operculum corneous,
paucispiral, nucleus excentrical. Animal ovoviviparous. Fresh water.
Cretaceous ----. Principal genera: _Melania_ (with many sections or
sub-genera), _Pachychilus_, _Claviger_ (= _Vibex_), _Hemisinus_,
_Pirena_, _Melanopsis_, _Tiphobia_, _Paludomus_ (subg., _Philopotamis_,
_Tanalia_, _Stomatodon_), _Hantkenia_ (Eocene), _Larina_ (?).

FAM. 30. _Pleuroceridae._--Mantle edge not festooned, no copulatory
organ, otherwise like Melaniidae; operculum with nucleus sub-marginal.
Animal oviparous. Fresh-water. Cretaceous ----. Genera: _Pleurocera_
(including _Io_, Fig. 12, p. 16, _Angitrema_, _Lithasia_,
_Strephobasis_), _Goniobasis_, _Anculotus_, _Gyrotoma_.

FAM. 31. _Pseudomelaniidae._--Shell resembling that of Melaniidae,
but marine. Genera: _Pseudomelania_, _Loxonema_, _Bourguetia_,
_Macrochilus_. Palaeozoic to Tertiary strata.

  [Illustration: FIG. 275.--_Melania confusa_ Dohrn, Ceylon.]

FAM. 32. _Turritellidae._--Mantle with a siphonal fold on the right
side; radula variable (p. 224); shell long, whorls many, slowly
increasing in size, transversely ribbed or striated, aperture small;
operculum corneous, nucleus central. Jurassic ----. Principal genera:
_Turritella_, _Mesalia_, _Protoma_, _Mathilda_ (?).

FAM. 33. _Coecidae._--Tentacles long, eyes sessile at their base; shell
small, spiral in the young form, spire generally lost in the adult,
the shell becoming simply a straight or curved cylinder; operculum
corneous, multispiral. Eocene ----. Single genus, _Coecum_.

FAM. 34. _Vermetidae._--Visceral sac greatly produced, irregularly
spiral, no copulatory organs (radula, Fig. 126, p. 223), shell tubular,
irregularly coiled, last whorls often free, aperture circular;
operculum corneous, circular, nucleus central. Carboniferous ----.
Principal genera: _Vermetus_; _Siliquaria_ (Fig 153, p. 248), a long
fissure, or series of holes, runs along a considerable part of the
shell, operculum with outer face spiral, elevated.

FAM. 35. _Strombidae._--Foot narrow, arched, metapodium greatly
produced, snout long, eye peduncles long, thick, eyes elaborate, siphon
short, penis prominent, bifurcate; central tooth with strong median
cusp, marginals falciform, slender, edge more or less denticulate;
shell solid, spire conical, outer lip generally dilated into wings
or digitations, channelled before and behind, a labial sinus at the
base, distinct from the anterior canal; operculum small for the
aperture, corneous, claw-shaped, edge notched. Lias ----. Genera:
_Strombus_ (Fig. 99, p. 200); _Pereiraea_ (Miocene), _Pteroceras_
(Fig. 277; digitations of the outer lip very strong), _Rostellaria_
(spire produced, anterior canal very long), _Rimella_, _Pterodonta_,
_Terebellum_ (base of shell truncate, spire short).

  [Illustration: FIG. 276.--Development of _Coecum_: =A=, showing
  the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_,
  first septum; _s´s´_, second septum. (After de Folin.) =B=, adult
  form of _C. eburneum_ Ad., Panama, x 10.]

FAM. 36. _Chenopodidae_ (= _Aporrhaidae_).--Foot flat; lateral and
marginal teeth not denticulate; shell resembling that of _Strombus_,
outer lip dilated, wing-like, no labial sinus. Jurassic ----. Genera:
_Chenopus_ (= _Aporrhais_, _Diastema_, _Malaptera_, _Harpagodes_,
_Alaria_) (last four from Secondary strata).

FAM. 37. _Struthiolariidae._--Radula allied to that of _Strombus_,
marginals occasionally multiplied; shell buccinoid, very solid, outer
lip thickened, canal short, operculum claw-shaped, notched, nucleus
terminal. Tertiary ----. Single genus, _Struthiolaria_ (subg.,
_Perissodonta_, marginal teeth multiplied).

FAM. 38. _Cypraeidae._--Mantle with two large lateral lobes reflected
and meeting over the shell, siphon small; central and lateral teeth
bluntly tricuspid or multicuspid, laterals fairly broad, edges cusped
or finely pectinate; shell polished, solid, spire generally concealed
in the adult or overlaid with enamel, aperture straight, narrow,
nearly as long as the shell, toothed at the sides, channelled at each
end, labium inflected; no operculum. Jurassic ----. Genera: _Ovula_
(including _Amphiperas_, _Transovula_, _Cyphoma_, _Radius_, _Simnia_),
_Pedicularia_, _Cypraea_ (with subg., _Cypraeovula_, _Cypraedia_, and
_Trivia_), and _Erato_.

FAM. 39. _Doliidae._--Foot expanded, wider and longer than the shell,
truncated and thickened in front, siphon very long and narrow; central
tooth with very strong median and small lateral and basal cusps,
lateral and marginals bluntly falciform; shell ventricose, without
varices, spire short, outer lip generally simple, anterior canal rather
wide, no operculum. Cretaceous ----. Genera: _Dolium_ (subg., _Malea_,
outer lip thickened, denticulate, reflected); _Pirula_, mantle with two
lateral lobes reflected over part of the shell, shell fig-shaped (Fig.
278).

  [Illustration: FIG. 277.--Three stages in the growth of
  _Pteroceras rugosum_ Sowb., E. Indies, showing the development of
  the ‘fingers.’]

FAM. 40. _Cassididae._--Foot broad, siphon long (radula, Fig. 125, p.
223); shell ventricose, with varices, spire short, outer lip reflected
or thickened, anterior canal short, recurved narrow; operculum
semi-lunar, with ribs radiating from a marginal nucleus. Cretaceous
----. Genera: _Cassis_ (subg., _Semicassis_ and _Cypraecassis_),
_Morio_ (= _Cassidaria_), _Oniscia_.

FAM. 41. _Columbellinidae._--Shell solid, ribbed, usually cancellated,
with an oblique posterior canal, columella callous, more or less
reflected. Genera: _Columbellina_, _Columbellaria_, _Zittelia_,
_Petersia_, _Alariopsis_ (?). Secondary strata only.

FAM. 42. _Tritonidae_.--Foot short, narrow; siphon short, not
prominent; radula allied to that of Cassididae; shell thick, varicose;
outer lip inflected and thickened, canal long, periostracum often
thick and hairy, operculum corneous, nucleus terminal or sub-marginal.
Cretaceous ----. Genera: _Triton_ (Fig. 191, p. 275; subg.,
_Epidromus_, _Plesiotriton_, _Simpulum_, _Ranularia_, _Argobuccinum_);
_Persona_, aperture toothed, narrow; columella reflected upon the last
whorl; _Ranella_, shell dorso-ventrally compressed, generally with two
continuous lateral varices, posterior canal present.

The position of the following four families is doubtful:--

FAM. 43. _Oocorythidae._--Siphon short, foot broad, eyes absent, radula
taenioglossate; shell buccinoid or cassidiform, operculum corneous,
spiral. Recent. Single genus, _Oocorys_.

FAM. 44. _Subulitidae._--Shell elongate, fusiform, smooth; suture
shallow, base truncate or rounded, aperture channelled or notched.
Ordovician to Trias. Genera: _Subulites_, _Fusispira_, _Euchrysallis_.

  [Illustration: FIG. 278.--_Pirula Dussumieri_ Val., Philippines.
  × ½.]

FAM. 45. _Seguenziidae._--Radula taenioglossate, shell trochiform,
aperture channelled, columella twisted, operculum multispiral, nucleus
central. Pliocene ----. Single genus, _Seguenzia_.

FAM. 46. _Choristidae._--Anterior tentacles united by a frontal veil,
posterior simple; eyes absent, foot with tentaculae before and behind;
three central teeth, outer marginal with a basal plate; shell helicoid,
suture deep, peristome continuous, operculum corneous, paucispiral.
Pliocene ----. Single genus, _Choristes_.

SECTION II. HETEROPODA.--Foot fin-shaped, not flat.

The Heteropoda are free-swimming Mollusca, being, like the Pteropoda,
Gasteropoda modified to suit their pelagic environment. Their nervous
system is streptoneurous, and they are therefore probably derived from
the Prosobranchiata, but they are highly specialised forms. Pelseneer
considers them far more widely removed from the Streptoneura than the
Pteropoda are from the Euthyneura. They swim on the surface “upside
down,” _i.e._ with the ventral side uppermost.

The tissues and shell are transparent, permitting observation of the
internal organs. In the Pterotrachaeidae the foot takes the form
of a fan-shaped disc, usually furnished with a sucker. The body is
compressed at the posterior end, often with a ventral “fin.” In
_Atlanta_ the foot consists of three very distinct parts: a propodium,
a mesopodium, on which is a small sucker, and a metapodium, which
carries the operculum. The branchiae are carried on the visceral sac,
and are free in _Pterotrachaea_, slightly protected by the shell in
_Carinaria_, and entirely covered in _Atlanta_; absent altogether in
_Firoloida_.

The head carries two tentacles (except in _Pterotrachaea_),
with large, highly organised eyes on short lobes at their outer
base. The alimentary tract consists of a long protrusible proboscis,
with a taenioglossate radula (Fig. 132, p. 227), a long oesophagus,
and a slightly flexured intestine. In _Atlanta_ the visceral
sac is spiral and protected by a spiral planorbiform shell; in
_Carinaria_ the visceral sac is small, conical, protected by a
very thin capuliform shell. There is no shell in _Pterotrachaea_
or _Firoloida_.

The Heteropoda are dioecious. In the male there is a flagellum
behind the penis, which is near the middle of the right side.
_Pterotrachaea_ lays long chains of granular eggs, and has
been noticed to produce a metre’s length in a day. The eggs of
_Atlanta_ are isolated. The embryo has a deeply bilobed velum.

FAM. 1. _Pterotrachaeidae._--Body long, with a caudal “fin;” branchiae
dorsal, free or partly protected by a shell; foot consisting of a
muscular disc, with or without a sucker.

_Pterotrachaea_ proper has no mantle, shell, or tentacles. The
branchiae are disposed round the visceral sac, at the upper part of
which is the anus. In _Firoloida_ the body is abruptly truncated
behind, with a long filiform segmented caudal appendage; visceral sac
at the posterior end: fin-sucker present or absent in both male and
female. _Cardiapoda_ resembles _Carinaria_, but the visceral sac is
more posterior and is only slightly protected by a very small spiral
shell. _Carinaria_ (Fig. 279) has a rugose translucent skin, visceral
sac sub-median, apparently pedunculated, covered by a capuliform shell.
The larval shell, which persists in the adult, is helicoid.

FAM. 2. _Atlantidae._--Shell spiral, operculate, covering the animal.
Branchiae in a dorsal cavity of the mantle; foot trilobed, with a small
sucker on the mesopodium.

The shell of _Atlanta_ is discoidal and sharply keeled, while that of
_Oxygyrus_ is nautiloid, with the spire concealed, no keel, aperture
dilated.

(_c_) GYMNOGLOSSA.--Radula and jaws absent; proboscis prominent, sexes
probably separate, penis present. The section is probably artificial
and unnecessary, the families composing it being, in all probability,
Taenioglossa which have lost their radula in consequence of changed
conditions of life (pp. 79, 225).

  [Illustration: FIG. 279.--_Carinaria mediterranea_ Lam., Naples:
  _a_, anus; _br_, branchiae; _f_, foot; _i_, intestine; _m_,
  mouth; _p_, penis; _s_, sucker; _sh_, shell; _t_, tentacles. × ½.]

FAM. 1. _Eulimidae._--Proboscis very long, retractile, mantle
forming a siphonal fold; shell small, long, subulate, polished;
suture shallow, aperture continuous, operculum present or absent.
Animal often parasitic, sucking the juices of its host by its long
proboscis. Trias----. Genera: _Eulima_ (subg., _Subularia_, _Arcuella_,
_Apicalia_, _Mucronalia_, _Stiliferina_, and others), _Stilifer_,
_Scalenostoma_, _Niso_, and _Hoplopteron_.

FAM. 2. _Pyramidellidae._--Tentacles auriform, proboscis as in
Eulimidae, a prominent _mentum_ or flap under the buccal orifice; shell
usually small, conical; suture shallow, apical whorls (the embryonic
shell) sinistral (p. 250), operculum corneous, paucispiral; nucleus
excentrical. Trias----. Genera: _Pyramidella_ (subg., _Syrnola_,
_Otopleura_, _Chrysallida_, _Mumiola_), _Odostomia_, _Eulimella_,
_Murchisoniella_, _Turbonilla_ (subg., _Dunkeria_ and _Cingulina_).

(_d_) RACHIGLOSSA (p. 220).--Proboscis long, retractile; siphon
distinct, radula without uncini, sometimes without laterals; teeth
strongly cusped; shell generally wholly external.

FAM. 1. _Muricidae._--Eyes sessile at the outer base of the tentacles,
penis large, behind the right tentacle, radula within the retractile
proboscis, central tooth (Fig. 119, p. 220) with at least three
strong cusps, laterals plain; shell solid, more or less tuberculate,
spiny and varicose, anterior canal varying from a mere notch to a
long channel. Cretaceous----. Principal genera: (i.) _Muricinae_,
nucleus of operculum sub-terminal; _Trophon_, _Typhis_, _Murex_ (with
many subdivisions), _Ocinebra_ (including _Cerastoma_, _Vitularia_,
and _Hadriania_), _Urosalpinx_, _Eupleura_, _Pseudomurea_. (ii.)
_Purpurinae_, nucleus of operculum lateral; _Rapana_ (including
_Latiaxis_), _Purpura_ (with subg., _Cuma_, _Iopas_, _Vexilla_, and
_Pinaxia_), _Monoceros_ (including _Chorus_), _Purpuroidea_ (Secondary
strata), _Pentadactylus_, _Sistrum_, _Concholepas_.

FAM. 2. _Coralliophilidae._--Animal living in Madrepores, resembling
_Purpura_, radula absent; shell variously shaped, often deformed
or tubular, operculum that of _Purpura_, if present. Miocene----.
Principal genera: _Rhizochilus_, _Coralliophila_, _Leptoconchus_,
_Magilus_ (Fig. 29, p. 75), _Rapa_.

FAM. 3. _Columbellidae._--(Radula, Fig. 123, p. 222.) Shell small,
solid, fusiform, aperture narrow, canal short, outer lip thickened.
Miocene----. Single genus, _Columbella_ (subg., _Nitidella_, _Anachis_,
_Meta_, _Strombina_, _Atilia_, _Conidea_, _Amphissa_, _Mitrella_, and
others).

FAM. 4. _Nassidae._--Foot long and broad, often with terminal
appendages; siphon long, eyes on outer base of tentacles, central
tooth of radula arched, multicuspid, lateral strongly bicuspid, with
small denticles between the cusps; shell rather small, buccinoid,
columella more or less callous, outer lip thickened, often toothed;
operculum corneous, edges often toothed. Miocene----. Principal genera:
_Nassa_ (with many sections), _Amycla_, _Desmoulea_, _Cyclonassa_,
_Canidia_ (subg., _Clea_ and _Nassodonta_), _Dorsanum_, _Bullia_ (=
_Buccinanops_, Fig. 62, p. 185), _Truncaria_.

FAM. 5. _Buccinidae._--Siphon rather long, eyes at outer base of
tentacles; central tooth of radula with 5 to 7 cusps, laterals
bicuspid or tricuspid (Fig. 118, p. 220); shell more or less fusiform,
thick, covered with a periostracum, canal of varying length, outer
lip simple or thickened; operculum corneous, nucleus variable in
position. Cretaceous----. Principal genera: Group i. _Chrysodomus_
(with sections _Neptunea_, _Volutopsis_, _Pyrolofusus_, _Jumala_),
subg., _Sipho_; _Siphonalia_ (subg., _Kelletia_). Group ii. _Liomesus_
(= _Buccinopsis_). Group iii. _Buccinum_ (Fig. 1 B, p. 6; subg.,
_Volutharpa_, _Neobuccinum_). Group iv. _Cominella_, _Tritonidea_,
_Pisania_, _Euthria_; _Anura_ (Miocene), _Genea_ (Pliocene), _Metula_,
_Engina_. Group v. _Phos_, _Hindsia_. Group vi. _Dipsaccus_ (=
_Eburna_), _Macron_. Group vii. _Pseudoliva_.

FAM. 6. _Turbinellidae._--Central tooth of radula tricuspid, median
cusp strong, lateral bicuspid, cusps unequal (Fig. 117, p. 220);
shell fusiform or pear-shaped, heavy, canal often long, operculum
corneous, claw-shaped, nucleus terminal. Miocene----. Principal
genera: _Turbinella_, _Cynodonta_, _Tudicla_ (subg., _Streptosiphon_);
_Piropsis_ (Cretaceous), _Perissolax_ (Cretaceous), _Strepsidura_
(Eocene, subg., _Whitneya_), _Melapium_, _Fulgur_ (= _Busycon_,
Fig. 150, p. 249, including _Sycotypus_), _Melongena_ (subg.,
_Pugilina_, _Myristica_); _Liostoma_ (Eocene), _Hemifusus_ (subg.,
_Megalatractus_), _Ptychatractus_, _Meyeria_.

  [Illustration: FIG. 280.--_Turbinella pyrum_ Lam., Ceylon. × ⅔.]

FAM. 7. _Fasciolariidae._--Eyes at the outer base of the tentacles
(radula, Fig. 121, p. 221); shell fusiform, spire long, canal often
very long, columella often with a fold at the base; operculum corneous,
nucleus terminal. Cretaceous----. Principal genera: _Fusus_ (including
_Sinistralia_, _Aptyxis_, _Troschelia_), with subg., _Serrifusus_
(Cretaceous), _Clavella_ (subg. _Thersites_), _Fasciolaria_, _Latirus_
(subg. _Polygona_, _Peristernia_, _Leucozonia_, _Lagena_; _Mazzalina_
(Eocene), _Chascax_).

  [Illustration: FIG. 281.--_Latirus_ (_Leucozonia_) _cingulatus_
  Wood, Panama.]

FAM. 8. _Mitridae._--Siphon rather long, with anterior appendages,
eyes on the side of the tentacles, proboscis very long; radula
variable, laterals sometimes lost (Fig. 120, p. 221); shell fusiform,
solid, spire more or less pointed, columella with several prominent
folds, the posterior the largest, aperture rather narrow, no
operculum. Cretaceous----. Principal genera: _Mitra_ (with many
sections), subg., _Strigatella_, _Mitreola_, _Mutyca_, _Dibaphus_;
_Plochelaea_ (Tertiary), _Thala_; _Turricula_ (with several sections),
_Cylindromitra_, and _Imbricaria_.

FAM. 9. _Volutidae._--Foot broad in front, head laterally dilated
into lobes, on which are placed the sessile eyes; siphon prominent,
with appendages at the base (radula, Fig. 122, p. 221); shell thick,
often shining, fusiform, globular or cylindrical, columella projecting
anteriorly, with several folds, the anterior of which is the largest,
aperture notched, canal not produced, operculum generally absent.
Cretaceous----. Principal genera: _Cryptochorda_ (Eocene), _Zidona_,
_Provocator_, _Guivillea_, _Yetus_ (= _Cymbium_), _Voluta_ (with many
sections), _Volutolithes_ (chiefly Eocene), _Volutolyria_, _Lyria_,
_Enaeta_, _Volutomitra_.

FAM. 10. _Marginellidae._--Foot broad, siphon without appendages,
mantle largely reflected over the shell; radula without laterals,
central tooth comb-like, cusps rather blunt; shell oval or conoidal,
polished, aperture narrow, outer lip thickened, columella with many
folds; no operculum. Eocene----. Principal genera: _Marginella_, with
many sections and so-called sub-genera; _Persicula_, _Pachybathron_
(?), _Cystiscus_, _Microvoluta_.

  [Illustration: FIG. 282.--_Voluta nivosa_ Lam., West Australia. ×
  ⅔.]

  [Illustration: FIG. 283.--_Oliva porphyria_ Lam., Panama.]

FAM. 11. _Harpidae._--Foot large, with a transverse groove, separating
off a semi-lunar propodium; mantle partly reflected over the shell;
shell ventricose, polished; spire short, strongly longitudinally
ribbed, ribs prolonged over the suture, columella callous; no
operculum. Eocene----. Single genus, _Harpa_ (subg., _Silia_).

FAM. 12. _Olividae._--Propodium semi-lunar, with a longitudinal groove
above, mesopodium reflected laterally over the shell; central tooth of
radula tricuspid on a very broad base, lateral simple, hooked; shell
sub-cylindrical or fusiform, polished; aperture narrow, operculum
present or absent. Cretaceous----. Principal genera: _Oliva_ (Figs.
283, and 98, p. 199), _Olivancillaria_ (including _Lintricula_ and
_Agaronia_), _Olivella_, _Ancilla_ (subg., _Ancillina_).

(_e_) TOXOGLOSSA (p. 218).--Radula with normal formula 1·0·1, teeth
large; oesophagus with a large poison gland; animal carnivorous,
exclusively marine.

  [Illustration: FIG. 284.--_Terebra subulata_ L., Ceylon.]

  [Illustration: FIG. 285.--_Pleurotoma tigrina_ Lam., E. Indies.]

FAM. 1. _Terebridae._--Eyes at the end of the tentacles, shell
subulate, many whorled, operculum with terminal nucleus. Eocene----.
Single genus, _Terebra_, with several sections.

FAM. 2. _Conidae._--Eyes on outer side of tentacles, siphon prominent;
shell conical or fusiform, aperture narrow. Cretaceous----. Principal
genera: _Conus_, shell solid, spire short, aperture narrow, straight,
internal partitions partly absorbed; _Conorbis_, _Genotia_ (with
several sections, chiefly Tertiary), _Pusionella_, _Columbarium_,
_Clavatula_, _Surcula_, _Pleurotoma_; _Borsonia_ (Eocene), _Drillia_
(subg., _Spirotropis_), _Bela_, _Mangilia_ (including _Daphnella_,
_Clathurella_, and others), _Halia_.

FAM. 3. _Cancellariidae._--Proboscis short, usually no radula, shell
oval, columella strongly plicate; no operculum. Cretaceous----. Single
genus, _Cancellaria_ (subg., _Merica_, _Trigonostoma_, _Admete_).




                              CHAPTER XV

        CLASS GASTEROPODA (_continued_): OPISTHOBRANCHIATA AND
                               PULMONATA


                    =Order III. Opisthobranchiata=

Visceral loop not twisted (except in _Actaeon_) in a figure of 8
(Euthyneurous type, p. 203), auricle usually behind the ventricle,
ctenidium often replaced by secondary branchiae, pallial cavity, if
existing, more or less open, shell present or absent, operculum absent
(except in _Actaeon_), animal hermaphrodite, with separate sexual
openings, marine only.--Carboniferous to present time.

The character of their nervous system decisively removes the
Opisthobranchiata from the Prosobranchiata, and approximates them to
the Pulmonata. _Actaeon_, however, which is streptoneurous, as well
as possessing an operculate shell with prominent spire, forms an
interesting link with the Prosobranchiata. At the opposite extreme
to _Actaeon_ stand forms like _Siphonaria_ and _Gadinia_, which are
probably close links with the Pulmonata (p. 19). The generative
system of the whole group, which is, as in the Basommatophora, of
the hermaphrodite type, without mutual fecundation, is another link
of connexion with the Pulmonata. The respiratory organs present the
most varied forms, sometimes consisting of one ctenidium (never two),
sometimes of secondary branchiae, variously placed, while sometimes no
special organ exists.

The prolongation of the foot into lateral epipodia or parapodia
(possibly to aid in swimming), and the effect of the epipodia upon the
shell, according as they involve it completely or partially, are among
the most instructive features of the Opisthobranchiata. If the epipodia
are developed on the anterior portion of the body, and do not become
reflected, they may, as in most Pteropoda Thecosomata, not directly
affect the shell. But when, as in the Tectibranchiata, the epipodia
are medio-lateral, and tend to envelope the shell, their effect may
be traced by a series of forms varying in proportion to the amount
of shell-surface covered by the epipodia. The two principal lines
along which modification takes place are the gradual reduction of the
spiral nature of the shell, and the gradual lessening of its solidity.
Both these changes are the direct result of the additional protection
afforded to the visceral mass by the reflected epipodia, which renders
the existence of a shell less and less necessary. A precisely similar
line of change is seen in the Pulmonata, culminating in forms like
_Arion_ (p. 174).

  [Illustration: FIG. 286.--Illustrating the transition of form
  in the shell of Tectibranchiata from the pointed spiral to the
  almost flattened plate: =A=, _Actaeon_; =B=, _Aplustrum_; =C=,
  _Cylichna_; =D=, _Atys_; =E=, _Philine_; =F=, _Dolabella_; =G=,
  _Aplysia_; =H=, _Pleurobranchus_. (Not drawn to scale.)]

  [Illustration: FIG. 287.--Illustrating the gradual covering of
  the shell in the Tectibranchiata by the epipodia and mantle: =A=,
  _Haminea_; =B=, _Scaphander_; =C=, _Aplustrum_; =D=, _Aplysia_;
  =E=, _Philine_; _c.d_, cephalic disc; _ep_, _ep_, epipodia; _sh_,
  shell. (Not drawn to scale.)]

The habits of life of the Opisthobranchiata are very varied. Some,
especially the heavier types, burrow in sand, and are then usually
furnished with a broad cephalic disc, as a digging apparatus; some
(certain _Bulla_) flit about in shallow pools on mud flats; others
(_Phyllirrhoe_ and the Pteropoda) swim freely in the open sea; others
(most Nudibranchiata) crawl slug-like on sea-weeds or corallines, and
in colour singularly harmonise with their environment (p. 71 f.);
others again (_Siphonaria_, _Gadinia_), stick limpet-like to rocks
between tide marks. As a rule, they occur only in clean salt water,
but _Embletonia_ has been found in the Victoria Docks at Rotherhithe,
as well as in parts of the Baltic, where the water has only 7 parts
of salt in 1000, while _Limapontia_ occurs in nearly fresh water at
Bornholm and Gothland.

Their food varies greatly. As a rule, they are frugivorous, but many
cases of carnivorous habit occur. _Scaphander_ has been seen to swallow
_Dentalium_ six at a time, and in six hours the shells of all were
reduced to tiny fragments. _Glaucus_ devours the soft portions of the
pelagic _Porpita_ and _Velella_; _Idalia elegans_ eats its way into
the test of Ascidians, and completely buries itself in the body of its
prey.[403]

The Opisthobranchiata may be classified as follows:--

                                             { _Bulloidea_
                        { 1. TECTIBRANCHIATA { _Aplysioidea_
                        {                    { _Pleurobranchoidea_
                        {                    { _Siphonarioidea_
                        {
    =Opisthobranchiata= { 2. ASCOGLOSSA
                        {
                        { 3. NUDIBRANCHIATA  { _Cladohepatica_
                        {                    { _Holohepatica_
                        {
                        { 4. PTEROPODA       { _Thecosomata_
                        {                    { _Gymnosomata_

=Sub-order I.= =Tectibranchiata.=--Right ctenidium usually present,
more or less concealed by the mantle fold, visceral ganglia united by a
very long commissure, shell variable in form, more or less enveloped in
folds of the mantle and foot, often becoming rudimentary.

SECTION I. BULLOIDEA.--Shell more or less spiral, internal or external,
epipodia more or less developed, a broad cephalic disc, distinct from
the dorsal region, usually no tentacles, eyes sessile.

FAM. 1. _Actaeonidae._--Shell spiral, solid, entirely covering the
animal; spire generally prominent, operculum corneous, visceral loop
streptoneurous, no epipodia, radula multiseriate, teeth numerous, very
small. Carboniferous ----. Genera: _Actaeon_ (Fig. 286a); _Volvaria_
(Tertiary), _Fortisia_ (Eocene) _Actaeonina_ (Carboniferous),
_Cylindrites_ (Secondary strata), _Actaeonella_ (Cretaceous).

FAM. 2. _Tornatinidae._--Shell spiral, cylindrical, entirely covering
the animal; spire concealed, cephalic disc with two large tentaculiform
appendages behind, no radula. Genera: _Tornatina_ (= _Utriculus_),
_Volvula_.

FAM. 3. _Scaphandridae._--Shell more or less external, covering all or
nearly all the animal, spire concealed, cephalic disc simple or notched
behind, epipodia well developed, radula with first lateral very large,
stomach sometimes with powerful gizzard. Genera: _Scaphander_ (Fig.
287 B); _Sabatia_ (Pliocene), _Smaragdinella_, _Atys_ (Fig. 286 D),
_Cylichna_ (Fig. 286 C), _Amphisphyra_.

FAM. 4. _Bullidae._--Shell external or partly internal, spire quite or
nearly hidden, cephalic disc broad, without appendages, epipodia often
large; radula usually multiseriate. Genera: _Bulla_ (subg. _Haminea_),
_Acera_, mantle with long filiform appendage, epipodia touching over
the shell; _Cylindrobulla_, _Volvatella_.

FAM. 5. _Aplustridae._--Shell partly internal, overlaid by the
posterior part of the cephalic disc, spire not prominent, epipodia
reflected, tentacles auriform. Single genus, _Aplustrum_ (Fig. 286 B;
subg. _Hydatina_).

FAM. 6. _Ringiculidae._--Shell small, solid, covering all the animal;
spire somewhat prominent, aperture narrow, plicated; peristome thick,
sometimes channelled, cephalic disc with a kind of posterior siphon.
Genera: _Ringicula_; _Avellana_ (Cretaceous).

FAM. 7. _Gastropteridae._--Shell completely internal, nautiloid, small;
epipodia very large, rounded, united behind; cephalic disc simple.
Single genus, _Gastropteron_.

FAM. 8. _Philinidae._--Shell completely internal, thin, slightly
spiral; epipodia thick, cephalic disc large, thick, simple; stomach
usually with powerful gizzard. Genera: _Philine_ (Fig. 287 E),
_Colpodaspis_, _Colobocephalus_, _Chelinodura_, _Phanerophthalmus_,
_Cryptophthalmus_.

FAM. 9. _Doridiidae._--Shell completely internal, a mere pellicle with
a small spiral nucleus, mantle with two posterior lobes and a caudal
filament, epipodia reflected. Single genus, _Doridium_.

SECTION II. APLYSIOIDEA.--Shell small, usually not spiral, sometimes
absent, no cephalic disc, head prominent, with two pairs of tentacles,
epipodia large, more or less reflected.

FAM. _Aplysiidae._--Characters those of the section. Genera: _Aplysia_
(Fig. 287 D), shell arched, flattened, animal large (the “sea hare”);
_Dolabella_, shell subtriangular (Fig. 286 F); _Dolabrifer_, shell
sub-quadrangular, not spiral; _Notarchus_, shell microscopic, spiral;
_Phyllaplysia_, body very depressed, oval, no shell.

SECTION III. PLEAUROBRANCHOIDEA.--Dorsal region protected by a wide
_notaeum_ or dorsal covering, or by a shell; no epipodia, ctenidium
large, external, between the right under surface of the notaeum or
shell and the foot; head short, shell present or absent.

FAM. 1. _Pleurobranchidae._--Shell internal or absent, notaeum with
spicules, radula multiseriate. Genera: _Pleurobranchus_ (Fig. 286 H),
(?) _Haliotinella_, _Pleurobranchaea_, (?) _Neda_.

FAM. 2. _Runcinidae._--Branchial lamellae few, under the posterior
right notaeum, no shell. Single genus, _Runcina_.

FAM. 3. _Umbrellidae._--Shell external, depressed patelliform, not
covering all the animal; foot very thick, ctenidium large, head
depressed, small; radula multiseriate, teeth innumerable, very small.
Genera: _Umbrella_ (Fig. 5A, p. 10), _Tylodina_.

SECTION IV. SIPHONARIOIDEA.--Shell patelliform, branchia replaced
wholly or in part by a pulmonary sac, pulmonary orifice closed by a
small lobe, radula multiseriate, teeth very small.

FAM. _Siphonariidae._--Characters those of the section. Genera:
_Siphonaria_ (branchia as well as pulmonary sac), _Gadinia_ (no
branchia). These genera, hitherto placed among the Pulmonata, have been
recently shown (see p. 19) to be modified Opisthobranchiata.

=Sub-order II.= =Ascoglossa.=[404]--Branchia, mantle cavity, and shell
generally wanting, liver ramified, rami enclosed in external papillae
(cerata) or beneath the dorsal surface, kidney not compact, branched;
radula with one series of strong teeth (Fig. 288), worn out teeth at
the front end not dropping off, but preserved in a special sac (ἀσκός).

According to Bergh, the Ascoglossa form a link between the
Tectibranchiata,--especially the Aplysiidae and Bullidae--and the
Cladohepatic Nudibranchs, while the Pleurobranchidae form a somewhat
similar link between the Holohepatic Nudibranchs and the other
Tectibranchiata.

FAM. 1. _Oxynoeidae._[405]--Animal long, tentacles auriform, epipodia
large, simple, or wing-like, a ctenidium and branchial chamber on right
side, shell small, thin, slightly spiral, not covering much of the
body. Genera: _Oxynoe_ (= _Lophocercus_), _Lobiger_.

FAM. 2. _Hermaeidae._--Body depressed, cerata in several rows, no
branchiae, no shell. Genera: _Hermaea_, _Phyllobranchus_, _Stiliger_,
_Alderia_.

FAM. 3. _Elysiidae._--Body depressed, head rather elevated, tentacles
auriform, sides of body dilated into two large wings, which enclose
branches of the liver and sometimes fold over the dorsal surface, no
branchiae, no shell. Genera: _Elysia_, _Thridachia_, _Placobranchus_.

  [Illustration: FIG. 288.--Radula of one of Ascoglossa (_Elysia
  viridis_ Mont. × 40).]

FAM. 4. _Limapontiidae._--Body slug-like, liver scarcely ramified, no
branchiae, shell, or appendages. Genera: _Limapontia_, _Actaeonia_,
_Cenia_.

=Sub-order III.= =Nudibranchiata.=--Shell absent in the adult, no
ctenidium proper, or osphradium, cerata dorsal or dorso-lateral,
nervous system concentrated, kidney not compact, ramified, penis
retractile, jaws and radula usually present.

SECTION I. CLADOHEPATICA.--Cerata usually latero-dorsal, elongated,
or arborescent, buccal mass strong, jaws present, liver generally
ramified, rami generally entering the cerata.

FAM. 1. _Aeolidiidae._--Body slug-like, head with tentacles and
rhinophores, dorsal area with rows of cerata, which usually contain
sting-cells, radula variable. Genera: _Aeolis_, _Cratena_, _Tergipes_,
_Coryphella_, _Favorinus_, _Facelina_, _Flabellina_, _Fiona_,
_Glaucus_, _Janus_, _Hero_, with many sub-genera.

FAM. 2. _Tethymelibidae._--Body slug-like, large, cerata very large,
no sting-cells, head large, cowl-shaped, no tentacles, rhinophores
much foliated, no radula. Genera: _Tethys_, _Melibe_. The cerata of
_Tethys_, which are capable of independent movement when severed,
have been described as parasitic worms. _Tethys_ feeds on molluscs and
Crustacea.

FAM. 3. _Lomanotidae._--Body slug-like, dorsum prominent, undulating
or lobed, with one row of small cerata, no tentacles, rhinophores
much foliated, radula with uncinated dentate laterals. Single genus,
_Lomanotus_.

FAM. 4. _Dotonidae._--Body slug-like, small, two rows of cerata, each
ceras surrounded by a ring of tubercles, rhinophores simple, radula
uniseriate. Single genus, _Doto_.

FAM. 5. _Dendronotidae._--Body slug-like, somewhat compressed, two rows
of arborescent cerata, no tentacles, frontal margin with arborescent
papillae, rhinophores arborescent, radula multiseriate. Genera:
_Campaspe_, _Dendronotus_.

FAM. 6. _Bornellidae._--Two rows of dorsal papillae, with branchiform
appendages at the base, rhinophores foliate, radula multiseriate.
Single genus, _Bornella_.

FAM. 7. _Scyllaeidae._--Body oblong, compressed, two large foliated
cerata with branchial appendages on the inner side, no tentacles,
rhinophores large, radula multiseriate. Single genus, _Scyllaea_.

FAM. 8. _Phyllirrhoidae._--Body much compressed, with bovine head and
neck, tail tapering, no tentacles, rhinophores simple, teeth few, no
marginals. Single genus, _Phyllirrhoe_.

FAM. 9. _Pleurophyllidiidae._--Body elongate-oval, snout broad, covered
by an arched shield with lateral angles prolonged, branchiae consisting
of two rows of lamellae placed between the notaeum and the foot, no
tentacles, rhinophores short, hidden, radula multiseriate. Single
genus, _Pleurophyllidia_.

FAM. 10. _Pleuroleuridae._--Animal resembling _Pleurophyllidia_, but
without the branchial lamellae. Single genus, _Pleuroleura_.

FAM. 11. _Tritoniidae._--Body long, two rows of unequal arborescent
cerata, rhinophores with ramose appendages, liver not prolonged into
the cerata. Genera: _Tritonia_, _Marionia_.

SECTION 2. HOLOHEPATICA.--Cerata medio-dorsal, retractile or not,
usually paucifoliate, liver never ramified, usually no jaws.

FAM. 1. _Dorididae._--Branchia consisting of a circle or semicircle
of pinnate leaves united at the base, surrounding the anus, almost
always retractile into a cavity, rhinophores foliate, no suctorial
proboscis, radula multiseriate. Genera: _Bathydoris_, _Hexabranchus_,
_Archidoris_ (Fig. 289), _Discodoris_, _Diaulula_, _Cadlina_,
_Centrodoris_, _Platydoris_, _Chromodoris_, _Miamira_, with many
sub-genera.

  [Illustration: FIG. 289.--_Doris_ (_Archidoris_) _tuberculata_
  L., Britain: _a_, anus; _br_, branchiae surrounding the anus;
  _m_, male organ; _rh_, _rh_, rhinophores. × ⅔.]

FAM. 2. _Doriopsidae._--Branchia and rhinophores as in _Dorididae_,
oral aperture pore-shaped, suctorial, no radula. Single genus,
_Doriopsis_.

FAM. 3. _Phyllidiidae._--Body oval, depressed, leathery, a ring of
branchial lamellae, only interrupted by the head and genital papilla,
under the pallial edge, oral aperture pore-shaped, suctorial, no
radula. Genera: _Phyllidia_, _Fryeria_. Bergh unites this and the
preceding family in the group _Porostomata_, which, with Fam. 1, form
the group _Dorididae cryptobranchiatae_.

FAM. 4. _Polyceridae._--Body slug-like, branchiae not retractile,
usually surrounding the anus, rhinophores foliate, tentacles simple,
radula variable, central tooth generally wanting. Genera: _Notodoris_,
_Triopella_, _Aegires_, _Triopa_, _Issa_, _Triopha_, _Crimora_,
_Thecacera_, _Polycerella_, _Palio_, _Polycera_, _Ohola_, _Trevelyana_,
_Nembrotha_, _Euplocamus_, _Plocamopherus_, _Kalinga_.

FAM. 5. _Goniodoridae._--Body oval, depressed, branchia multifoliate,
usually disposed in shape of a horse-shoe, rhinophores foliate,
retractile or not, mouth with a large suctorial proboscis, radula
variable. Genera: _Akiodoris_, _Doridunculus_, _Acanthodoris_,
_Adalaria_, _Lamellidoris_, _Calycidoris_, _Goniodoris_, _Idalia_,
_Ancula_, _Drepania_.

FAM. 6. _Corambidae._--Body otherwise _Doris_-like, but with
two posterior branchiae under the mantle edge, jaws present, no
central tooth, about five laterals. Single genus, _Corambe_ (=
_Hypobranchiaea_). Bergh unites this and the two preceding families in
the group _Dorididae phanerobranchiatae_.

=Sub-order IV.= =Pteropoda.=--The Pteropoda are pelagic animals in
which the lateral portions of the foot are modified into fins, which
are innervated by the pedal ganglia. Their systematic position has
undergone recent revision. It has been the custom to regard them as an
Order of equivalent value to the other four, while some have held them
to be a subdivision of Cephalopoda. Modern authorities, chief among
whom is Pelseneer, regard the Pteropoda not as a primitive, but as a
derived and recent group. They are “Gasteropoda in which the adaptation
to pelagic life has so modified their external characters as to give
them an apparent symmetry.”

The principal point which relates the Pteropoda to the Gasteropoda is
the asymmetry of the visceral organs, intestine, heart, kidney, and
genital gland, which results from their development on one side only
of the body. Their hermaphroditism and the structure of their nervous
system relate them to the Euthyneura rather than to the Streptoneura.
Resemblances in the organs of circulation and generation approximate
them to the Opisthobranchiata rather than to the Pulmonata, while
of the two groups of the former, they tend to closer relationship
with the Tectibranchiata than with the Nudibranchiata. The two
sections of Pteropoda have been considered of distinct origin, the
Thecosomata being derived from the Bulloidea, the Gymnosomata from the
Aplysioidea.[406]

Thus the Pteropoda are a group whose true relations are masked by the
special conditions of their existence, which have tended towards the
development of certain organs, the so-called “wings” and the shell,
which give them an apparent symmetry; this symmetry disappears on a
closer investigation of the internal organs. They are hermaphrodite;
the genital gland has a single efferent duct (except in some
_Cavolinia_), a seminal groove leading to the copulatory organ,
which in the Thecosomata is on the right side of the head, in the
Gymnosomata on the right side of the foot. The genital system resembles
that of the Opisthobranchiata and of the “digonoporous” Pulmonata.

SECTION 1. THECOSOMATA.--Shell or cartilaginoid test always present,
fins united by an intermediate lobe, ctenidia as a rule absent,
replaced by secondary branchiae, no very distinct head or eyes, one
pair of tentacles; cerebral ganglia on the sides of and under the
oesophagus; radula with three rather large teeth in a row, generally
unicuspid, jaw in two pieces, stomach with horny plates, anus generally
on the left side.

The Thecosomata feed on Protozoa and the lower Algae; they have no
proboscis, and the intestine is flexured. The fins are always closely
connected with the head, or what answers to it. About 42 species are
known, belonging to 8 genera.

FAM. 1. _Limacinidae._--Fins very large, branchial chamber dorsal, anus
on right side; shell spiral, sinistral (ultra-dextral, see p. 249),
operculate. Genera: _Limacina_, shell helicoid, deeply umbilicated (_L.
helicina_ swarms in Arctic seas and furnishes food for many Cetacea);
_Peraclis_, spire turreted, aperture large, elongated, produced
anteriorly, no umbilicus; operculum sinistral, in spite of the shell
being ultra-dextral.

FAM. 2. _Cavoliniidae._--Fins large, branchial chamber ventral, shell
a non-spiral cone, angular or round, very thin, embryonic portion
distinct, or formed of two separate plates.

  [Illustration: FIG. 290.--Illustrations of Pteropoda Thecosomata:
  =A=, _Limacina australis_ Eyd.; =B=, _Cleodora cuspidata_ Bosc.
  (shell only); =C=, _Cuvierina columnella_ Rang; =D=, _Creseis
  virgula_ Rang; =E=, _Clio balantium_ Rang; _f_, _f_, fins; _l_,
  liver; _o_, ovary; _sh_, shell. (After Souleyet.)]

In _Cavolinia_ (= _Hyalaea_, Fig. 5 B, p. 10) the shell consists of two
plates, the ventral being convex, with one to three sharp spines at
the posterior end, the dorsal flatter, without spines. The aperture is
broad, contracted dorso-ventrally. Two long pointed prolongations of
the mantle project from the lateral slits of the shell, and probably
serve to balance the bulky body when swimming. Fins trilobed at the
margin. _Cleodora_ has only rudimentary lateral prolongations, fins
bilobed, shell triangular, angles greatly produced, aperture very
wide, dorsal side keeled. In _Cuvierina_ the shell is straight,
sub-cylindrical, with a median partition, slightly expanding towards
the apex, which is truncated in the adult. The principal sub-genera of
_Clio_ are _Creseis_, which has an elongated sub-cylindrical shell,
sometimes slightly curved, smooth or grooved; and _Clio_ proper, in
which the shell is long, angular, with a dorsal rib, apex (= embryonic
shell) rounded, constricted. _Styliola_ and _Hyalocylix_ also belong to
this group.

FAM. 3. _Cymbuliidae._--Test (which is not homologous with the shell of
other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening
of the mantle; embryo with a calcareous, spiral, operculate shell.
Genera: _Cymbulia_, _Cymbuliopsis_, _Gleba_.

Three other families, Hyalithidae, Pterothecidae, and Conulariidae,
from Palaeozoic strata, are generally added to the Thecosomata. All
are fossil only, and it is doubtful whether they are really Molluscan.
Pelseneer holds that no true fossil Pteropoda occur until the lower
Tertiaries.

SECTION 2. GYMNOSOMATA.--Mantle and shell absent in the adult, fins not
connected by a lobe, no branchial chamber, head well developed, with
two pairs of tentacles, eyes on the posterior pair; cerebral ganglia
above the oesophagus; buccal cavity provided with a pair of protrusible
“hook-sacs,” radula generally with 4 to 12 hooked laterals, central
tooth triangular, jaw in one piece, composed of horny plates, no horny
plates in stomach, anus on the right side.

The Gymnosomata are carnivorous, feeding on Thecosomata and other
pelagic animals, being provided for this purpose with a formidable
buccal armature of hook-sacs and suckers. The intestine, as usual in
carnivorous groups, passes straight from the stomach to the anus; the
fins are not attached to the head, but to the anterior part of the
body. The larva has a straight shell, which disappears in the adult.
About 21 species are known, belonging to 7 genera.

FAM. 1. _Pneumodermatidae._--Animal fusiform, fins rather small, head
prominent, anterior part of buccal cavity protrusible, with suckers on
the ventral side, hook-sacs well marked; branchia on right side, skin
soft, pigmented. Genera: _Dexiobranchaea_, no posterior gill, hook-sacs
short; _Spongiobranchaea_, posterior gill circular; _Pneumoderma_, gill
tetraradiate, hook-sacs long.

FAM. 2. _Clionopsidae._--Body barrel-shaped, proboscis three times the
length of the body, no buccal appendages; hook-sacs short, no lateral
gill, posterior gill tetraradiate, skin not pigmented. _Clionopsis_ is
the single genus.

FAM. 3. _Notobranchaeidae._--Body ovate, buccal appendages conical,
no lateral gill, posterior gill with three radiating crests, skin
pigmented. _Notobranchaea_ is the single genus.

FAM. 4. _Clionidae._--Body long, angulated behind, proboscis short,
mouth with two or three pairs of appendages, no jaw, no gills.

_Clione limacina_ is so abundant in Arctic seas as at times to colour
the surface for miles. Each of the cephalic appendages has about 60,000
minute pedicellated suckers.

  [Illustration: FIG. 291.--=A=, Anterior portion of _Pneumoderma_;
  =B=, _Clione limacina_ Phipps; =C=, _Halopsyche Gaudichaudi_
  Soul.; _f_, _f_, fins; _h.s_, _h.s_, hook-sacs; _l.f_, lobe of
  the foot; _s_, _s_, suckers; _o_, posterior genital orifice; _t_,
  _t_, tentacles. (After Souleyet.)]

FAM. 5. _Halopsychidae._--Body ovate, thick, rounded behind, no gill
or proboscis, fins long, narrow, broadened at the ends, epidermis
sub-cartilaginoid.

_Halopsyche_ (= _Eurybia_) has the power of withdrawing its head
completely into a sort of pocket, which is closed by an anterior fold
of the mantle. There are two long non-retractile buccal appendages.


                         =Order IV. Pulmonata=

Gasteropoda with two pairs of tentacles, visceral loop euthyneurous,
ganglia concentrated round the oesophagus; breathing air by a
pallial cavity formed by the union of the front edge of the mantle
with the cervical region, sexes united, shell present or absent, no
operculum[407] (except in _Amphibola_).

=Sub-order I.= =Basommatophora.=--Eyes generally at the base of the
tentacles, which are not retractile, male and female genital orifices
separate, radula (p. 235) multiseriate, shell always present, external.
Fresh water or quasi-marine.

FAM. 1. _Auriculidae._--Breathing organ a pulmonary sac or true lung;
shell spiral, conoidal, internal partitions usually absorbed, aperture
more or less strongly toothed. Jurassic----. Genera: _Auricula_,
_Carychium_, _Scarabus_, _Alexia_, _Tralia_, _Plecotrema_, _Cassidula_,
_Melampus_, _Leuconia_, _Pedipes_ (Fig. 292).

  [Illustration: FIG. 292.--Examples of the _Auriculidae_: =A=,
  _Auricula Judae_ Lam., Borneo; =B=, _Scarabus Lessoni_ Blainv.,
  E. Indies; =C=, _Cassidula mustelina_ Desh., N. Zealand;
  =D=, _Melampus castaneus_ Mühlf., S. Pacific; =E=, _Pedipes
  quadridens_ Pfr., Jamaica.]

FAM. 2. _Otinidae._--Shell auriform, spire very short. Genera: _Otina_,
_Camptonyx_.--Recent only.

FAM. 3. _Amphibolidae._--A pulmonary sac on right side of neck,
eyes almost pedunculate, shell turbinate, rudely sculptured,
operculate.--Recent. Genus, _Amphibola_ (Fig. 293); subg. _Ampullarina_.

FAM. 4. _Limnaeidae._--Pulmonary sac protected by an external lobe;
shell variable, fragile. Jurassic----. (i.) _Ancylinae_, shell more or
less limpet-shaped. Genera: _Ancylus_, _Gundlachia_, _Latia_. (ii.)
_Limnaeinae_, shell spiral. Genera: _Limnaea_, _Amphipeplea_, _Erinna_,
_Lantzia_, _Pompholyx_, _Choanomphalus_ (with _Carinifex_). (iii.)
_Planorbinae_, shell sinistral, spire flattened or elevated. Genera:
_Planorbis_, _Isidora_ (= _Bulinus_).

FAM. 5. _Physidae._--Mantle more or less reflected over the shell
(radula, Fig. 141C, p. 235); shell sinistral, lustrous. Jurassic----.
Genera: _Physa_, _Aplecta_.

  [Illustration: FIG. 293.--_Amphibola avellana_ Chem.]

FAM. 6. _Chilinidae._--Lobe of pulmonary sac large, tentacles broad;
shell ventricose, rather solid; columella plicate. Miocene----. Single
genus, _Chilina_.

=Sub-order II.= =Stylommatophora.=--Two pairs of retractile tentacles
(except in _Janella_), eyes at the tip of the upper pair, male and
female orifices united (except in Vaginulidae and Onchidiidae), no
distinct osphradium.

FAM. 1. _Testacellidae._--Animal carnivorous, slug-like or spirally
coiled, no jaw (whence the name _Agnatha_, often given to this group),
radula with usually few, large, sickle-shaped teeth (p. 232), shell
variable, rarely absent, usually external. Cretaceous----. Principal
genera: _Chlamydephorus_ (shell a simple plate, internal), _Apera_,
_Testacella_ (slug-like, shell terminal), _Strebelia_, _Streptostyla_,
_Glandina_, _Salasiella_, _Petenia_, _Pseudosubulina_, _Streptostele_,
_Tomostele_, _Streptaxis_ (Fig. 203), _Gibbus_, _Ennea_, _Daudebardia_
(Fig. 193), _Schizoglossa_, _Guesteria_, _Aerope_, _Paryphanta_,
_Rhytida_ (subg. _Diplomphalus_, _Elaea_ and _Rhenea_).

  [Illustration: FIG. 294.--=A=, _Ennea_ (_Gibbulina_) _palanga_
  Fér; =A´= young of same; =B=, Gibbus lyonetianus Pall.]

FAM. 2. _Selenitidae._--Shell internal, external, or absent; jaw
present, radula Testacellidan, central tooth present. Tertiary----.
Genera: _Selenites_, _Plutonia_, _Trigonochlamys_, _Pseudomilax_ (?),
_Rathouisia_ (?).

FAM. 3. _Limacidae._--Shell present or absent, internal or external,
spiral or not, tail often with a mucus pore, jaw (Fig. 107 A, p. 211)
with projecting rostrum on cutting edge, radula with central tooth
tricuspid, laterals bi- or uni-cuspid, marginals aculeate. Eocene----.
Genera: _Otoconcha_, _Urocyclus_, _Mariaella_ (subg. _Tennentia_),
_Parmarion_, _Helicarion_, _Cystopelta_, _Aspidelus_, _Estria_,
_Vitrinopsis_ (subg. _Vitrinoidea_, _Parmella_), _Damayantia_, _Nanina_
(= _Ariophanta_, including _Pachystyla_, _Rhysota_, _Hemiplecta_,
_Trochonanina_, _Euplecta_, _Orpiella_, _Xesta_, _Macrochlamys_,
_Microcystis_, _Sitala_, _Kaliella_, _Durgella_, _Austenia_, _Girasia_,
_Parmacochlea_, _Africarion_, _Sesara_, _Macroceras_, and others),
_Vitriniconus_, _Parmacella_, _Limax_ (subg. _Amalia_ and many
sections), _Vitrina_ (subg. _Vitrinozonites_, _Velifera_), _Zonites_
(subg. _Stenopus_, _Moreletia_, _Mesomphix_, _Hyalinia_, _Gastrodonta_,
_Pristiloma_, _Poecilozonites_, _Thyrophorella_).

FAM. 4. _Philomycidae._--Shell absent, jaw limacidan, radula
helicidan, shield covering all the body. Single genus, _Philomycus_ (=
_Tebennophorus_), with subg. _Pallifera_.

FAM. 5. _Helicidae._--Shell present or absent, internal or external;
jaw of various types, radula with central tooth tricuspid, equal in
size to the first laterals, laterals bi- or tricuspid, marginals
smaller, cusped. Eocene----. Principal genera: _Oopelta_ (no shell),
_Arion_ (shell absent or formed of calcareous granules), _Ariolimax_,
_Geomalacus_, _Anadenus_ (subg. _Prophysaon_), _Hemphillia_,
_Cryptostracon_, _Binneya_, _Helix_ (see below), _Cochlostyla_,
_Bulimus_ (subg. _Borus_, _Orphnus_, _Dryptus_, _Strophochilus_,
_Pachyotus_, and possibly _Caryodes_, _Leucotaenia_, _Liparus_,
_Livinhacea_, _Pachnodus_, _Rachis_, _Atopocochlis_, _Cerastus_,
_Clavator_ belong here, or with _Buliminus_), _Berendtia_, _Rhodea_.
Pilsbry proposes[408] to group _Helix_ as follows:

A. Eggs or young very large at birth:

(1) _Macroön_, incl. _Acavus_, _Pyrochilus_ (= _Phania_), _Stylodonta_,
_Helicophanta_.

B. Eggs or young smaller or minute at birth:

(2) _Belogona._--Female genital system with dart sac and mucus
gland. _Helix_ [restricted] (with sections _Arionta_, _Campylaea_,
_Chilotrema_, _Pomatia_, _Macularia_, _Tachea_, _Iberus_, _Leptaxis_,
_Eulota_, _Fruticicola_, _Xerophila_; _Dorcasia_, _Acusta_,
_Plectotropis_, _Aegista_, _Cathaica_, _Satsuma_, _Euhadra_;
_Lysinoe_), _Gonostoma_, _Leucochroa_, _Allognathus_, _Cochlostyla_,
_Polymita_, _Hemitrochus_ (with sections _Plagioptycha_, _Dialeuca_,
_Coryda_, _Jeanerettia_), _Glyptostoma_, _Acanthinula_, _Vallonia_.

(3) _Teleophalla._--Female system without accessories, male with
flagellum and appendix on penis; no epiphallus. _Sagda_, _Cysticopsis_.

(4) _Epiphallophora._--Female system without accessories, male
with epiphallus on penis; no appendix. _Caracolus_ (with sections
_Lucerna_, _Dentellaria_, _Isomeria_, _Labyrinthus_, _Eurycratera_,
_Parthena_, _Polydontes_, _Theliodomus_, _Cepolis_), _Camaena_ (incl.
_Phoenicobius_), _Obba_, _Chloritis_ (incl. _Hadra_), _Papuina_,
_Planispira_ (subg. _Cristigibba_).

  [Illustration: FIG. 295.--Example of the _Macroön_ group of
  _Helix_. _Helicophanta Souverbiana_ Fisch., Madagascar, showing
  large embryonic shell. × ⅔.]

(5) _Haplogona._--All accessory organs absent, jaw soldered into one
piece. _Polygyra_ (incl. _Daedalochila_, _Triodopsis_, _Mesodon_,
_Stenotrema_), _Endodonta_ (incl. _Libera_, _Charopa_, _Gerontia_,
_Therasia_, and others), _Patula_, _Trochomorpha_, _Anoglypta_.

(6) _Polyplacognatha._--All accessory organs absent, jaw composed
of 16–24 separate plates. _Punctum_, _Laoma_.

  [Illustration: FIG. 296.--_Odontostomus pantagruelinus_ Moric.,
  S. Brazil. × ½.]

Genera of doubtful position: _Strobilops_, _Ampelita_, _Pedinogyra_,
_Polygyratia_, _Macrocyclis_, _Solaropsis_.

FAM. 6. _Orthalicidae._--Radula, p. 233. Shell external, large,
bulimoid. Single genus, _Orthalicus_; subg. _Liguus_, _Porphyrobaphe_,
_Corona_.

FAM. 7. _Bulimulidae._--Radula, p. 233; jaw, p. 211; shell usually
external. Genera: _Bulimulus_ (incl. _Plecochilus_, _Goniostomus_,
_Drymaeus_, _Liostracus_, _Otostomus_, _Navicula_, _Scutalus_,
_Peronaeus_, _Eurytus_, _Eudioptus_, _Plectostylus_, _Mesembrinus_,
_Mormus_, etc.; _Thaumastus_, _Nesiotis_), _Placostylus_ (incl.
_Charis_), _Amphidromus_, _Partula_, _Calycia_ (?), _Peltella_ (animal
limaciform, shell internal), _Pellicula_, _Amphibulimus_ (incl.
_Simpulopsis_).

FAM. 8. _Cylindrellidae._--Radula, p. 233; shell many whorled, long
turriculate, last whorl often detached, apex often truncated (Fig. 169,
p. 260). Eocene----. Genera: _Cylindrella_ (with sections _Callonia_,
_Thaumasia_), _Leia_, _Macroceramus_, _Pineria_.

FAM. 9. _Pupidae._--Radula, p. 233; shell external, spire usually long,
aperture often narrowed, more or less toothed, often with internal
lamellae. Carboniferous----. Genera: _Anostoma_ (Fig. 154, p. 248),
_Hypselostoma_ (Fig. 202A, p. 302); _Anastomopsis_ (Cretaceous),
_Lychnus_ (Cretaceous), _Boysia_, _Odontostomus_ (incl. _Tomigerus_),
_Buliminus_ (incl. _Petraeus_, _Napaeus_, _Zebrina_, _Mastus_,
_Chondrula_, _Ena_, and perhaps _Rachis_, _Pachnodus_, _Hapalus_,
and others), _Pupa_ (incl. _Torquilla_, _Pupilla_, _Sphyradium_,
_Leucochila_, etc.), _Zospeum_, _Vertigo_, _Megaspira_, _Strophia_,
_Holospira_, _Eucalodium_ (incl. _Coelocentrum_), _Coeliaxis_,
_Perrieria_, _Balea_; _Rillya_ (Eocene), _Clausilia_ (with many
sub-genera), _Rhodina_ (?).

  [Illustration: FIG. 297.--=A=, _Clausilia crassicosta_ Ben.,
  Sicily; =B=, _Clausilia macarana_ Zieg., Dalmatia; =B´=,
  clausilium of same.]

FAM. 10. _Stenogyridae._--Radula, p. 234; shell long, spiral,
shining, more or less translucid, apex blunt, sometimes decollated.
Eocene----. Genera: _Stenogyra_ (subg. _Rumina_, _Obeliscus_, _Opeas_,
_Melaniella_, _Spiraxis_, _Leptinaria_, _Nothus_, _Subulina_,
_Glessula_), _Ferussacia_ (subg. _Cionella_, _Azeca_), _Caecilianella_
(subg. _Geostilbia_). _Achatina_ (shell large, ventricose, columella
strongly truncate), with the sub-genera _Perideris_, _Limicolaria_,
_Columna_, _Pseudachatina_, _Homorus_, probably belongs to a distinct
family.

FAM. 11. _Achatinellidae._--Radula, p. 234; shell small, bulimoid,
indifferently dextral or sinistral. Genera: _Achatinella_ (subg.
_Auriculella_, _Amastra_, _Carelia_), _Tornatellina_.

FAM. 12. _Succineidae._--Radula, p. 234; lower pair of tentacles
wanting or small; shell internal or external, thin, spiral or not, last
whorl large. Eocene----. Genera: _Succinea_, _Homalonyx_, _Hyalimax_,
(?) _Lithotis_, (?) _Catinella_.

FAM. 13. _Janellidae._--Radula, p. 234; animal slug-like, no lower
tentacles, shell an internal plate. Single genus, _Janella_ (=
_Athoracophorus_), with subg. _Aneitea_.

FAM. 14. _Vaginulidae._--Radula, p. 234; animal slug-like, covered with
a coriaceous mantle, lower tentacles bifid, genital orifices widely
separated, male behind the lower right tentacle, female on inferior
median part of right side, anus and pulmonary orifice nearly terminal;
shell absent. Single genus, _Vaginula_ (= _Veronicella_).

  [Illustration: FIG. 298.--_Achatina zebra_ Lam., S. Africa. × ½.]

FAM. 15. _Onchidiidae._--Body oval, mantle thick, often warty,
sometimes set with “eyes” (p. 187), two tentacles, genital orifices
widely separate, anus and pulmonary orifice as in _Vaginula_; no shell.
Genera: _Peronia_, _Onchidium_, _Onchidiella_. The family appears to be
an instance of Pulmonata reverting to marine habits of life.




                              CHAPTER XVI

                   CLASSES SCAPHOPODA AND PELECYPODA


                           CLASS SCAPHOPODA

Head rudimentary, mantle edges ventrally concrescent, forming a tube
opening before and behind, and covered with a shell of the same shape;
sexes separate.

  [Illustration: FIG. 299.--Anatomy of _Dentalium_: _a_, anterior
  aperture of mantle; _f_, foot; _g_, genital gland; _k_, kidney;
  _l_, liver. (After Lacaze-Duthiers.)]

The Scaphopoda form a small but very distinct class, whose organisation
is decidedly of a low type. The body is usually slightly curved, the
concave side being the dorsal; muscles near the posterior end attach
the body to the shell. The foot, which can be protruded from the
anterior or wider aperture, is rather long, pointed, and has sometimes
two lateral lobes (_Dentalium_), sometimes a terminal retractile disc
(_Siphonodentalium_), sometimes a retractile disc with a central
tentacle (_Pulsellum_). The cephalic region, as in Pelecypoda, is
covered by the mantle. The mouth is situated on a kind of projection
of the pharynx; the buccal mass, containing the radula (p. 236), is
at the base of the foot, and the intestine branches forward from the
front part of the stomach. The liver (Fig. 299) is paired, and consists
of a number of symmetrical, radiating coeca. There are no eyes, but
on each side of the mouth are small bunches of exsertile filaments
(_captacula_), which appear to act as tactile organs for the seizing
of food. There is no special respiring apparatus, heart or arterial
system, breathing being conducted by the walls of the mantle. The
nervous system has already been described (p. 205).

Two kidneys open on either side of the anus. The genital gland is
large, occupying nearly all the posterior part of the body, the sexual
products being emitted through the right kidney. The veliger has
already been figured (p. 131, Fig. 44). The embryonic shell is formed
of two calcareous laminae, which subsequently unite to form the tube.

With regard to their general relationships, the Scaphopoda resemble
the Gasteropoda in their univalve shell, and in the possession of a
radula; while the pointed foot, the non-lobed velum in the veliger, the
generative system, the bilateral symmetry of the organs generally, and
the absence of any definite head, eyes, or tentacles, are points which
approximate them to the Pelecypoda.

The Scaphopoda are known from Devonian strata to the present time. They
are found at a depth of a few fathoms to very deep water. The only
three genera are _Dentalium_, _Siphonodentalium_ (subg. _Cadulus_), and
_Pulsellum_, which differ in the structure of the foot, as described
above.


                           CLASS PELECYPODA

Cephalic region rudimentary, mantle consisting of two symmetrical right
and left lobes, covering the body and secreting a bivalve shell hinged
at the dorsal margin; no radula, sexes usually separate. Reference has
already been made to the reproductive system (p. 145), breathing organs
(p. 164 f.), mantle (p. 172), nervous system (p. 205), digestive system
(p. 237 f.), and nomenclature of the various parts of the shell (p. 269
f.).

The shape of the shell, in many Pelecypoda, involving as it does the
position, size, and number of the adductor muscles, is probably due to
mechanical causes, depending on the habits and manner of life of the
individual genus. Thus in a typical dimyarian or two-muscled bivalve,
_e.g._ _Mya_ (Fig. 300, A), the adductor muscles lie well towards each
end of the long axis of the shell, with the hinge about midway between
them. In this position they are best placed for effectually closing the
valves, and since they are nearly equidistant from the axis of motion,
_i.e._ from the hinge, they do an equal amount of work, and are about
equal in size. But in a form like _Modiola_, where the growth of the
shell is irregular in relation to the hinge-line, the anterior muscle
is brought nearer and nearer to the umbones, where its power to do
work, and therefore its size, becomes less and less. But the work to
be done remains the same, and the posterior muscle has to do it nearly
all; hence it moves farther and farther away from the hinge-line, and
at the same time gains in size. In shells like _Ostrea_, _Pecten_,
and _Vulsella_, the anterior muscle, having drawn into line with the
hinge and the posterior muscle, becomes atrophied, while the posterior
muscle, having double work to do, has doubled its size.[409]

  [Illustration: FIG. 300.--Illustrating changes in the position
  and size of the adductor muscles according to the shape of the
  shell: =A=, _Mya_; =B=, _Modiola_; =C=, _Vulsella_. The upper
  dotted line shows the hinge-line, the lower connects the two
  muscles.]

The development of the foot, again, largely depends upon habits of
life. It is well developed in burrowing forms, while in sessile genera
(_Ostrea_, _Chama_, _Spondylus_) it becomes unnecessary and aborts.
Even in _Pecten_, which does not become sessile, but has ceased to use
the foot as an organ of progression, a similar result follows. Forms
which burrow deeply often “gape” widely, sometimes at one end only,
sometimes at both. _Venus_, _Donax_, _Tellina_, _Mactra_, which are
shallow burrowers, do not gape; _Solen_, _Lutraria_, and to a less
degree _Mya_, burrow deeply and gape widely. In order to burrow deeply
the foot must be highly developed, and the larger it becomes, the
more will it tend to keep the valves apart at the place where it is
habitually protruded. Burrowing species always remain in communication
with the surface by means of their siphons, the constant extension of
which tends to keep the valves apart at the end opposite to the foot.
Burrowing species, again, tend to burrow in such a way as to descend
most easily, and not be impeded by their own shells; in other words,
they act as a wedge, and descend with their narrowest part foremost.
But the burrowing organ, the foot, has to follow suit, and gradually
draws round to the narrowest part of the shell, so that the habitual
deep burrower, such as _Lutraria_, lies with its long axis exactly at
right angles to the surface, its siphons protruding from, and keeping
open, the uppermost or posterior margin of the shell, and the foot
producing the same effect upon the lower or anterior margin. The deeper
the burrower, the more elongated does the shell become, until, through
forms like _Pholas_ and _Saxicava_, we arrive at _Solen_, the most
highly specialised burrower of all, in which the breadth of the shell
is equal throughout, and no obstructive curve exists to impede its
rapid ascent or descent.

The Pelecypoda have been classified in various ways; by the
completeness or sinuation of the pallial line, depending on the absence
or presence of siphons, by the number of adductor muscles, by the
character of the hinge-teeth, and by the number of the branchiae.
For various reasons, none of these methods have proved entirely
satisfactory. That adopted here was suggested by Pelseneer, and
depends upon the character of the branchiae themselves, as suggesting
successive stages of development (p. 166 f.).


                      =Order I. Protobranchiata=

Branchial filaments not reflected, the two rows inclined at a right
angle (more or less), ventral surface of foot more or less flattened,
byssogenous apparatus little developed, a single anterior aorta,
kidneys distinct, sexes separate, each genital gland opening into the
corresponding kidney.

FAM. 1. _Nuculidae._--Labial palps very large, rows of branchial
filaments at right angles to one another, mantle edges open, siphons
contracted, foot disc-shaped, elongated; shell equivalve, oval, or
produced, interior generally nacreous, hinge with numerous saw-like
teeth. Silurian ----. Principal genera: _Nucula_ (heart dorsal to the
rectum); _Palaeoneilo_ (Devonian), (?) _Sarepta_, _Leda_, _Yoldia_,
_Malletia_; _Tyndaria_ (Upper Tertiary), _Lyrodesma_ (Silurian),
_Actinodonta_ (Silurian), _Babinka_ (Silurian).

FAM. 2. _Solenomyidae._--Labial palps united, one row of branchial
filaments pointing dorsally, the other ventrally; mantle edges in
great part united postero-ventrally, a single siphonal orifice with
two very long tentacles, foot proboscidiform, with a round denticulate
disc at the end; shell equivalve, resembling a Solen, with a strong
corneous periostracum; no hinge-teeth, ligament internal. Single genus,
_Solenomya_. (?) Cretaceous ----].


                      =Order II. Filibranchiata=

Rows of branchial filaments parallel, pointing ventrally, reflected,
and provided with interfilamentary ciliated junctions, foot usually
with a well-developed byssogenous apparatus.

=Sub-order I.= =Anomiacea.=--Heart dorsal to the rectum, a single
aorta, foot small, anterior adductor very small; shell ostreiform, no
hinge-teeth, fixed by a calcified byssus traversing the right valve
(Fig. 173, p. 262).

FAM. _Anomiidae._--Jurassic ----. Genera: _Anomia_, _Placunanomia_;
_Carolia_ (Eocene), _Placuna_; _Hypotrema_ (Jurassic), _Placunopsis_
(Oolite).

=Sub-order II.= =Arcacea.=--Mantle edge open, both adductors well
developed, heart with two aortae, branchiae free, without interlamellar
junctions, no siphons; renal and generative apertures distinct.

FAM. 1. _Arcadae._--Mantle edge with composite eyes; shell round or
trapezoidal, solid, often with stout bushy periostracum; ligament
often external, on a special area; hinge with numerous lamelliform
teeth. Ordovician ----. Principal genera: _Arca_ (incl. _Barbatia_,
_Scaphula_, and _Cucullaea_), heart dorsal to rectum; _Pectunculus_,
_Glomus_, _Limopsis_; _Trinacria_ and _Nuculina_ (Tertiary).

FAM. 2. _Trigoniidae._--Foot large, hatchet-shaped, with ventral
disc; no byssus, mantle edge with ocelli; shell sub-trigonal,
hinge-teeth few, strong; interior violet-nacreous. Devonian ----.
Genera: _Trigonia_; _Myophoria_ and _Schizodus_ (Trias), _Cyrtonotus_
(Devonian).

  [Illustration: FIG. 301.--_Trigonia pectinata_ Lam., Sydney,
  N.S.W.]

=Sub-order III.= =Mytilacea.=--Mantle edges fused at one point, anal
orifice distinct, anterior terminal adductor small, one aorta,
branchiae with interfoliary junctions, genital glands penetrating the
side of the mantle and opening by the side of the kidneys.

FAM. _Mytilidae._--Byssus well developed, shell more or less equivalve,
oval, broad; hinge-teeth evanescent. Devonian----. Principal genera:
_Mytilus_?, _Myalina_, _Septifer_, _Modiola_, _Lithodomus_, _Crenella_,
_Dacrydium_, _Myrina_, _Idas_, _Modiolaria_, _Modiolarca_.


                 =Order III. Pseudolamellibranchiata=

Mantle edges entirely open, foot little developed, anterior adductor
usually aborted, branchial filaments reflected, with interlamellar
junctions, which are sometimes vascular; genital glands opening into
the kidneys or close to the apertures of the kidneys.

FAM. 1. _Aviculidae._--Foot long, tongue-shaped, byssogenous apparatus
well developed, branchiae concrescent with the mantle, adductor muscle
sub-central, at times a small anterior adductor, siphons absent;
shell usually inequivalve, dorsal margin straight, often very long,
winged, lateral teeth much prolonged; structure of shell cellular,
inside prismatic, outside nacreous. Palaeozoic ----. Principal genera:
_Avicula_, including _Meleagrina_, _Malleus_; _Vulsella_ (no wings or
hinge-teeth); _Perna_, including _Crenatula_, _Inoceramus_ (ligaments
in a number of fossettes); _Aucella_ and _Monotis_ (Palaeozoic and
Secondary); _Pterinaea_ and _Ambonychia_ (Palaeozoic); _Pinna_;
_Aviculopinna_ (Carboniferous).

  [Illustration: FIG. 302.--_Avicula heteroptera_ Lam., Australia,
  showing the inequivalve shell and byssal sinus (_bs_).]

FAM. 2. _Prasinidae._--Shell very small, umbones anterior, incurved,
anterior side depressed, hinge-teeth replaced by dentiform projections
of the lunule fitting into corresponding grooves. Recent. Single genus,
_Prasina_.

FAM. 3. _Ostreidae._--Heart generally ventral to the rectum, branchiae
concrescent with the mantle, no byssus; shell inequivalve, fixed by the
left valve, form irregular. Jurassic ----. Genera: _Ostrea_; _Heligmus_
(Oolite), _Naiadina_ (Cretaceous), _Pernostrea_ (Jurassic).

FAM. 4. _Pectinidae._--Byssus usually absent, mantle edge open,
duplicated, folded back, with pallial ocelli; branchiae not concrescent
with the mantle; shell with unequal “ears” at the umbo, hinge-teeth
lamelliform, often obscure. Silurian----. Principal genera:
_Pedum_, _Chlamys_, _Hinnites_, _Hemipecten_, _Amussium_, _Pecten_;
_Aviculopecten_ (Palaeozoic), _Crenipecten_.

FAM. 5. _Limidae._--Mantle edge as in _Pecten_, tentaculate; shell
sub-equivalve, eared, fixed by a byssus or free. Carboniferous----.
Genera: _Lima_ (Fig. 85, p. 179), _Limea_.

FAM. 6. _Spondylidae._--Foot with a peduncular appendage, no byssus,
numerous pallial ocelli; shell fixed by right valve, surface often
very spinose, two cardinal teeth in each valve. Jurassic----. Genera:
_Plicatula_, _Spondylus_; _Terquemia_ (Lias).

  [Illustration: FIG. 303.--_Pecten pallium_ L., East Indies.]

  [Illustration: FIG. 304.--_Spondylus petroselinum_ Sowb.,
  Mauritius; on a coral.]

FAM. 7. _Dimyidae._--Shell ostreiform, fixed, hinge with or without
symmetrical teeth, two muscular impressions. Single genus, _Dimya_
(Tertiary).


                    =Order IV. Eulamellibranchiata=

Mantle edges united at one or more points, branchiae with
interfilamentary junctions which are always vascular, genital glands
not opening into the kidneys, usually two adductor muscles.

=Sub-order I. Submytilacea.=--Mantle edges more or less open, anal
orifice distinct, usually no siphons, pallial line usually simple,
cardinal and lateral teeth well marked.

FAM. 1. _Carditidae._--Foot with a byssus or groove, branchiae large,
unequal; shell equivalve, solid, radiately grooved, one or two oblique
cardinal teeth, one or two laterals. Silurian ----. Principal genera:
_Venericardia_, _Cardita_, _Carditella_, _Carditopsis_, _Milneria_;
_Pleurophorus_ (Palaeozoic), _Anodontopsis_ (Silurian).

FAM. 2. _Astartidae._--A short anal siphon, labial palps large;
shell triangular, thick, ligament external, hinge with two or three
cardinals in each valve, laterals obscure.? Devonian ----. Principal
genera: _Astarte_; _Pachytypus_ (Jurassic), _Plesiastarte_ (Eocene),
_Parastarte_, _Woodia_, _Opis_ (Secondary strata), _Prosocoelus_
(Devonian).

FAM. 3. _Crassatellidae._--Mantle with anal orifice or open; shell
equivalve, thick, subtriangular, ligament in an internal fossette,
hinge with two cardinals, laterals produced. Cretaceous----. Principal
genus, _Crassatella_.

FAM. 4. _Cardiniidae._--Shell equivalve, oval or triangular,
ligament external, cardinal teeth small, laterals fairly strong.
Devonian----Oolite. Principal genera: _Cardinia_, _Anthracosia_,
_Carbonicola_, _Anoplophora_.

FAM. 5. _Cyprinidae._--Anal and branchial orifices complete, papillose,
foot thick; shell variable, equivalve, thick, umbones often spiral,
hinge-teeth very variable, ligament external. Jurassic----. Principal
genera: _Cyprina_; _Pygocardia_ (Crag), _Veniella_ (Cretaceous),
_Venilicardia_ (Secondary strata), _Anisocardia_ (Jurassic),
_Isocardia_, _Libitina_, _Coralliophaga_; _Basterotia_ (Eocene).
The families _Pachydomidae_ (Palaeozoic) and _Megalodontidae_
(Palaeozoic--Secondary) are probably related to the Cyprinidae.

  [Illustration: FIG. 305.--_Isocardia vulgaris_ Reeve, China.]

FAM. 6. _Aetheriidae._--Anal orifice complete, foot absent, labial
palps large; shell irregular, free or fixed, no hinge-teeth.
Fluviatile, recent only. Genera: _Aetheria_, _Mülleria_, _Bartlettia_.

FAM. 7. _Unionidae._--Foot large and thick, no byssus, anal siphon
short, branchial orifice complete or not, siphon present or absent,
embryo of certain groups passing through a _glochidium_ stage (p. 146);
shell equivalve, sometimes very thick, nacreous within, hinge variable.
Fluviatile. Jurassic----. Principal genera: _Unio_ (subg. _Arconaia_),
_Monocondylaea_, _Pseudodon_, _Anodonta_, _Solenaia_, _Mycetopus_,
_Mutela_, _Spatha_, _Hyria_, _Castalia_, _Leila_.

FAM. 8. _Dreissensiidae._--Both siphons prominent, foot tongue-shaped,
byssiferous; shell mytiliform, with small internal septum. Genera:
_Dreissensia_; _Dreissensiomya_ (Tertiary). The common _Dreissensia
polymorpha_ Pall. was distributed over large parts of Europe in later
Tertiary times. From unknown causes it died out, and has during the
past two hundred years been regaining its position, migrating N. and
W. from its original habitat, the Caspian, by the Volga and its Oka
confluent.

FAM. 9. _Modiolopsidae_.--Shell mytiliform, ligament exterior,
hinge-teeth small, rather numerous. Palaeozoic----. Principal genera:
_Modiolopsis_, _Cyrtodonta_, _Mytilops_, _Ptychodesma_.

FAM. 10. _Lucinidae._--Anal orifice sometimes with a siphon, branchial
orifice complete or not, sometimes a single branchia; foot very long,
vermiform, no byssus, anterior adductor long; shell rounded, equivalve,
blanched, hinge with two cardinals and two laterals in each valve,
sometimes toothless, ligament more or less internal. Silurian----.
Principal genera: _Lucina_, _Corbis_, _Axinus_, _Diplodonta_,
_Montacuta_.

FAM. 11. _Ungulinidae._--Anal orifice complete, foot vermiform, no
byssus, two branchiae; shell equivalve, subcircular, hinge-teeth
variable, no laterals, adductor impressions long, continuing the
pallial line. Tertiary----. Single genus, _Ungulina_.

FAM. 12. _Unicardiidae._--Shell equivalve, round or oval, cardinal
shelf large, a single cardinal in each valve, ligament external.
Carboniferous--Cretaceous. Genera: _Unicardium_, _Scaldia_,
_Pseudedmondia_.

FAM. 13. _Kellyellidae._--Anal siphon prolonged, no marked branchial
orifice; shell very small, oval or round, anterior lateral very strong,
under the cardinal. Eocene----. Genera: _Kellyella_; _Allopagus_ and
_Lutetia_ (Tertiary), _Turtonia_.

FAM. 14. _Erycinidae._--Mantle edges with three apertures, branchial
orifice on the buccal margin, foot long, broadened, with a byssus,
animal usually viviparous. Tertiary----. Genera: _Erycina_, _Kellia_,
_Pythina_, _Lasaea_, _Lepton_.

FAM. 15. _Galeommidae._--Mantle edges more or less reflected
over the shell, apertures and foot as in Erycinidae; shell thin,
equilateral, hinge with few teeth or none. Tertiary----. Genera:
Galeomma, _Scintilla_, _Sportella_, _Chlamydoconcha_, _Hindsiella_,
_Ephippodonta_ (Fig. 32, p. 81).

FAM. 16. _Cyrenidae._--Siphons short, foot large, no byssus; shell
equivalve, subtriangular, with periostracum, hinge with two or three
cardinals, laterals present; animal hermaphrodite, viviparous. Fresh
or brackish water. Jurassic----. Genera: _Cyrena_, _Corbicula_
(subg. _Batissa_, _Velorita_), _Sphaerium_ (= _Cyclas_), _Pisidium_,
_Galatea_, _Fischeria_.

The families _Cyrenellidae_ (single genus, _Cyrenella_) and _Rangiidae_
(single genus, _Rangia_) are probably to be placed here.

=Sub-order II. Tellinacea.=--Siphons long, separate, foot and labial
palps very large, pallial sinus deep, two adductor muscles.

FAM. 1. _Tellinidae._--External branchial fold directed dorsally,
foot with byssogenous slit, but no byssus, branchiae small; shell
compressed, equivalve, ligament external, at least two cardinals in
each valve, laterals variable. Cretaceous----. Principal genera:
_Tellina_ (with many sections), _Gastrana_.

FAM. 2. _Scrobiculariidae._--Animal as in _Tellina_; shell orbiculate
or long oval, equivalve, hinge-teeth weak, ligament in an internal
cavity. Tertiary----. Principal genera: _Scrobicularia_, _Syndosmya_,
_Theora_, _Cumingia_, _Semele_.

FAM. 3. _Donacidae._--External branchial fold directed ventrally; shell
equivalve, subtriangular, solid, smooth, two or three cardinals in each
valve, laterals variable, ligament external. Jurassic----. Genera:
_Donax_, _Iphigenia_, _Isodonta_.

  [Illustration: FIG. 306.--_Tellina rastellum_ Hanl., East Indies.]

FAM. 4. _Tancrediidae._--Shell donaciform, ligament external, cardinals
usually two in each valve, posterior laterals strong. Trias----.
Genera: _Tancredia_ (Secondary strata), _Hemidonax_.

FAM. 5. _Cardiliidae._--Shell heart-shaped, hinge as in Mactridae,
posterior adductor resting on a myophore or shelf. Single genus,
_Cardilia_. Tertiary----.

FAM. 6. _Mesodesmatidae._--Mantle edges largely united, with three
orifices, foot byssiferous or not; shell regular or irregular,
usually one cardinal and strong lateral teeth. Tertiary----. Genera:
_Mesodesma_, _Ervilia_.

FAM. 7. _Mactridae._--External branchial fold directed ventrally,
siphons fused, foot tongue-shaped; shell equivalve, triangular-oval,
hinge with ligament in an internal fossette, another portion external,
a bifurcated cardinal tooth in the left valve, fitting into a branching
tooth in the right valve, laterals present. Jurassic----. Genera:
_Nactra_, _Harvella_, _Raëta_, _Eastonia_, _Heterocardia_, _Vanganella_.

=Sub-order III.= =Veneracea.=--Branchiae slightly folded, foot
compressed, siphons generally short, pallial line variable, two
adductor muscles.

FAM. 1. _Veneridae._--Siphons free or partly united, foot seldom
byssiferous; shell solid, equivalve, hinge usually with three cardinal
teeth, laterals variable. Jurassic----. Principal genera: _Cytherea_,
_Circe_; _Grateloupia_ (Tertiary), _Meroe_, _Dosinia_ (= _Artemis_),
_Cyprimeria_, _Cyclina_, _Venus_, _Clementia_, _Lucinopsis_; _Thetis_
(Cretaceous), _Tapes_, _Venerupis_.

FAM. 2. _Petricolidae._--Animal perforating rocks; shell oval, slightly
gaping behind, two or three cardinals, no laterals, pallial sinus well
marked. Recent----. Genera: _Petricola_, _Naranio_.

  [Illustration: FIG. 307.--_Cytherea dione_ Lam., Peru.]

FAM. 3. _Glaucomyidae._--Siphons long, united, foot small; shell
produced, thin, hinge with three cardinals, no laterals, pallial sinus
well marked. Recent----. Genus, _Glaucomya_ (incl. _Tanysiphon_).

=Sub-order IV.= =Cardiacea.=--Branchiae much folded back, mantle edges
with three apertures, foot cylindroidal, more or less produced, siphons
present or absent, one or two adductor muscles, pallial line variable.

FAM. 1. _Cardiidae._--Siphons rather long, foot long, no byssus;
shell equivalve, more or less radiately ribbed, hinge with one
or two cardinals in each valve, laterals variable, ligament
external, two adductors. Brackish water or marine. Devonian----.
Genera: _Byssocardium_ and _Lithocardium_ (Tertiary), _Conocardium_
(Palaeozoic), _Cardium_ (with many sections, including _Hemicardium_),
_Limnocardium_ (subg. _Didacna_, _Monodacna_, _Adacna_).

[Illustration: FIG. 308.--_Cardium_ (_Hemicardium_) _cardissa_ L., East
Indies.]

FAM. 2. _Lunulicardiidae._--Shell equivalve, very inequilateral
subtriangular, anterior margin short or truncated, with a deep lunule.
Single genus, _Lunulicardium_ (Palaeozoic).

FAM. 3. _Tridacnidae._--Mantle orifices widely separated, foot short,
byssiferous, no anterior adductor; shell equivalve, large, thick,
usually gaping in front, one cardinal tooth and one or two posterior
laterals in each valve, no pallial sinus. Miocene----. Genera:
_Tridacna_, _Hippopus_. The muscular power of the great _Tridacna_ is
immense. Once caught between their gaping valves, a man’s hand or foot
can scarcely be withdrawn. Two valves of _T. gigas_ in the British
Museum weigh respectively 154 and 156 lbs.

FAM. 4. _Chamidae._--Mantle orifices widely separated, foot short, no
byssus, both adductors present, ovary invading the mantle lobes; shell
fixed, irregularly inequivalve, umbones spiral, ligament external,
cardinal teeth often a mere ridge, anterior lateral strong, nearly
central, no pallial sinus. Jurassic----. Genera: _Chama_; _Diceras_
(Jurassic), attached by one umbo, umbones very prominent, teeth strong;
_Heterodiceras_ (Jurassic), _Requienia_ (Cretaceous), left valve widely
spiral, attached by the umbo, right valve small, fitting on the other
as an operculum, teeth obsolete; _Toucasia_, _Apricardia_, _Matheronia_
(all Secondary strata).

  [Illustration: FIG. 309.--=A=, _Requienia ammonea_ Goldf.,
  Neocomian, × ½; =B=, _Hippurites cornu-vaccinum_ Goldf.,
  Cretaceous, × ¼. _a_, right valve; _f_, point of fixture. (From
  Zittel.)]

The four succeeding families require special study in a work on
Palaeontology.

FAM. 5. _Monopleuridae._--Shell very inequivalve, left valve
operculiform, right conical or spiral, fixed at the apex, ligament
prolonged in external grooves. Cretaceous----. Genera: _Monopleura_,
_Valletia_.

FAM. 6. _Caprinidae._--Shell very inequivalve, thick, free or fixed
by apex of right valve, which is spiral or conical, left valve
spiral or not, often perforated by radial canals from the umbo to
the free margin. Neocomian and Cretaceous----. Principal genera:
_Plagioptychus_, _Caprina_, _Ichthyosarcolites_, _Caprotina_,
_Polyconites_.

FAM. 7. _Hippuritidae_ (= _Rudistae_).--Shell very inequivalve,
externally as in Caprinidae, umbo central in left valve, no ligament
proper, left valve with strong hinge-teeth and grooves, two adductor
impressions on prominent myophores, shell structure of the two valves
differing. Cretaceous only. Single genus, _Hippurites_ (Fig. 309, B).

FAM. 8. _Radiolitidae._--Shell inversely conical, biconical, or
cylindrical, general aspect of _Hippurites_, umbo of left valve central
or lateral, right valve with a thick outer layer, often foliaceous,
umbonal cavity partitioned off by laminae. Cretaceous only. Genera:
_Radiolites_, _Biradiolites_.

=Sub-order V.= =Myacea.=--Branchiae much folded back, mantle edges
usually with three openings, foot compressed, siphons large, united or
not, two adductor muscles, pallial line variable.

FAM. 1. _Psammobiidae._--Siphons long, not united, foot large, not
byssiferous; shell equivalve, long, oval, slightly gaping at the ends,
ligament external, prominent, two cardinal teeth in each valve, no
laterals, a deep pallial sinus. Jurassic----. Genera: _Psammobia_,
_Solenotellina_, _Sanguinolaria_, _Asaphis_, _Elizia_, _Quenstedtia_
(Jurassic).

FAM. 2. _Myidae._--Pedal orifice small, siphons long, united in great
part; shell inequivalve, gaping at one or both ends, periostracum
more or less extensive, ligament internal, resting on a prominent
shelf; hinge-teeth variable. Cretaceous----. Genera: _Mya_, _Tugonia_,
_Sphenia_, _Corbula_, _Lutraria_ (for which latter some propose a
separate family).

FAM. 3. _Solenidae._--Foot long, powerful, more or less cylindrical, no
byssus, siphons usually short, united or not, branchiae narrow; shell
equivalve, long and narrow, gaping at both ends, with periostracum,
umbones flattened, ligament external, hinge-teeth variable.?
Devonian----. Genera: _Solecurtus_, _Pharella_, _Pharus_, _Cultellus_,
_Siliqua_, _Ensis_, _Solen_, _Orthonota_ (?), _Palaeosolen_ (?).

FAM. 4. _Glycimeridae._--Pedal orifice very narrow, siphons long,
united in great part, often covered with periostracum; shell more
or less equivalve, gaping at both ends, hinge toothless or with
two weak cardinals, ligament external; animal free or perforating.
Cretaceous----. Genera: _Glycimeris_, _Saxicava_, _Cyrtodaria_.

FAM. 5. _Gastrochaenidae._--Foot small, cylindrical, no byssus,
branchiae narrow, siphons long; shell perforating or cemented to
a shelly tube, gaping widely on the anterior and ventral sides,
no hinge-teeth, a deep pallial sinus. Cretaceous----. Genera:
_Gastrochaena_, _Fistulana_ (tube with a median diaphragm, perforated
by the siphons).

=Sub-order VI.= =Pholadacea.=--Mantle edges largely closed, siphons
long, united, foot short, truncated, disc-shaped, ligament absent, two
adductor muscles; animal perforating.

FAM. 1. _Pholadidae._--Organs contained within the valves, ctenidia
prolonged into the branchial siphon, shell more or less gaping, thin,
dorsal margin in part reflected over the umbones, one or more dorsal
accessory pieces, no hinge-teeth, an interior apophysis proceeding
from the umbonal cavity. Jurassic----. Genera: _Pholas_, _Talona_,
_Pholadidea_ (posterior extremity of the valves prolonged by a corneous
appendage, a passage to the long tube of _Teredo_), _Jouannetia_,
_Xylophaga_, _Martesia_; _Teredina_ (Eocene).

  [Illustration: FIG. 310.--_Teredo navalis_ L.: =V=, valves of
  shell; =T=, tube; =P=, pallets; =SS=, siphons. (After Möbius.)]

FAM. 2. _Teredinidae._--Animal vermiform, ctenidia mainly within the
branchial siphon, siphons very long, with two calcareous appendages
(“pallets”) near the anterior end, shell very small, continued into a
long calcareous tube, valves deeply notched, internal apophysis as in
Pholadidae. Lias----. Single genus, _Teredo_ (Fig. 310).

=Sub-order VII.= =Anatinacea.=--External branchial fold directed
dorsally, not reflected, sexes united, ovaries and testes with separate
orifices, mantle edges largely united, byssus usually absent, two
adductor muscles, pallial line variable, shell usually nacreous within.

FAM. 1. _Pandoridae._--Siphons short, largely united, foot
tongue-shaped; shell free or fixed, inequivalve, semi-lunar, or
subtriangular, ligament often with calcareous ossicle, pallial line
complete or with slight sinus. Cretaceous----. Genera: _Pandora_,
_Myodora_, _Myochama_.

FAM. 2. _Chamostreidae._--Shell fixed, _Chama_-like, thick, umbones
spiral, ligament with ossicle. Single genus, _Chamostrea_.

FAM. 3. _Verticordiidae._--Siphons not prolonged; shell heart-shaped,
umbones prominent, spiral, ligament with an ossicle, pallial line
complete. Miocene----. Genera: _Verticordia_, _Mytilimeria_,
_Lyonsiella_.

FAM. 4. _Lyonsiidae._--Foot short, byssiferous, siphons short,
separate, shell inequivalve, hinge-teeth usually absent, ligament and
ossicle in an internal groove. Eocene----. Single genus, _Lyonsia_.

  [Illustration: FIG. 311.--_Myochama Stutchburyi_ A. Ad., attached
  to _Circe undatina_ Lam., Moreton Bay.]

FAM. 5. _Ceromyidae._--Shell inequivalve, large, heart- or
wedge-shaped, hinge toothless, ligament internal in one valve, external
in the other. Secondary strata----. Genera: _Ceromya_, _Gresslya_.

FAM. 6. _Arcomyidae._--Shell equivalve, thin, surface finely
granulated, hinge toothless, cardinal edge dentiform, ligament
external. Secondary and Tertiary strata----. Genera: _Arcomya_,
_Goniomya_, _Pleuromya_, _Machomya_.

FAM. 7. _Anatinidae._--A fourth (?byssal) pallial orifice, siphons
long, separate or fused; shell thin, sometimes inequivalve, exterior
often granulose, ligament often with ossicle, hinge toothless or with
lamellae. Jurassic----. Genera: _Anatina_, _Plectomya_ (Secondary),
_Periploma_, _Cochlodesma_, _Thracia_, _Tyleria_, _Alicia_,
_Asthenothaerus_.

FAM. 8. _Grammysiidae._--Shell equivalve, oval, ligament external,
cardinal margin straight, toothless, pallial line complete.
Palaeozoic----. Principal genus, _Grammysia_, with many other genera of
toothless hinge, but whose exact affinities are uncertain.

FAM. 9. _Praecardiidae._--Shell thin, equivalve or not, radiately
ribbed, margins dentated, sub-umbonal area as in Arca, hinge toothless.
Palaeozoic----. Principal genus, _Praecardium_.

FAM. 10. _Pholadomyidae._--A fourth pallial orifice, siphons very
long, united, foot small; shell thin, equivalve, with radiating ribs,
ligament external, hinge toothless, pallial line sinuate. Jurassic----.
Single genus, _Pholadomya_.

FAM. 11. _Clavagellidae._--Foot rudimentary, siphons long, united,
contained in a long calcareous tube; shell small, one or both
valves soldered in the tube, tube with a centrally fissured disc
at the anterior end, more or less frilled at the posterior end.
Cretaceous----. Genera: _Clavagella_, _Brechites_ (= _Aspergillum_).


                      =Order V. Septibranchiata=

Mantle edges united at three points, branchiae replaced by a muscular
septum extending from the anterior adductor to the point of separation
of the siphons, septum with symmetrical orifices.

FAM. 1. _Poromyidae._--Branchial septum with groups of lamellae between
the orifices, labial palps large, foot long and narrow, siphons short,
papillose, separated, animal hermaphrodite; shell small, slightly
inequivalve, rounded or oblong, nacreous within. Eocene----. Genera:
_Poromya_, _Silenia_.

FAM. 2. _Cuspidariidae._--Siphons long, united in part, foot short,
animal dioecious; shell small, slightly inequivalve, rostrate, not
nacreous, each valve with ligamentary cartilage spoon-shaped, with
a calcareous ossicle, cardinal and lateral teeth present or absent.
Jurassic----. Single genus, _Cuspidaria_, with many sections.




                              BRACHIOPODA

                                PART I

                          RECENT BRACHIOPODA

                                  BY

                        ARTHUR E. SHIPLEY, M.A.

            Fellow and Tutor of Christ’s College, Cambridge




                             CHAPTER XVII

                          RECENT BRACHIOPODA

   INTRODUCTION--SHELL--BODY--DIGESTIVE SYSTEM--BODY
   CAVITY--CIRCULATORY SYSTEM--EXCRETORY
   ORGANS--MUSCLES--NERVOUS SYSTEM--REPRODUCTIVE
   SYSTEM--EMBRYOLOGY--HABITS--DISTRIBUTION--CLASSIFICATION


                            =Introduction=

The group Brachiopoda owes its chief interest to the immense variety
and great antiquity of its fossil forms. Whereas at the present time
the number of extant species amounts to but about 120, Davidson in
his admirable monograph[410] on the British Fossil Brachiopoda has
enumerated close upon 1000 fossil species, found within the limits of
the United Kingdom alone.

The amount of interest that the group in question has excited amongst
naturalists is evinced by the invaluable Bibliography of Brachiopoda,
prepared by the same author and his friend W. H. Dalton.[411] This
monument of patient research contains over 160 quarto pages, each with
the titles of from eighteen to twenty separate papers dealing with
Brachiopods, published between the years 1606 and 1885.

Probably the first reference to Brachiopods in zoological literature
is to be found in a work entitled _Aquatilium et Terrestrium aliquot
Animalium_, published in the year 1606 by Prince Fabio Colonna at Rome.
This work contains the first description of a Brachiopod under the
name of _Concha diphya_. In a second edition, which is not so rare in
our libraries as the first, the author mentions three more species
of Brachiopods. Towards the end of the same century, Martin Lister of
Oxford, in his _Historia sive Synopsis methodica Conchyliorum_ which
appeared in parts, described and figured a considerable number of
Brachiopods, which, under the name of _Anomia_, were until the present
century regarded as Molluscs, and placed in the subdivision Pelecypoda
(Lamellibranchiata).

The first satisfactory figure and description of a _Terebratula_ were
published in the year 1766, in Pallas’ _Miscellanea Zoologica_, still
under the name _Anomia_. In 1781 O. F. Müller figured a _Crania_, under
the name _Patella anomala_, the generic name being subsequently altered
by Cuvier into _Orbicula_.

Bruguière in the year 1789 was the first to recognise the relationship
between _Lingula_ and the other Brachiopods. He for the first time
saw the stalk of this genus, and compared it with that of the stalked
Barnacles, a class of animals which has been more than once associated
with our group.

Cuvier in his _Mémoire sur l’Anatomie de la Lingule_, 1797, gave
the first account of the internal anatomy of a Brachiopod. The same
naturalist first described the nephridia, although his mistake in
considering them lateral hearts was not rectified until the middle of
the present century, when Huxley pointed out that these structures
serve as excretory ducts for the genital products.

Duméril in 1807 proposed the somewhat unfortunate name of Brachiopoda;
and although efforts have been made by de Blainville, who suggested
Palliobranchiata, and more recently by Haeckel, who proposed
Spirobranchiata, to arrive at a name which would be both grammatically
and physiologically more correct, the older name has maintained its
position, and is now universally in use.

In 1834 and 1835 Professor Owen published the results of his researches
into the anatomy of the Brachiopoda. He investigated in these years the
structure of _Waldheimia flavescens_, of a species of _Lingula_ and of
a _Discina_, called by him _Orbicula_. He regarded the group as midway
between the Pelecypoda and the Ascidians. The structure of _Lingula_
was further investigated by Carl Vogt, who in 1851 also supported the
view that the Brachiopoda were related to the Mollusca. But already
in 1847 and 1848 Steenstrup had thrown doubts upon this relationship,
and had maintained that the Order was more closely related to certain
members of the Chaetopoda, a view which afterwards found its ablest
supporter in the American naturalist Morse.

D’Orbigny seems to have been the first observer who drew attention
to the resemblances alleged to exist between the Brachiopoda and the
Polyzoa, and Hancock, in his masterly works _On the Anatomy of the
Fresh-water Bryozoa_ (_Polyzoa_) and in his _Organisation of the
Brachiopoda_, dwelt on these resemblances, and placed the Brachiopoda
between the Polyzoa on the one hand and the Ascidians on the other; a
collocation which subsequently resulted in their inclusion in the now
discarded group of Molluscoidea.

In 1854 Huxley[412] published what is, with the possible exception of
Hancock’s monograph, mentioned above, the most important work upon the
anatomy of the Brachiopoda with which we are acquainted. He corrected
numerous errors of his predecessors and added many new facts to our
knowledge of the group. He was the first to describe the true nature
of the lateral hearts of Cuvier, and to describe the true heart,
afterwards so carefully figured by Hancock.

A further step was made in 1860 and 1861 by the discovery and
description of the larvae of Brachiopoda, by F. Müller and
Lacaze-Duthiers. Since that time we owe what little advance has been
made in the embryology of the group to the researches of Morse and of
Kowalevsky.

Modern methods of research--section cutting, etc.--were first applied
to the group by the Dutch naturalist, van Bemmelen,[413] from whose
admirable historical account of our knowledge of the group many of the
above facts have been gathered. These methods have thrown considerable
light upon the histology of the group, but have not added very much to
our knowledge of the structure or the affinities of the Brachiopoda.
The modern views as to the latter point may be best discussed after
some account of the anatomy of the various genera has been given.


                              =The Shell=

The body of a Brachiopod is enclosed within a bivalve shell, but the
two halves are not, as they are in the Pelecypoda, one on each side
of the body, but occupy a different position with regard to the main
axes of the body. What this position is, has formed the subject of a
good deal of discussion. For our purpose, however, it will suffice
to distinguish the two valves by the most commonly accepted terms of
_dorsal_ and _ventral_. The former is, as a rule, the smaller
of the two, and usually lies on the lower surface of the animal in
life. Adopting the orientation indicated above, the stalk by means of
which the Brachiopoda are attached to the rocks and stones, etc., upon
which they live, becomes posterior, and the broader edge of the two
shells, which are capable of being opened to some extent, is anterior.

  [Illustration: _Fig. 312._--Four specimens of _Terebratulina
  caput serpentis_, attached to a waterlogged piece of wood, from
  the Clyde area.]

The posterior end of the shell usually narrows, and the ventral valve
projects behind the dorsal, and may be produced into a sort of beak
or funnel, through the aperture of which the stalk protrudes. This
aperture may be completed by the ventral shell, or the latter may only
be notched, in which case the hole is completed by the posterior edge
of the dorsal shell.

The nature of the shell has been used in classifying the group into two
orders:--

   I. The Ecardines, whose shell is chitinous but slightly
   strengthened by a deposit of calcareous salts. There is no hinge
   and no internal supports for the arms. The alimentary canal
   terminates in an anus.

   II. The Testicardines, whose shells are composed of calcareous
   spicules. The valves are hinged together, and there is usually
   an internal skeleton supporting the arms. There is no anus.

The outside of the shell of recent Brachiopods is often smooth, but
many are ridged. In a recent species, _Rhynchonella Döderleini_ from
Japan, Davidson[414] has described a number of spines arranged in
concentric circles on the ribbed shell. They are not so long as the
spines irregularly scattered on the shell of _Rh. spinosa_ from the
Jurassic formations. Some shells are brightly coloured, as, for
instance, the various species of _Cistella_ which live on the coralline
rock in the Mediterranean; these exhibit bands or rays of alternate
orange and bright pink. On the other hand, the shells of _Terebratula
vitrea_ are of a slightly translucent white, and of the utmost
delicacy. They are very large, so that the cavity within the valves is
of much greater size than the body of the animal, but in other genera
the soft parts are packed very closely, and there is but a very small
mantle-cavity or space within the shell unoccupied by the body of the
animal. It is, however, more common for the shells of Brachiopods to be
of a dull yellowish colour, and to be somewhat massive. Most species
are attached by a pedicle or stalk to some rock or stone at the bottom
of the sea, but in some, as in _Crania_, the ventral valve becomes
closely adherent to its support, so much so that it is difficult, or
impossible, to remove the animal without leaving the ventral valve
behind. _Lingula_, like _Crania_, one of the Ecardines, lives in sand
(Fig. 321, p. 483), and does not use its long pedicle to adhere to any
fixed object. . The outline of the shell varies extremely. It may be
almost round or prolonged along either axis; the edges of the valves
may be smooth or fluted in correspondence with the ridges and grooves
of the outside of the shell.

  [Illustration: FIG. 313.--Three specimens of _Crania anomala_ on
  a stone dredged in Loch Fyne. The topmost specimen is seen in
  profile.]

On the inner surface of the shell of the Testicardinate Brachiopoda,
at the hinder end of the ventral valve, are two lateral teeth, which
fit into corresponding sockets in the dorsal valve. These form a
hinge, which in many cases is so arranged as to permit the shell
to be opened to only a very limited extent. There are also certain
plate-like processes which project into the lumen of the shell, and
help to support various portions of the body; and in _Terebratula_,
_Waldheimia_, etc., these form a complicated band-like loop, which
increases in complexity with advancing age, and serves to support the
arms. In the extinct _Spirifera_ the internal skeleton takes the form
of two spirally coiled lamellae, which almost entirely fill the cavity
of the shell; the apices of the spirals point outwards (Fig. 330). The
inner surface of the shell also bears the marks of the insertion of the
numerous muscles which govern its movements.

Microscopic examination of thin sections of the shell shows that it
consists of small prisms or spicules of calcareous substance, whose
long axis lies, roughly speaking, at right angles to the surface of the
shell. These spicules are held together by an organic matrix, in which,
however, no cellular elements can be detected. In sections made through
a decalcified shell the position of the spicules which have been
dissolved by the acid is indicated by spaces, and the matrix remains
as a network of fibrils, which end on the outside in a thin cuticular
layer of organic matter. In _Lingula_ and _Discina_ the organic matter
takes a much larger share in the formation of the shell, which in
these genera consists of a number of alternating layers of horny and
calcareous matter. The former is described by Gratiolet as fibrillated,
the fibrils being obliquely placed, whilst the latter consists of a
number of small prisms set at right angles to the surface of the shell.

In many genera, as in _Terebratula_, _Terebratella_, _Cistella_,
_Waldheimia_, _Crania_, etc., the shell is pierced by a number of small
canals (Fig. 314), which in the dried specimens form so many open
pores, but in the living animal contain prolongations of the mantle
or body wall which secretes the shell. They contain extensions of the
layer of epithelial cells which lines and secretes the shell. The
canals come to the surface and at their outer end are often slightly
swollen. They are closed by the cuticular layer which is mentioned
above as covering the shell externally. They are not found in the loops
or other internal processes of the shell. In _Crania_ the canals depart
to some extent from the usual type; instead of running a straight
course to a somewhat swollen outer end, they break up into a number of
very fine branching tubules, which form a very minute meshwork near
the surface of the shell. These fine branches contain protoplasmic
fibrils, which have their origin in the epithelial cells which lie in
the tubules.

By carefully counting the number of tubules in a given area of young
and old specimens of _Waldheimia cranium_, van Bemmelen[415] was able
to show that the spaces between the tubules did not increase with age.
He therefore reasoned that the shells of Brachiopoda do not increase by
intussusception, and that their increase in size must be entirely due
to additions made round their free edge.

The function of the tubules has been a matter of some discussion.
They have been regarded as respiratory organs, but it would seem more
reasonable to suppose that they serve as organs to supply nourishment,
etc., to the organic matrix of the shell.[416]

With the exception of the genus _Crania_, it is usual for Brachiopods
to bear round the edge of their mantle rows or bundles of chitinous
setae or bristles (Figs. 315 and 319). The length and arrangement of
these structures vary in the different species; they are secreted from
little pits in the edge of the mantle. It seems probable that they
serve to some extent as organs of defence, especially in the larva,
where they make their appearance at an early stage; possibly they also
serve as a filter, and prevent the entrance of foreign bodies into
the shell. Their presence has been taken to indicate a certain degree
of affinity between Brachiopods and Chaetopods, since setae are very
characteristic of the last-named group.


                              =The Body=

The shell of a Brachiopod is secreted partly by the general surface of
the body which is situated at the hinder end of the shell, and partly
by the two leaf-like extensions of the body, which are termed the
dorsal and ventral mantles. These are, in fact, folds of the body wall,
and into them the body cavity and certain of its contents, such as the
liver and generative glands, etc., extend. The space between the two
folds of the mantle, which is limited behind by the anterior wall of
the body, is termed the pallial or mantle cavity. On each side of the
middle line the anterior wall of the body is produced into two “arms,”
which occupy as a rule a considerable part of the mantle cavity. These
arms may be but flattened portions of the general body wall, which
occupies a large part of what in other genera is the mantle of the
dorsal valve, as in _Cistella_ and _Argiope_;[417] or they may be
outgrowths of the body wall in the form of long processes, which are
coiled and twisted in a very characteristic manner in the various
genera. In any case the cross section of the arm shows a groove, one
side of which forms a continuous lip, and the other takes the form of
a single row of tentacles, which are richly ciliated and capable of
considerable movement. The whole arm in _Rhynchonella_ can be protruded
from the shell, as was noted years ago by O. F. Müller, and although
his statement to this effect has often been doubted, its truth was
confirmed by Professor Morse,[418] who writes: “In the year 1872,
while studying living _Rhynchonella_ in the St. Lawrence, I observed a
specimen protrude its arms to a distance of 4 c.m. beyond the anterior
borders of the shell, a distance nearly equalling twice the length of
the shell.” The same observer also mentions that _Lingula_ has the
power of partially protruding its arms. In most genera the cirrhi or
tentacles can alone be protruded.

  [Illustration: FIG. 314.--View of the left half of _Cistella_
  (_Argiope_) _neapolitana_, which has been cut in two by a median
  longitudinal incision, to show the disposition of the organs.
  Partly diagrammatic. The inorganic part of the shell only is
  shown. The tubular extensions of the mantle and the organic outer
  layer are not indicated, and hence the pores appear open.

    1. The ventral valve.
    2. The dorsal valve.
    3. The stalk.
    4. The mouth.
    5. Lip which overhangs the mouth and runs all round the tentacular
         arms.
    6. Tentacles.
    7. Ovary in dorsal valve.
    8. Liver diverticula.
    9. Occlusor muscle; its double origin is shown.
    10. Internal opening of left nephridium.
    11. External opening of left nephridium.
    12. Ventral adjuster. The line from 10 crosses the dorsal adjuster.
    13. Divaricator muscle.]

The cilia which clothe the tentacles keep up a constant flow of water
into the mantle cavity. This stream not only serves to aerate the blood
of the animals--a process which probably takes place through the thin
inner lining of the mantle--but it also brings with it a number of
diatoms and other minute organisms which serve as food. These particles
become entangled in the tentacles, and are ultimately lodged in the
groove at their base, and passing along this by the action of the cilia
they find their way into the wide mouth, into which the groove deepens
in the posterior median line.


                        =The Digestive System=

The mouth leads into an oesophagus; this widens into a chamber which
may be termed the stomach (Fig. 314), and which receives the openings
of two large branching glands usually known as the liver. The stomach
passes into a short intestine which is usually bent at about a right
angle with the oesophagus. In the Testicardines the intestine ends
blindly, but in the Ecardines it is of much greater length, and
terminates in an anus, situated posteriorly in the median line in
_Crania_, but asymmetrical and to the right of the body in _Lingula_
(Fig. 315) and _Discina_; in both cases, however, the opening is into
a portion of the mantle cavity. The alimentary canal is supported by
a median dorsal and ventral mesentery, and by two pairs of lateral
mesenteries which pass to the body wall. The lateral mesenteries are
not always quite distinct. When they are, the anterior pair are known
as the gastro-parietal bands, and the posterior as the ileo-parietal.
In _Rhynchonella_ there are two pairs of renal organs, and each of
these mesenteries bears the internal openings of one pair. In all other
Brachiopods there is only one pair, and they are supported by the
ileo-parietal bands.

The alimentary canal is ciliated throughout, and some interesting
observations have been made by Schulgin[419] on the shortening of
these cilia in _Argiope_ (_Cistella_) when the animal is well fed,
and their elongation when the animal is hungry. Amongst the ciliated
cells certain glandular cells have been described. The so-called liver
consists of two more or less branching glands, which open by wide
apertures, one on each side of the stomach. It seems probable that a
good deal of digestion is carried on in these glands, since the diatoms
and other minute organisms upon which the Brachiopoda live are usually
found in the branches of these glands, and the glandular cells lining
the tubules vary much in appearance according to the animal’s state of
nutrition.


                           =The Body Cavity=

The alimentary canal and liver occupy a considerable portion of the
body cavity or general space of the body; this space is to some extent
cut up by the various mesenteries above mentioned. It also lodges the
reproductive organs and the excretory ducts. Its walls are ciliated,
and the action of the cilia keeps in motion the corpusculated fluid
that bathes the various organs in the body cavity. The mantles, which
are nothing but flattened leaf-like extensions of the body wall lining
the shell, also contain diverticula of the body cavity, which may be
simple flattered spaces or may be broken up into definite channels, as
in _Lingula_ (Fig. 315). It seems not improbable that the body
cavity fluid is aerated through the thin inner layer of the mantle.

  [Illustration: FIG. 315.--View of the inner side of a valve
  of _Lingula anatifera_ (after François), to show the definite
  arrangement of the channels in the mantle: _a_, position of
  mouth; _b_, position of anus.]

Running along the base of each arm are two canals, a small one at the
base of the tentacles, which we may term the tentacular canal, and a
larger one, the canal of the lip. The former sends a prolongation into
each tentacle. The latter is, according to Blochmann, a closed canal
in _Crania_, _Lingula_, _Rhynchonella_, and others; but according
to Joubin,[420] it communicates in _Crania_ at one point with the
tentacular canal. It is probably originally a part of the body cavity.
Blochmann[421] states in very definite terms that in _Crania_ neither
the large canal nor the small canal communicates with the general body
cavity, but he admits that in _Lingula_ the small canal opens into that
space.


                       =The Circulatory System=

The details of the discovery of the central circulatory organ of
Brachiopods form a curious and instructive chapter in the history of
modern morphological inquiry. Hancock, in his monograph on the group,
described and figured on the dorsal surface of the alimentary canal a
well-developed heart, which had been previously noticed by Huxley, who
first showed that the organs which up to his time had been regarded
as hearts were in reality excretory organs. In connexion with this
heart Hancock described numerous arteries, distributed to various
parts of the body. The observers who have written upon the anatomy of
Brachiopods since Hancock’s time, in spite of the fact that they had
at their disposal such refined methods of research as section cutting,
which was quite unknown at the time his monograph was written, have
almost all failed to find this circulatory system, and many of them
have been tempted to deny its existence. Blochmann,[422] however,
in the year 1885 stated that he had found the heart, and had seen
it pulsating in several species of Brachiopoda which he had rapidly
opened whilst alive. Joubin has also described it in large specimens of
_Waldheimia venosa_, and recently Blochmann has published a detailed
account of his work on this subject. Both these authors describe
the heart as a vesicle with muscular walls, situated dorsal to the
alimentary canal. From this, according to Blochmann, a vessel--the
branchio-visceral of Hancock--runs forward as a triangular split in the
dorsal mesentery supporting the alimentary canal. This vessel divides
into two at the oesophagus, and passing through some lacunae in the
walls of this tube, opens into the tentacular canal, and consequently
supplies the tentacles with blood. These two canals which diverge
from the median artery are connected ventrally by a vessel which runs
below the oesophagus; the latter is therefore surrounded by a vascular
ring. Blochmann also describes two pairs of vessels that were seen
and figured by Hancock. A pair of these pass over the gastro-parietal
mesenteries and into the dorsal mantle sinus, the second pair pass
over the ileo-parietal mesenteries and into the ventral mantle sinus;
each of these four arteries runs to one of the four generative
glands, which, as is so usually the case in the animal kingdom, have
thus a specially rich blood supply. If this description should prove
to be correct, the vascular system of Brachiopods shows a striking
resemblance to that of the closed vascular system of the unarmed
Gephyrea, except that the former group has specialised genital vessels.
The blood is colourless.

Joubin’s description of the vascular system of _W. venosa_ differs
in some respects from that of Blochmann. He regards the heart as
collecting the lymph which it receives from numerous lacunar spaces in
the walls of the alimentary canal, and distributing it through various
vessels, which in the main correspond with those of Blochmann, and
which run both to the “arms” and to the generative glands. The latter
vessels, however, open freely into the body cavity, and the fluid which
is forced out from their openings freely bathes the organs found in
the body cavity. Whichever of these accounts should prove to be more
closely in accordance with the facts, there is little doubt that in
addition to the true blood there is a corpusculated fluid in the body
cavity which is to some extent kept in motion by the ciliated cells
that line its walls.


                        =The Excretory Organs=

The excretory organs (kidneys) which were at one time regarded by
Cuvier and Owen as hearts, are typical nephridia--that is to say,
they are tubes with glandular excretory walls which open at one end
by a wide but flattened funnel-shaped opening into the body cavity,
and at the other end by a circular pore to the exterior (Fig. 314).
In _Rhynchonella_, where there are two pairs of these tubes,--the
only evidence that the group presents of any metameric repetition of
parts,--the inner ends of the anterior pair are supported by the
gastro-parietal mesenteries, and those of the posterior pair by the
ileo-parietal mesenteries. In all other Brachiopods the posterior pair
alone exists. The external opening of these nephridia is near the base
of the anus; in _Cistella_ it is at the bottom of a brood-pouch formed
by the tucking in of the body wall in this neighbourhood, and in this
brood-pouch the eggs develop until the larval stage is reached.

The walls of these nephridia are lined by ciliated cells, amongst
which are some excretory cells, in which numerous brown and yellow
concretions are to be seen; these are probably the nitrogenous excreta
of the animal, and pass out of the body, being washed away by the
stream of water which is constantly passing between the shells.

As in so many other animals, the nephridia act as genital ducts, and
through them the ova and spermatozoa, which break off from the genital
glands and fall into the body cavity, find their way to the outer world.


                        =The Stalk and Muscles=

The body cavity of a Brachiopod is traversed by several pairs of
muscles, which are very constant in position, and whose contraction
serves to open and close the shell, to move the animal upon its stalk,
and to govern the movements of the arms.

The stalk is absent in _Crania_, and the members of this genus are
attached to the rocks on which they are found by the whole surface of
their ventral valve. In _Lingula_ (Fig. 315) the stalk is long and
hollow, containing what is probably a portion of the body cavity,
surrounded by muscular walls. _Lingula_ is not a fixed form, but lives
half-buried in the sand of the sea-shore (Fig. 321). _Discina_, the
other member of the Ecardines, has a peduncle which pierces the ventral
valve and fixes the animal to its support. Amongst the Testicardines,
_Thecidium_ is also fixed to its supports by the surface of its ventral
valve; the other genera, however, are provided with stalks, which,
being the means of the fixation of the animals, become at the same
time the fixed points upon which their very limited movements can be
effected. The stalk protrudes through the notch or aperture at the
posterior end of the ventral valve, and it probably belongs to the
ventral side of the body. It is in _Cistella_, and doubtless in other
genera, in close organic connexion with both valves, and it seems to
consist of an unusually large development of the supporting tissue
which occurs so frequently in the body of Brachiopods. The surface of
the peduncle is produced into several irregularities and projections
which fit into any depressions of the rock upon which the animal is
fixed.

In _Cistella_ there are four pairs of muscles, two connected with
opening and closing the shell, and two with the movement of the body
upon the stalk (Fig. 314). The most considerable of these muscles are
the two occlusors, which have their origin, one on each side of the
middle line of the dorsal valve, and their insertion by means of a
tendon into the ventral valve. In the species in question each of these
muscles arises by a double head, the two muscles thus formed probably
representing the anterior and posterior occlusors of other forms. The
contraction of these muscles undoubtedly serves to close the shell,
which is opened by a small pair of divaricators arising from the
ventral valve, and inserted into a portion of the dorsal shell which
is posterior to the axis of the hinge. Contraction of these muscles
would thus serve to approximate the posterior edges of the valves and
divaricate the anterior edges and thus to open the shell.

The adjustors are four in number, a ventral pair running from the
ventral valve to be inserted into the stalk, and a corresponding dorsal
pair from the dorsal valve. The simultaneous contraction of either pair
would tend to raise the valve, whilst the alternate contraction of the
muscles of each side would tend to rotate the shell upon the peduncle.
The muscles of _Waldheimia flavescens_ are shown in Fig. 329, and
described briefly on p. 502.

The muscles of the Ecardines differ from those of the Testicardines
inasmuch as they do not terminate in a tendon, but the muscle fibres
run straight from shell to shell. They are also more numerous. In
_Crania_ there is an anterior and a posterior pair of occlusor
muscles, and two pairs of oblique muscles, which seem when they
contract together to move the dorsal shell forwards, or when they
contract alternately to slightly rotate it. In this genus there are
also a pair of protractors and a pair of retractors, and two levators
of the arms, whose function is to draw forward or retract the arms, and
an unpaired median or levator ani muscle. In addition to these bundles
of muscles there are certain muscles in the body wall, and it seems
probable that by their contraction, when the adductors are relaxed,
the body may become somewhat thicker and the valves of the shells will
slightly open.

  [Illustration: FIG. 316.--A semi-diagrammatic figure of the
  muscular system of _Crania_ (after Blochmann): _a_, anterior
  occlusor; _b_, posterior occlusor; _c_, superior oblique; _d_,
  inferior oblique; _e_, retractor of the arms; _f_, elevator of
  the arms; _g_, protractor of the arms; _h_, unpaired median
  muscle. The dorsal valve is uppermost.]

In _Lingula_ (Fig. 322) the muscular system is more complicated;
in addition to the anterior (= anterior laterals) and posterior
(= centrals) pairs of occlusors, there is a single divaricator (=
umbonal), whose contractions in conjunction with those of certain
muscles in the body wall press forward the fluid in the body cavity,
and thus force the valves of the shell apart; and there are three pairs
of adjustor muscles. These latter are called respectively the central
(= middle laterals), external (= external laterals), and posterior
(= transmedians) adjustors, whose action adjusts the shells when all
contract together, and brings about a certain sliding movement of the
shells on one another when they act independently of each other.[423]


                         =The Nervous System=

The nervous system of Brachiopods is not very clearly understood, and
there are considerable discrepancies in the accounts of the various
investigators, even when they are dealing with the same species. So
much, however, seems certain, that there is a nervous ring surrounding
the oesophagus, that this ring is enlarged dorsally, or, in other
words, near the base of the lip, into a small and inconspicuous dorsal
ganglion, and again ventrally or just behind the base of the tentacles
into a ventral or sub-oesophageal ganglion. The latter is, contrary
to what is usual in Invertebrates, of much larger size than the
supra-oesophageal ganglion, but like the last named, it has retained
its primitive connexion with the ectoderm or outermost layer of the
skin. Both ganglia give off a nerve on each side which runs to the
arms and along the base of the tentacles and lips. The sub-oesophageal
ganglion also gives off nerves which supply the dorsal and ventral
folds of the mantle, the muscles, and other parts.

The modified epithelium in connexion with the ganglia may possibly have
some olfactory or tactile function, but beyond this the Brachiopoda
would appear to be devoid of eyes, ears, or any other kind of sense
organs,--a condition of things doubtless correlated with their sessile
habits, and with the presence of a bivalved shell which leaves no part
of their body exposed.


                       =The Reproductive System=

The majority of Brachiopods are bisexual, and many authorities regard
the separation of sex as characteristic of the group; on the other
hand, _Lingula pyramidata_ is stated to be hermaphrodite, and it is not
impossible that other species are in the same condition.

The generative organs are of the typical sort, that is, they are formed
from modified mesoblastic cells lining the body cavity. These cells are
heaped up, usually in four places, and form the four ovaries or testes
as the case may be (Fig. 314). The generative glands usually lie partly
in the general body cavity and partly in the dorsal and ventral mantle
folds, two on each side of the body. Along the axis of the heaped-up
cells runs a blood-vessel, which doubtless serves to nourish the
gland, the outer surface of which is bathed in the perivisceral fluid.
Every gradation can be found between the ripe generative cell and the
ordinary cell lining the body cavity. When the ova and spermatozoa
are ripe they fall off from the ovary and testis respectively into
the body cavity, thence they are conveyed to the exterior through the
nephridia. The ova in certain genera, such as _Argiope_, _Cistella_,
and _Thecidium_, develop in brood-pouches which are either lateral
or median involutions of the body wall in the neighbourhood of the
external opening of the nephridia; they are probably fertilised there
by spermatozoa carried from other individuals in the stream of water
which flows into the shell. In other species the ova are thrown out
into the open sea, and their chances of meeting with a spermatozoon is
much increased by the gregarious habits of their sessile parents, for
as a rule considerable numbers of a given species are found in the same
locality.


                           =The Embryology=

We owe what little we know of the Embryology of the group chiefly to
Kowalevsky,[424] Lacaze-Duthiers,[425] and Morse.[426] The Russian
naturalist worked on _Cistella_ (_Argiope_) _neapolitana_, the French
on _Thecidium_, and the American chiefly on _Terebratulina_.

Although this is not known with any certainty, it seems probable that
the eggs of Brachiopods are fertilised after they have been laid, and
not whilst in the body of the mother. The spermatozoa are doubtless
cast out into the sea by the male, and carried to the female by the
currents set up by the cilia clothing the tentacles.

In _Thecidium_, _Cistella_, and _Argiope_ the first stages of
development, up to the completion of the larva, take place in
brood-pouches; in _Terebratulina_ the eggs pass out of the shell of
the mother and hang in spermaceti-white clusters from her setae and on
surrounding objects. In the course of a few hours they become ciliated
and swim about freely. The brood-pouch in _Thecidium_ is median, in
the convex lower shell, in _Cistella_ it is paired, and arises by the
pushing in of the lateral walls of the body in the region just behind
the horse-shoe-shaped tentacular arms; the renal ducts, which also
serve as oviducts, open into these lateral recesses.

In the female _Thecidium_ (Fig. 317) the two median tentacles which lie
just behind the mouth are enlarged and their ends somewhat swollen;
they are bent back into the brood-pouch, and to them the numerous
larvae are attached by a short filament inserted into the second of the
four segments into which the larva is divided. In _Cistella_ a similar
filament attaches the larvae to the walls of the brood-sac; thus they
are secured from being washed away by the currents constantly flowing
through the mantle cavity of the mother.

  [Illustration: FIG. 317.--Brood-pouch of _Thecidium
  mediterraneum_. (After Lacaze-Duthiers.) Part of the wall of the
  pouch has been removed to show the clusters of larvae.

   1. Mouth, overhung by lip.

   2. One of the two median tentacles which are enlarged and
   modified to bear the larvae.

   3. Wall of brood-pouch into which the median tentacles are
   folded.

   4. Larva attached to the swollen end of the tentacles.]

In _Cistella_ the larva consists at first of two segments, but the
anterior one divides into two, so that in the free swimming larva
we find three segments, the hindermost somewhat longer and narrower
than the others and destined to form the stalk. About the time of
the appearance of the second segment four red eye-spots arise in the
anterior segment, which tends to become constricted off from the
others, and may now be termed the head. It gradually becomes somewhat
umbrella-shaped, develops cilia all over its surface and a special ring
of large cilia round its edge.

In the meantime the second or mantle segment has grown down and
enveloped the stalk, and four bundles of setae have arisen from its
edge. In this stage the larva leaves its mother’s shell and swims
out into the world of water to look for a suitable place on which to
settle down. This is the only stage in the life history of a Brachiopod
when the animal is locomotor, and can serve to spread its species.
The extreme minuteness of the larva and the short time it spends in
this motile condition probably accounts for the fact that Brachiopods
are extremely localised. Where they do occur they are found in great
numbers, rocks being often almost covered with them, but they are not
found over large areas. When viewed under a microscope the larvae seem
to be moving with surprising rapidity, but judging from the analogy of
other forms, it seems doubtful if they swim a yard in an hour.

  [Illustration: FIG. 318.--Young larva of _Cistella neapolitana_,
  showing three segments, two eye-spots, and two bundles of setae.
  (After Kowalevsky.)]

  [Illustration: FIG. 319.--Full-grown larva of _Cistella
  neapolitana_, with umbrella-shaped head, ciliated. (After
  Kowalevsky.)]

Frequently the larva stands on its head for some time, as if
investigating the nature of the rocks on which it may settle; it
is extremely contractile, turning its head from time to time, and
seldom retaining the same outline for any length of time; the setae
are protruded, and at times stick out in every direction; they are
possibly defensive in function. When fully stretched out the larva is
about ⅓ mm. long, but it frequently shortens its body to two-thirds
of this length. The larvae are of a pinkish red colour, with eye-spots
of ruby red. Their colour renders them difficult to discern when they
are swimming over the red coralline rocks upon which they frequently
settle. After swimming about for a few hours the larva fixes itself
finally, apparently adhering by some secretion produced by the stalk
segment. The folds of the second or body segment then turn forward
over the head, and now form the ventral and dorsal mantle folds; these
at once begin to secrete the shell on their outer surfaces. The head
with its eye-spots must be to some extent absorbed, but what goes on
within the mantle is not accurately known. The setae drop off and the
tentacular arms begin to appear as a thickening on the dorsal lobe of
the mantle. They are at first circular in outline. The various changes
which the larva passes through are well illustrated by Morse for
_Terebratulina_, which spawns at Eastport, Me., from April till
August. The different stages are represented in outline in Fig. 320,
taken from his paper.

  [Illustration: FIG. 320.--Stages in the development of the larva
  of _Terebratulina septentrionalis_. (After Morse.) The youngest
  larva has two segments, a third then appears, the larva then
  fixes itself, and the second segment folds over the first and
  develops bristles round its edge.]


                               =Habits=

There is little to be said about the habits and natural history of the
Brachiopoda. When once the larva has settled down, the animal never
moves from the spot selected; occasionally it may rotate slightly from
side to side on its stalk, and from time to time it opens its shell. As
so frequently is the case with sessile animals, the sense organs are
reduced to a minimum, the eyes of the larva disappear, and the only
communication which the animal has with the world around it is by
means of the currents set up by the cilia on the tentacles.

In spite of the absence of any definite eyes, _Thecidium_, according to
Lacaze-Duthiers, is sensitive to light; he noticed for instance that,
when his shadow fell across a number of these animals he was watching
in a vessel, their shells, which had been previously gaping, shut up at
once.

In _Cistella_ the tentacles can be protruded from the open shell, and
in _Rhynchonella_ the spirally-coiled arms can be unrolled and extruded
from the shell, but this does not seem to have been observed in other
genera, with the possible exception of _Lingula_. The food of these
animals consists of minute fragments of animal and vegetable matter, a
very large proportion of it being diatoms and other small algae.

  [Illustration: FIG. 321.--Figures illustrating the tubes in which
  _Lingula anatifera_ lives. The upper figure is a view of the
  trilobed opening of the tube. The lower figure shows the tube
  in the sand laid open and the animal exposed. The dotted line
  indicates the position of the body when retracted. The darker
  portion is the tube of sand agglutinated by the secretion of the
  stalk. (After François.)]

_Lingula_ differs markedly from the other members of the group,
inasmuch as it is not firmly fixed to a rock or some such body by a
stalk or by one of its valves, but lives in a tube in the sand. Some
recent observations of Mons. P. François[427] on living specimens of
_Lingula anatifera_ which he found living in great numbers on the
sea-shore at Nouméa in New Caledonia may be mentioned. The presence of
the animal is shown by a number of elongated trilobed orifices which
lead into the tube in which the _Lingula_ lives. The animals, like most
other Brachiopods, live well in captivity, and he was able to watch
their habits in the aquaria of his laboratory. The _Lingula_ place
themselves vertically; the anterior end of the body just reaches the
level of the sand; the three lobes into which the orifice of the tube
is divided corresponding with the three brushes of setae which project
from the anterior rim of the mantles. These setae are described by
Morse as projecting in the form of three funnels; currents of water are
seen continually passing in at the side orifices and out through the
central. The tube consists of two portions: an upper part, which is
flattened to correspond with the flat shape of the body, and a lower
part, in which the stalk lies. The upper part is lined with a layer of
mucus, but the sand is not glued together to form a definite tube. The
lower part of the stalk, or the whole when the animal is contracted, is
lodged in a definite tube composed of grains of sand agglutinated by
mucus, probably secreted from the walls of the stalk. At the least sign
of danger the stalk is contracted violently, and the body is withdrawn
to the bottom of the upper portion of the tube. The rapid retreat of
the animal is followed by the collapse of the sand at the mouth of the
tube, and all trace of the presence of the _Lingula_ is lost.

The shells of this species are frequently rotated through a small angle
upon one another, a movement which is prevented in the Testicardines
by the hinge. In very young transparent specimens François was able
to observe the movements of the fluid in the system of tubules which
penetrate the mantle; these tubules are figured by him, and Fig. 315 is
taken from his illustration.

Davidson in his Monograph on the British Fossil Brachiopoda states
that the largest “recent Brachiopod which has come under my notice is
a specimen of _Waldheimia venosa_ Solander, measuring 3 inches 2 lines
in length, by 2 inches in breadth, and 1 inch 11 lines in depth.” It
was found in the outer harbour of Fort William, Falkland Islands, in
1843. A specimen of _Terebratula grandis_ from the Tertiary deposits,
however, exceeds this in all its dimensions. Its length was 4½ inches,
its breadth 3 inches 2 lines, and its depth 2 inches 2 lines.


                        =Distribution in Space=

Brachiopods are very localised; they live in but few places, but when
they are found they usually occur in great numbers. During the cruise
of the _Challenger_, dredging was conducted at 361 stations; at only
38 or 39 of these were Brachiopoda brought up. Mr. Cuming, quoted
by Davidson, records that after a great storm in the year 1836, he
collected as many as 20 bushels of _Lingula anatifera_ on the sea-shore
at Manilla, where, he relates, they are used as an article of food. It
has been suggested above that their abundance in certain localities is
due to their limited powers of locomotion, which are effective but for
a few hours, the larva being, moreover, so minute that unless borne by
a current it could not travel far from its parent. When once settled
down it has little to fear from the attacks of other animals. The
size of its shell relative to its body would deter most animals from
regarding it as a desirable article of food, and as far as is known
at present the Brachiopoda suffer but little from internal parasites,
the only case I know being a minute parasitic Copepod belonging to a
new and as yet unnamed genus which I found within the mantle cavity
of _Cistella_ (_Argiope_) _neapolitana_ in Naples. Their slight value
as an article of diet has doubtless helped to preserve them through
the long periods of geological time, through which they have existed
apparently unchanged.

Two of the recent genera of the family Lingulidae, _Lingula_ and
_Glottidia_, are usually found between tide-marks or in shallow water
not exceeding 17 fathoms. _Discina_ is also found about the low-tide
level, but one species at any rate, _Discinisca atlantica_, has been
dredged, according to Davidson, “at depths ranging from 690 to nearly
2425 fathoms.” Their larvae frequently settle on the shells of their
parents, and thus numbers of overlapping shells are found clustered
together. _Crania_ is usually dredged from moderate depths down to 808
fathoms, adhering to rocks, lumps of coral, stones, and shells.

Of the Testicardines, _Terebratula Wyvillei_ has probably been found at
the greatest depth, _i.e._ 2900 fathoms, in the North Pacific. It is
interesting to note that its shell is glassy and extremely thin. The
Brachiopoda are, however, as a rule, found in shallower water; they
abound up to a depth of 500 or 600 fathoms, after which they rapidly
diminish with increasing depth. About one-half the named species occur
at a depth of less than 100 fathoms.

The vertical range of depth of certain species is great; _Terebratula
vitrea_ is recorded from 5 to 1456 fathoms, _T. Wyvillei_ from 1035
to 2900 fathoms. This is to some extent explicable since, after a
certain depth has been reached, many of the external conditions, such
as absence of temperature and light, must remain constant even to the
greatest depths of the ocean.

The area of the ocean explored by dredging forms such an infinitesimal
fraction of the whole, that it seems superfluous to consider the
horizontal distribution of Brachiopods. A few facts may, however,
be mentioned. Certain species, as _Terebratula vitrea_, _T. caput
serpentis_, _Waldheimia cranium_, _Megerlia truncata_, and _Discinisca
atlantica_, have a very wide if not cosmopolitan distribution. The
second of the above named extends as far north as Spitzbergen, and as
far south as Kerguelen Island. Many species are, on the other hand,
very localised, and have hitherto only been found in one place. A very
considerable number of these have been dredged off Japan and Korea, and
this region may be to some extent regarded as the headquarters of the
group.

The following species have been obtained within the limits of the
British Area, as defined by Canon Norman, who has been good enough to
revise the list, which is founded on that drawn up by Davidson in his
Challenger Report. Their range of bathymetric distribution is given in
the column on the left.

    Depth in
    Fathoms

   0 to 1180.   _Terebratulina caput           Oban, and off Cumbrae
                   serpentis_ Lin.             Islands, Loch Torridon,
                                                Scotland, off Belfast

   8 to   25.   _Terebratula (Gwynia)          Belfast Bay, E. and S.
                   capsula_ Jeff.              coast of Ireland,
                                               Plymouth, Weymouth, and
                                               Guernsey

   5 to  690.   _Waldheimia cranium_ Müller    North British seas. Off
                                               Shetland

  75 to  725.   _Waldheimia septigera_ Lovén   North British seas. Off
                                               Shetland

  20 to  600.   _Terebratella                  N.N.W. of Unst, Shetland
                   spitzbergenensis_ Dav.

  18 to  364.   _Argiope decollata_ Chemnitz   Two miles east of
                                               Guernsey

  20 to   45.   _Cistella cistellula_ S. Wood  Shetland, near Weymouth,
                                               S. coast of England

 650 to 1750.   _Atretia gnomon_ Jeff.         W. of Donegal Bay in 1443
                                               faths. Between Ireland
                                               and Rockall, in 1350
                                               faths.

  10 to  690.   _Rhynchonella psittacea_       Shetland and near Dogger
                   Gmelin.                     Bank. This species is
                                               possibly fossil as well
                                               as recent

   3 to  808.   _Crania anomala_ Müller        Loch Fyne, North of
                                               Scotland

 690 to 2425.   _Discinisca atlantica_ King    W. of Donegal Bay in 1366
                                               faths., W. of Ireland in
                                               1240 faths., off Dingwall
                                               Bay


                           =Classification=

The table of classification here appended is that suggested by Mr.
Davidson in his Monograph on the Recent Brachiopoda.

                           I. TESTICARDINES

         Family

    A. TEREBRATULIDAE.     This includes the majority of genera and of
                           species, the latter, without counting
                           uncertain species, amounting to sixty-eight.
                           Examples: _Terebratula_, _Terebratella_,
                           _Terebratulina_, _Waldheimia_,_Megerlia_,
                           _Argiope_, _Cistella_.

    B. THECIDIIDAE.        This family contains one genus, _Thecidium_,
                           with two species.

    C. RHYNCHONELLIDAE.    This family is made up of eight species,
                           six of which belong to the genus
                           _Rhynchonella_, and two to _Atretia_.

                             II. ECARDINES

    D. CRANIIDAE.          This family comprises the four species of
                           _Crania_.

    E. DISCINIDAE.         This family contains one species of
                           _Discina_ and six of _Discinisca_.

    F. LINGULIDAE.         This family consists of eight species of
                            _Lingula_ and three of _Glottidia_.

It is impossible to come to any satisfactory conclusion as to the
position of the group Brachiopoda with relation to the rest of the
animal kingdom. They have, in accordance with the views of various
investigators, been placed in close connexion with many of the large
groups into which the Invertebrates are split up. The Mollusca, the
Tunicata, the Polyzoa, the Chaetopoda, the Gephyrea, and of recent
times such isolated forms as _Phoronis_ and _Sagitta_, have all in turn
had their claims advanced of relationship to this most ancient group.
As far as I am in a position to judge, their affinities seem to be
perhaps more closely with the Gephyrea and with _Phoronis_ than with
any of the other claimants; but I think even these are too remote to
justify any system of classification which would bring them together
under a common name. Investigation into the details of the embryology
of the group, more especially into that of the Ecardines, might throw
some light on this subject, and it is much to be desired that this
should be undertaken without delay. That the group is a most ancient
one, extending from the oldest geological formations, we know, that the
existing members of it have changed but little during the vast lapse
of time since their earliest fossil ancestors flourished, we believe;
but we are in almost total ignorance of the origin or affinities of
the group, and we can hardly hope for any light on the subject except
through embryological research.




                              BRACHIOPODA

                                PART II
                   PALAEONTOLOGY OF THE BRACHIOPODA

                                  BY

                    F. R. COWPER REED, B.A., F.G.S.

                      Trinity College, Cambridge




                             CHAPTER XVIII

                   PALAEONTOLOGY OF THE BRACHIOPODA

   INTRODUCTION--DIVISION I. ECARDINES--EXTERNAL
   CHARACTERS--INTERNAL CHARACTERS--DIVISION II.
   TESTICARDINES--EXTERNAL CHARACTERS--INTERNAL
   CHARACTERS--SYNOPSIS OF FAMILIES--STRATIGRAPHICAL
   DISTRIBUTION--PHYLOGENY AND ONTOGENY


                            =Introduction=

The wide distribution and vast abundance of the Brachiopoda throughout
the whole series of geological formations make this group of especial
importance to the student of the past history of the earth; and the
zoologist must always regard the fossil forms with peculiar interest,
because they not only largely outnumber the living representatives,
but comprise numerous extinct genera, and even families, exhibiting
types of structure and characters entirely absent in the modern
members of the group. It is a most fortunate circumstance that the
excellent state of preservation in which we frequently find them, and
the immense amount of material at our disposal, enable us to determine
with accuracy and certainty the internal characters of the shells in
the great majority of cases. But it is only since the beginning of the
present century that our knowledge of the anatomy of the soft parts
of the living animal has rendered any tracing of homologies possible.
In the case of features in fossil extinct types the interpretation
must be to some extent doubtful. Barrande, Clarke, Davidson, Hall,
King, Oehlert, Waagen, de Verneuil, and a host of other workers have
contributed to the information which we now possess; and their works
must be consulted for details of the subject.[428]

Since all Brachiopods are inhabitants of the sea, the geologist at once
recognises as a marine deposit any bed which contains their remains.
Under favourable conditions they swarmed in the seas of Palaeozoic
and Mesozoic times. Beds of limestone are frequently almost entirely
composed of their shells, as, for instance, some of the Devonian
limestones of Bohemia. Often they give the facies to the fauna and
outnumber in species and individuals all the other organisms of the
period. The Ungulite Sandstone (Cambrian) of Russia and the Productus
Limestone of the Salt Range in India of Carboniferous and Permian age
are well-known examples.

Many species seem to have been gregarious in habit; thus _Productus
giganteus_ of the Carboniferous Limestone may generally be found in
crowded masses, as in some localities in Yorkshire.

The fact that certain species of Brachiopods characterise definite
stratigraphical horizons or “zones” gives them occasionally an
importance equal to that of Graptolites; for instance, the Ecardinate
species _Trematis corona_ marks a set of beds in the Ordovician, and
the isolated _Stringocephalus Burtini_ is restricted to the upper part
of the Middle Devonian, giving to the limestone on that horizon its
distinctive name. It is noteworthy also how certain species affect a
sandy and others a calcareous sea-bottom, so that beds of the same
age show differences in their Brachiopod fauna owing to a dissimilar
lithological composition.

While few of the recent Brachiopods reach a large size, some of the
extinct species measure several inches in breadth, but the great
_Productus giganteus_ attained the width of even a foot.

The bright colours of the shells of the living animals are not
generally preserved amongst the fossil species from the older rocks;
yet in a Carboniferous _Terebratula_ we can even now detect the purple
bands in some specimens, and a Cretaceous _Rhynchonella_ similarly
exhibits its original colour.

The Brachiopoda are evidently a group in its decline, as the geological
record shows; but they date back from the earliest known fossiliferous
rocks, in which the Ecardinate division is alone represented. As we
ascend through the stratigraphical series the number and variety of
genera and species belonging to both divisions rapidly increase until
in the united Ordovician and Silurian there are nearly 2000 species and
about 70 genera. From this point of maximum development down to the
present day there is a gradual decrease in numbers.

According to Davidson, at least 17 Upper Tertiary species are still
living on our sea-bottoms; and many recent Mediterranean forms occur in
the Pliocene rocks of the islands and shores of that sea, and in the
Crags of East Anglia.

A brief review of the chief characteristics of fossil Brachiopoda
is given below. Those genera which have the greatest zoological or
geological importance can alone be noticed owing to the exigencies of
space.


                             I. ECARDINES


                         =External Characters=

A considerable diversity of external form is met with even in this
division, from the limpet-like _Discina_ to the flattened tongue-shaped
_Lingula_. The valves have most commonly a smooth external surface with
delicate growth-lines; but sometimes pittings (_Trematis_) or radiating
ribs (_Crania_) are present, and in a few forms the shell is furnished
with spines (_Siphonotreta_), which perhaps serve to anchor it in the
soft mud of the sea-bottom. The usual mode of fixation was by means of
the pedicle (= peduncle or stalk), which either (1) passed out simply
between the posterior gaping portion of the valves (_Lingula_), or (2)
lay in a slit in the ventral valve (_Lingulella_), or (3) pierced the
substance of the latter valve by a definite foramen (_Discina_). The
first-mentioned condition of the pedicle seems the most primitive.
Rarely the pedicle was absent, and the shell was attached by the whole
surface of the ventral valve (_Crania_, p. 467).

The two valves in the fossil Ecardines were held together by muscular
action, though in some families (_Trimerellidae_) we see traces of
articulating processes. The “hinge line,” or line along which the
valves worked as on a hinge, is in most forms more or less curved. A
“hinge area” (_i.e._ that portion of the shell generally smoother than
other parts of the valves, more or less triangular in form, and lying
between the beaks on one or both sides of the hinge line), is usually
absent in the Ecardines.

  [Illustration: FIG. 322.--Muscle-scars of _Lingula anatina_.
  Inner surface of =A=, Pedicle-valve or ventral valve. =B=,
  Brachial or dorsal valve; _p.s_, parietal scar; _u_, umbonal
  muscle; _t_, transmedians; _c_, centrals; _a.m.e_, laterals (_a_,
  anteriors; _m_, middles; _e_, externals).]

  [Illustration: FIG. 323.--_Trimerella._ (After Davidson and
  King.) =A=, Inner surface of pedicle-valve or ventral valve: _a_,
  pseudo-deltidium; _b_, deltidial slope; _c_, deltidial ridges;
  _d_, areal borders; _e_, cardinal callosities; _f_, cardinal
  facet; _g_, lozenge; _i_, umbonal chambers separated by cardinal
  buttress; _j_, platform; _k_, platform vaults; _l_, median plate;
  _m_, median scars; _n_, anterior scars; _o_, lateral scars; _p_,
  post-median scars; _q_, crown crescent; _r_, side or lateral
  crescent; _s_, end or terminal crescent; _t_, transverse scars;
  _u_, archlet (vascular sinuses); _w_, sub-cardinal scars; _x_,
  umbo-lateral scars. =B=, Brachial or dorsal valve: _e_, cardinal
  sockets; _j_, platform; _k_, platform vaults; _l_, median plate;
  _m_, median scars; _n_, anterior scars; _q_, crown crescent; _r_,
  side or lateral crescent; _s_, end or terminal crescent; _t_,
  transverse scars; _u_, archlet (vascular sinuses); _v_, cardinal
  scars; _w_, sub-cardinal scars.]


                         =Internal Characters=

Owing to the rarity of well-preserved interiors of valves in this
division, our knowledge of their internal characters is still far
from satisfactory. The arrangement of the muscular impressions varies
greatly amongst extinct genera, but we are often able to interpret
them with a considerable amount of certainty by a study of the scars
and the muscles of the well-known recent _Lingula_ (Fig. 322). The
extreme specialisation of the muscles in many of the earliest genera
(_e.g._ _Lingula_) is remarkable, and points to a long but so far
undiscovered ancestry in pre-Cambrian times.[429] In fossil species
of _Crania_ and _Lingula_ the muscle-scars correspond closely with
those in the living representatives of these genera. In the most
highly specialised family of the Ecardines--the _Trimerellidae_--we
meet with features of peculiar interest.[430] The muscle-scars in this
family (Fig. 323, A, B) are most remarkable for the development of the
so-called “crescent,” (_q.r.s._) which skirts the posterior margin of
both valves as a sub-cardinal impression. It is believed to be the
trace of a strong post-parietal muscular wall, analogous in position
to that of _Lingula_. The three pairs of “lateral” muscle-scars in
the latter genus seem to be represented by the “terminal” (_s_) and
“lateral” (_r_) scars on the crescent of the _Trimerellidae_. A
pair of “transverse” scars (_t_) occurs in each valve between the
“terminals” and the antero-lateral edge of the “platform” (_j_).
“Cardinal” (_v_), “sub-cardinal” (_w_), and “umbo-lateral” (_x_) scars
also occur. The median impression which covers the “platform” (_j_)
consists of a central, lateral, and usually an anterior pair of scars;
and the impressions of the genital organs, according to Davidson
and King, lie medianly posterior to the “platform.” The “platform”
itself is a more or less conspicuous central calcareous elevated area
occurring in each valve, but most developed in the dorsal; in some
cases it is double-chambered with tubular cavities (“platform vaults,”
Fig. 323, A, B, _k_), in others it is more or less solid. It appears to
have originated through a posterior shifting of the central muscular
bands, that they might be inserted behind the liver; at the same time a
deposition of shelly material, to form fulcra to work the heavy valves,
took place at these points. The tunnelling-out of the platform was
probably due to the continual pressure of the lobes of the liver. The
division of the umbonal cavity into definite chambers in _Monomerella_,
and to a less extent in other members of this family, appears,
according to Davidson and King, to have been caused by pressure of the
ovarian lobes.

In connexion with the foregoing remarks on the development of the
“platform,” it may be mentioned that the paths along which the
muscle-bands move, as the shell of Brachiopods increases in size, are
marked by elongated scars, and often by shelly deposits; and when the
members of a muscle-pair come into juxtaposition these shelly deposits
(which act as fulcra for the muscles) combine, and by the growth
of the shell form a septum, as in the case of the median septum of
_Lingulepis_.

The _Obolidae_ show some important features in the internal
impressions. _Obolella crassa_ (Hall) may be taken as a well-known
type of the family. In this species a pair of small scars, one on each
side of the pedicle-groove, lies close under the hinge line in the
ventral valve. There is also a well-marked scar for the insertion of
the pedicle-muscle at the end of the pedicle-groove. A pair of much
elongated lateral impressions extending forward from the “cardinals”
may be homologous with the “laterals” of _Lingula_; and the two small
central scars between them may be compared with the “centrals” of
_Lingula_ which are in a somewhat similar position. In the dorsal valve
of _O. crassa_ a pair of “cardinals” is found, and on each side of a
low median rounded ridge are two small “central” scars. Indistinct
“lateral” scars arise close to or in the central area, and diverge
anteriorly.

Sometimes a great concentration of muscle-scars occurs round the
foramen in the ventral valve, as in _Siphonotreta_.

As regards the minute structure and composition of the shell in
the Ecardines, we find that the _Lingulidae_ and _Discinidae_ have
their shell composed of alternating layers of phosphate of lime and
a corneous substance; the former layers are pierced by microscopic
canals. The _Craniidae_ have calcareous shells traversed by tubules,
which divide into many fine branches near the external surface; a thin
periostracum covers the exterior. The _Trimerellidae_ have heavy thick
calcareous shells, for which they required the previously-described
elaborate arrangement of muscles to open and shut them.


                           II. TESTICARDINES


                         =External Characters=

It is to this division that the great majority of the Brachiopoda
belong; and the diversity of form, of ornamentation, and of internal
characters is correspondingly greater than in the Ecardines.

A transversely or longitudinally oval shape of shell is the commonest;
but sometimes it is triangular, as in _Rhynchonella_ (Fig. 327), or
bilobed, as in _Pygope_ (= _Terebratula diphya_). The ventral valve is
usually more convex than the dorsal, and the former may be prolonged
into a tube by the accelerated growth and infolding of the anterior
and lateral margins, producing a very abnormal form (_Proboscidella_).
The external surface of the valves is frequently ornamented with more
or less prominent radiating ribs; and fine concentric growth-lines
are commonly shown, and may be developed into coarse ridges or
wrinkles, particularly in old individuals. The members of the family
_Productidae_ are usually furnished with tubular spines, which are
sometimes of great length, and served to anchor the free shells in the
mud, or were twisted round Crinoid stems and similar objects.

In the ventral valve of many genera there is a median sinus, with
a corresponding fold in the dorsal valve, and rarely _vice versâ_;
sometimes the fold and sinus are double.

The hinge line is either curved or straight, and the valves are
articulated by means of a pair of “hinge-teeth” (Fig. 329, _t_) in the
ventral valve, which fit into corresponding sockets in the opposite
valve. Some genera have the teeth very rudimentary, or have lost them
altogether. The teeth are frequently supported by “dental plates,” and
the sockets by “socket plates” (_e.g._ _Conchidium_, Figs. 324, 325).
A few genera with a long hinge line have the whole of it denticulated
(_Stropheodonta_). In the dorsal valve medianly close under the hinge
line is a shelly protuberance--the “cardinal process”--to which
the diductor muscles are attached. It is sometimes of great length
and forked (_Stringocephalus_, Fig. 326), or tripartite, or even
quadripartite; but in _Rhynchonella_ and some other genera it is
rudimentary.

  [Illustration: FIG. 324.--_Conchidium galeatum._ Wenlock
  Limestone.]

  [Illustration: FIG. 325.--_Conchidium galeatum._ Transverse
  section. _d_, Dorsal valve; _d.s_, dorsal septum; _s_, socket
  plate; _v_, ventral valve; _v.s_, ventral septum; _d.p_, dental
  plate.]

  [Illustration: FIG. 326.--_Stringocephalus Burtini._ (Modified
  from Woodward.) Devonian. =A=, Interior of dorsal valve. =B=,
  Side view of interior of shell; _a_, adductor (= occlusor) scars;
  _c_, crura; _c.p_, cardinal process; _d.s_, dorsal septum; _h.p_,
  hinge plate; _l_, brachial loop; _s.p_, shelly processes; _t.s_,
  dental sockets; _v.s_, ventral septum.]

A “hinge area” (Fig. 334, _c.a_) is often present on one or both
valves, and may be of great size, as in _Clitambonites_, but in
_Productus_ it is wholly absent. In those genera that possess it a
triangular fissure--the “deltidial fissure”--frequently traverses it
on both valves; in the dorsal valve the fissure is merely the space
between the dental sockets, and may be occupied by the cardinal-
process (Fig. 334, C) or covered by a shelly plate--the “chilidium.” In
the ventral valve it gives passage to the pedicle, and may be partly
or entirely closed by a similar plate (Fig. 334, _d_) known as the
“pseudo-deltidium,” especially large in _Clitambonites_, or remain
open (_Orthis_). This pseudo-deltidium is a primitive character, and
arises in an early stage of the development as a shell-growth on the
dorsal side of the animal, becoming attached to the ventral valve
subsequently. The pedicle in many genera passes out through a special
foramen in the beak of the ventral valve; and its proximal portion
is often embraced by a pair of small plates--the deltidial plates or
“deltidium”--which are formed on lateral extensions of the ventral
mantle lobe, according to Beecher. These plates lie on each side of the
pedicle, or grow round and unite in front of it (_Rhynchonella_, Fig.
327), or constitute merely its anterior border (_Terebratula_, Fig.
328). In some cases this foramen becomes closed in old age.

  [Illustration: FIG. 327.--_Rhynchonella Boueti._ (Cornbrash.)
  _d_, Deltidium; _f_, foramen.]

  [Illustration: FIG. 328.--_Terebratula sella._ (Lower Greensand.)
  _d_, Deltidium; _f_, foramen.]

The dorsal valve in a few cases has its beak perforated by a
foramen--the “visceral foramen.” This foramen is in no way connected
with the pedicle foramen, but points perhaps to the existence in
the early Testicardinate genera of an anal aperture. In _Athyris
concentrica_ (Devonian) this foramen is connected internally with a
cylindrical tube, which extends longitudinally to about one-third the
length of the valve. In _Centronella_ the aperture in the cardinal
plate is rounded and complete; and in _Strophomena_ and its allies
the opening lies between the cardinal processes. If this feature is
correctly interpreted, it suggests a retrogression of the group since
Palaeozoic times not only in numbers, but in structure; and other
evidence points the same way.


                         =Internal Characters=

The interior of the shell is sometimes more or less divided up by
septa. A median septum occurs in one or both valves of many genera as
a low ridge or strongly developed partition (_Waldheimia_, Fig. 329,
_ss_; and Stringocephalus, Fig. 326, B, _v.s_). _Conchidium_ (Fig. 325)
has its dental plates of great size, and uniting to form a V-shaped
chamber or “spondylium,” supported by a median double septum; and by
means of these with a pair of septa and the large socket-plates in the
dorsal valve the interior of the shell of this genus is divided up into
several chambers.

The interiors of several other genera are somewhat similarly divided up.

  [Illustration: FIG. 329.--_Waldheimia (Magellania) flavescens._
  =A=, Interior of ventral valve: _a_, adductor scars; _v.a_,
  ventral adjustors; _d_, divaricators; _a.d_, accessory
  divaricators; _p_, peduncular muscle; _dm_, deltidium; _f_,
  foramen; _t_, teeth. =B=, Interior of dorsal valve: _a.a_,
  anterior adductor (occlusor) scars; _a.p_, posterior adductor
  (occlusor) scars; _c.p_, cardinal process; _cr_, crura; _d.s_,
  dental sockets; _hp_, hinge-plate; _l_, brachial loop; _ss_,
  septum. (After Davidson.)]

In the Carboniferous genus _Syringothyris_ two special plates, situated
between the dental plates, are rolled into an incomplete tube, so as
to enclose probably the anal extremity of the alimentary canal; and
in several genera a sub-umbonal “cardinal plate” is present, which is
perforated (_Athyris_) or slit in some cases for the passage of the
anal tube.

For the support of the fleshy “spiral arms” the calcareous structures
forming the “brachial apparatus” are of two main types--(1) the loop
type; (2) the spiral-cone type. In the _Strophomenidae_ no special
calcareous support seems to have been usually present (Fig. 334),
though in some species of _Leptaena_ spirally-grooved elevated areas
supported the fleshy arms; in the _Productidae_ it is probable that the
ridges enclosing the “reniform impressions” (Fig. 333, _i_) served for
a similar purpose.

The _Terebratulidae_ show the “loop type” of brachial apparatus. In
_Waldheimia_ (Fig. 329), which may be taken as an example, we notice
first in the dorsal valve the “crura” (_cr_), from which arise the two
“descending branches” which run forwards and then are bent back to form
the “ascending branches” which are united by the “transverse band.” In
some genera the “ascending branches” may be reduced to mere points,
and the “transverse band” become a median vertical plate; the “crura,”
too, may be fused so as to form a “crural band”; and the “descending
branches” may be connected by a cross band--the “jugal band.” In
_Stringocephalus_ (Fig. 326, _l_, _s.p_) the loop is furnished on its
inner edge with radiating processes; and in _Argiope_ the loop is
simple, not reflected, and fused with marginal septa; while in the
_Thecidiidae_ it is more or less fused with the shell itself, and with
the mass of calcareous spicules secreted by the mantle.

The “spiral-cone type” of brachial apparatus is found in the
_Spiriferidae_, _Atrypidae_, and _Koninckinidae_, and consists of two
spirally-enrolled calcified lamellae, forming two cones with their
apices directed laterally (_Spirifera_, Fig. 330), or towards the
interior of the dorsal valve (_Atrypa_, Fig. 332), or towards each
other (_Glassia_); or forming two flat spirals in the same plane
(_Koninckinidae_). A “jugal band” is generally present, but varies much
in position, and in some genera has complicated posterior processes.

The _Rhynchonellidae_ have no loop or spiral cones, but merely a pair
of short “crura.”

  [Illustration: FIG. 330.--_Spirifera striata._ (Carboniferous
  Limestone.) Showing brachial spires.]

The principal modifications in the attachments of the muscles in the
_Testicardines_ are illustrated by _Productus giganteus_ (Fig. 333),
_Leptaena rhomboidalis_ (Fig. 334), and _Waldheimia flavescens_ (Fig.
329).

In _Productus_ (Fig. 333) we see in the ventral valve a pair of
dendritic occlusor, often called adductor, impressions and a pair of
large flabellate divaricator impressions. In the dorsal valve the large
“cardinal process” served for the attachment of the divaricator, and a
low median septum separated the dendritic occlusor scars, which are
rarely divisible into anterior and posterior pairs.

  [Illustration: FIG. 331.--_Atrypa reticularis._ (Wenlock
  Limestone.)]

  [Illustration: FIG. 332.--Interior of the same, seen from the
  dorsal side, showing brachial spires. (After Hall.)]

In _Leptaena_ (Fig. 334) the occlusor scars (_a_) in the ventral valve
are narrow and median, and are enclosed by a pair of flabelliform
divaricator impressions (_d.v_); in the dorsal valve two pairs of
occlusor scars (_a.a_, _p.a_) are well marked, and accessory posterior
occlusor scars are traceable in some specimens. The vascular sinuses
(_v.s_) and genital areas are conspicuous in many species of this and
other genera.

  [Illustration: FIG. 333.--_Productus giganteus._ (After
  Woodward.) Carboniferous Limestone. =A=, Interior of dorsal
  valve. =B=, Interior of ventral valve. =C=, Transverse section
  of valves. =D=, Hinge line of =A=: _a_, occlusor scars; _d_,
  divaricator scars; _i_, “reniform impressions”; _ca_, cardinal
  process; _h_, hinge line; _p_, brachial prominence; _s_, cavity
  for spiral arms; _do_, dorsal valve; _ve_, ventral valve.]

In _Waldheimia_ (Fig. 329) a sub-umbonal “peduncular muscle” scar
(_p_) in the ventral valve has before it a pair of “accessory
divaricator” scars (_a.d_) flanked by a pair of “ventral adjustor”
(_v.a_) and a pair of “divaricator” impressions (_d_), between which
lie the two occlusor scars (_a_). In the dorsal valve anterior and
posterior pairs of occlusor scars (_a.a_, _a.p_) are visible.

The minute structure of the calcareous shell of the Testicardines is
of flattened fibrous prisms inclined at a very acute angle to the
surfaces. In many forms minute tubes more or less closely arranged
pierce through the fibrous shell-substance; but in some genera
(_Productus_) they do not reach the outer surface (see p. 468). Allied
genera, however, differ much in the punctate or impunctate character of
the shell.

  [Illustration: FIG. 334.--_Leptaena rhomboidalis._ (Silurian.)
  =A=, External view of ventral valve. =B=, Interior of ventral
  valve: _a_, occlusor scars; _d_, pseudo-deltidium; _d.v_,
  divaricator scars; _c.a_, hinge area; _t_, teeth. =C=, Interior
  of dorsal valve: _a.a_, anterior occlusor scars; _p.a_, posterior
  occlusor scars; _c.a_, hinge area; _c.p_, cardinal process; _d_,
  chilidium; _s_, dental sockets; _v.s_, vascular sinuses.]


SYNOPSIS OF FAMILIES


                             I. ECARDINES


                         Family. _Lingulidae_

   Shell elongated, composed of alternating chitinous and
   calcareous layers, the latter of which are perforated. Attached
   by a pedicle passing between apices of valves.

   Arms have no calcified supports.

   (For muscles see Fig. 322.)

   RANGE.--Lower Cambrian to Recent.

   PRINCIPAL GENERA.--_Lingula_, _Lingulella_, _Lingulepis_.


                          Family. _Obolidae_

   Shell varies in shape. Ventral valve provided with pedicular
   groove or foramen. Cardinal border thickened. No brachial
   supports. Shell composed of alternating chitinous and calcareous
   layers.

   (For muscles see p. 496.)

   RANGE.--Lower Cambrian to Devonian.

   PRINCIPAL GENERA.--_Obolus_, _Obolella_, _Kutorgina_,
   _Linnarssonia_, _Siphonotreta_, _Acrotreta_, _Neobolus_.


                         Family. _Discinidae_

   Shell rounded, valves more or less conical, fixed by pedicle
   passing through slit or tubular foramen in ventral valve. No
   calcified brachial supports. Shell structure chitino-calcareous.

   RANGE.--Ordovician to Recent.

   PRINCIPAL GENERA.--_Discina_, _Orbiculoidea_, _Trematis_.


                          Family. _Craniidae_

   Shell calcareous, subcircular; fixed by surface of ventral
   valve; dorsal valve the larger, depressed-conical. Shell
   structure punctate.

   Four principal muscular scars in each valve, with central
   triangular protuberance in ventral valve (see p. 476).

   RANGE.--Ordovician to Recent.

   PRINCIPAL GENUS.--_Crania_.


                        Family. _Trimerellidae_

   Shell thick, calcareous, inequivalve; beak of ventral valve
   usually prominent; rudimentary teeth maybe present; hinge area
   well developed, with pseudo-deltidium. In interior of valves
   muscular platform, “crescent,” and sometimes sub-umbonal
   chambers (see p. 494, Fig. 323).

   RANGE.--Ordovician and Silurian; maximum in Wenlock.

   PRINCIPAL GENERA.--_Trimerella_, _Monomerella_, _Dinobolus_,
   _Rhinobolus_.


                           II. TESTICARDINES


                         Family. _Productidae_

   Shell entirely free, or fixed by ventral valve or spines.
   Concavo-convex, more or less covered with tubular spines. Hinge
   line straight. Hinge-teeth absent or rudimentary.

   Cardinal process prominent.

   Reniform impressions in dorsal valve.

   (For muscular impressions see p. 501, Fig. 333.)

   RANGE.--Silurian to Permian. Genus _Productus_ very
   characteristic of the Carboniferous.

   PRINCIPAL GENERA.--_Productus_, _Chonetes_, _Strophalosia_,
   _Proboscidella_, _Aulosteges_.


                       Family. _Strophomenidae_

   Shell very variable in shape; concavo-convex, plano-convex,
   or biconvex; hinge line usually straight; frequently with an
   area on each valve; foramen may or may not be present. Shell
   structure near always punctate. Ventral valve usually furnished
   with hinge-teeth; and dorsal valve with cardinal process.

   Brachial supports completely absent or very rudimentary.

   (For muscular impressions see p. 502, Fig. 334.)

   RANGE.--Wholly Palaeozoic.

   PRINCIPAL GENERA.--_Orthis_, with many sub-genera,
   _Clitambonites_, _Skenidium_, _Strophomena_, _Orthothetes_,
   _Leptaena_, _Stropheodonta_, _Plectambonites_.


                        Family. _Koninckinidae_

   Shell plano-convex or concavo-convex. Brachial apparatus
   composed of two lamellae spirally enrolled in the same plane, or
   in the form of depressed cones, with the apices directed into
   the ventral valve.

   RANGE.--Silurian to Lias.

   PRINCIPAL GENERA.--_Koninckina_, _Koninckella_, _Coelospira_,
   _Davidsonia_.


                        Family. _Spiriferidae_

   Shell biconvex. Brachial apparatus consisting essentially of two
   descending calcareous lamellae which by spiral enrolment form a
   pair of laterally-directed cones (Fig. 330).

   RANGE.--Chiefly Palaeozoic, but a few forms pass up into the
   Lias.

   PRINCIPAL GENERA.--_Spirifera_, _Cyrtia_, _Uncites_, _Athyris_,
   _Merista_.


                          Family. _Atrypidae_

   Brachial apparatus consists of two descending calcareous
   lamellae which bend outwards at the extremity of the crura and
   are coiled into two spiral cones, the apices of which either
   converge towards each other (_Glassia_) or towards the dorsal
   valve (_Atrypa_, Fig. 332), or diverge towards the dorsal valve
   (_Dayia_); shell structure impunctate.

   RANGE.--Ordovician to Trias.

   PRINCIPAL GENERA.--_Atrypa_, _Dayia_, _Glassia_.


                       Family. _Rhynchonellidae_

   Shell biconvex, hinge line usually curved.

   Beak of ventral valve incurved, with foramen.

   Calcareous brachial supports reduced to a pair of short curved
   crura.

   The septa, dental and socket plates may be highly developed and
   divide up the cavity of the shell into chambers (_Stenochisma_,
   _Conchidium_).

   Shell structure fibrous, rarely punctate; muscular impressions
   as in _Terebratulidae_.

   RANGE.--Ordovician to Recent: majority of the genera are
   Palaeozoic.

   PRINCIPAL GENERA.--_Rhynchonella_ (Fig. 327), _Stenochisma_,
   _Stricklandia_, _Conchidium_.


                       Family. _Terebratulidae_

   Shell structure punctate.

   Arms supported by a calcareous loop, usually bent back on itself.

   (For muscular impressions see p. 502, Figs. 328, 329.)

   Beak of ventral valve perforated by foramen, furnished with
   deltidium.

   RANGE.--Devonian to Recent; maximum development in Mesozoic
   times.

   PRINCIPAL GENERA.--_Terebratula_, _Terebratulina_, _Waldheimia_,
   _Terebratella_, _Kingena_, _Magas_, _Centronella_.


                         Family. _Argiopidae_

   Large foramen for passage of pedicle. Marginal septa present
   in both valves. Calcareous brachial loop follows margin of
   shell and is more or less fused with the septa. Shell structure
   punctate.

   RANGE.--Jurassic to Recent.

   PRINCIPAL GENERA.--_Argiope_, _Cistella_.


                      Family. _Stringocephalidae_

   Shell subcircular, punctate. Cardinal process highly developed,
   bifid. Brachial apparatus composed of two calcareous free
   lamellae, prolonged at first downwards, then bent back, upwards
   and outwards to run parallel to margin of shell and to unite in
   front, thus constituting a wide loop.

   RANGE.--Silurian and Devonian.

   SOLE GENUS.--_Stringocephalus_.


                         Family. _Thecidiidae_

   Shell usually fixed by beak of ventral valve, plano-convex.
   Sub-cardinal apophysis in ventral valve for attachment of
   occlusors. Marginal septa in dorsal valve. Calcareous brachial
   loop more or less fused with shell, and with calcareous spicules
   of mantle. Shell structure: inner layer fibrous, outer layer
   tubulated.

   RANGE.--Carboniferous to Recent.

   PRINCIPAL GENERA.--_Thecidium_, _Oldhamina_.


              STRATIGRAPHICAL DISTRIBUTION OF BRACHIOPODA

It is remarkable that some of the earliest types of Brachiopoda exist
generically unchanged at the present day. Such are _Lingula_, ranging
from the Cambrian; _Discina_ and _Crania_, ranging from the Ordovician;
and amongst the hinged forms _Terebratula_ from the Devonian, and
_Rhynchonella_ from the Ordovician.

In the lowest Cambrian (Olenellus beds) the most important genera
are _Linnarssonia_ and _Kutorgina_. The hinged forms appear in the
Cambrian, being represented by _Orthis_; but the majority in this
formation belong to the Ecardines. _Lingula_, _Lingulella_, and
_Obolella_ are characteristic.

In the Ordovician many new genera of the Testicardines make
their appearance, such as _Strophomena_, _Leptaena_, _Atrypa_,
_Rhynchonella_, _Clitambonites_, etc., but the extraordinary abundance
and variety of _Orthis_ is most remarkable. The Ecardines are
reinforced by such forms as _Trematis_ and _Siphonotreta_. It is,
however, in the Silurian that the Testicardinate Brachiopoda attain
their maximum, for in addition to a great development of species
amongst the older forms, a host of new genera for the first time occur
here (_Spirifera_, _Athyris_, _Conchidium_, _Stricklandia_, _Chonetes_,
_Cyrtia_, etc.); and the _Trimerellidae_ are especially characteristic
of the Wenlock.

With the commencement of Devonian times many species and genera
become extinct, but new forms come in (_Terebratula_, _Orthothetes_,
_Productus_, etc.), and some genera are wholly confined to this
formation (_Uncites_, _Stringocephalus_). The Carboniferous is
marked by the maximum development of _Productus_ and _Spirifera_;
_Orthothetes_, _Stenochisma_, and _Athyris_ are also abundant, but
there is a considerable extinction of the older genera and species, and
a great diminution in the number of individuals and species of those
that persist.

A further reduction occurs in the Permian, where the most important
genera are _Productus_, _Strophalosia_, and _Stenochisma_; but
_Aulosteges_ is a new form peculiar to this period. In the Trias a new
era commences; the principal families and genera of the older rocks
disappear entirely; a few spire-bearing genera persist (_Spiriferina_,
_Athyris_), and the genus _Koninckina_ is restricted to this formation.

The enormous development of species of the _Terebratulidae_ and
_Rhynchonellidae_ is the most noticeable feature in Jurassic times; and
a few ancient types linger on into the Lias (_Spiriferina_, _Suessia_,
a sub-genus of _Spirifera_); _Koninckella_ here occurs.

The Cretaceous Brachiopoda are closely allied to the Jurassic; _Magas_
and _Lyra_ are peculiar to the period, and the _Terebratulidae_ and
_Rhynchonellidae_ are very abundant, together with the Ecardinate genus
_Crania_.

With the commencement of Tertiary times the Brachiopoda have lost their
geological importance, and have dwindled down into an insignificant
proportion of the whole Invertebrate fauna.

       *       *       *       *       *

The distribution of the Brachiopoda in past time is shown in the
following table:--

 +------------------------------------+-----------------------+-----------+-------+
 |                                    |       Palaeozoic      |  Mesozoic |       |
 |                                    |   |   |   |   | C |   |   |   |   |   |   |
 |                                    |   |   |   |   | a |   |   |   |   |   |   |
 |                                    |   |   |   |   | r |   |   |   |   |   |   |
 |                                    |   | O |   |   | b |   |   |   | C |   |   |
 |                                    |   | r |   |   | o |   |   |   | r |   |   |
 |                                    | C | d | S | D | n |   |   | J | e | T |   |
 |                                    | a | o | i | e | i | P |   | u | t | e |   |
 |                                    | m | v | l | v | f | e |   | r | a | r | R |
 |                                    | b | i | u | o | e | r | T | a | c | t | e |
 |                                    | r | c | r | n | r | m | r | s | e | i | c |
 |                                    | i | i | i | i | o | i | i | s | o | a | e |
 |                                    | a | a | a | a | u | a | a | i | u | r | n |
 |                                    | n | n | n | n | s | n | s | c | s | y | t |
 |              ECARDINES             +---+---+---+---+---+---+---+---+---+---+---+
 | Lingulidae         Lingula         |___|___|___|___|___|___|___|___|___|___|___|
 |                    Lingulella      |___|   |   |   |   |   |   |   |   |   |   |
 | Obolidae           Obolus          |   |___|___|   |   |   |   |   |   |   |   |
 |                    Obolella        |___|___|   |   |   |   |   |   |   |   |   |
 |                    Kutorgina       |___|___|   |   |   |   |   |   |   |   |   |
 |                    Linnarssonia    |___|   |   |   |   |   |   |   |   |   |   |
 |                    Trematis        |   |___|___|   |   |   |   |   |   |   |   |
 |                    Siphonotreta    |   |___|___|   |   |   |   |   |   |   |   |
 |                    Acrotreta       |   |___|   |   |   |   |   |   |   |   |   |
 | Discinidae         Discina         |   |___|___|___|___|___|___|___|___|___|___|
 | Craniidae          Crania          |   |___|___|___|___|___|___|___|___|___|___|
 | Trimerellidae      Trimerella      |   |   |___|   |   |   |   |   |   |   |   |
 |                    Dinobolus       |   |___|   |   |   |   |   |   |   |   |   |
 |                                    |   |   |   |   |   |   |   |   |   |   |   |
 |            TESTICARDINES           |   |   |   |   |   |   |   |   |   |   |   |
 | Productidae        Productus       |   |   |   |___|___|___|   |   |   |   |   |
 |                    Chonetes        |   |   |___|___|___|   |   |   |   |   |   |
 |                    Strophalosia    |   |   |   |___|___|___|   |   |   |   |   |
 | Strophomenidae     Orthis          |___|___|___|___|___|   |   |   |   |   |   |
 |                    Skenidium       |   |___|___|   |   |   |   |   |   |   |   |
 |                    Clitambonites   |   |___|   |   |   |   |   |   |   |   |   |
 |                    Strophomena     |   |___|___|   |   |   |   |   |   |   |   |
 |                    Stropheodonta   |   |___|___|___|   |   |   |   |   |   |   |
 |                    Leptaena        |   |___|___|___|___|   |   |   |   |   |   |
 |                    Orthothetes     |   |   |   |___|___|___|   |   |   |   |   |
 |                    Davidsonia      |   |   |   |___|   |   |   |   |   |   |   |
 | Koninckinidae      Koninckina      |   |   |   |   |   |   |___|   |   |   |   |
 |                    Koninckella     |   |   |   |   |   |   |   |___|   |   |   |
 | Spiriferidae       Spirifera       |   |   |___|___|___|___|   |   |   |   |   |
 |                    Spiriferina     |   |   |   |___|___|___|___|___|   |   |   |
 |                    Cyrtia          |   |   |___|___|___|   |   |   |   |   |   |
 |                    Syringothyris   |   |   |   |   |___|   |   |   |   |   |   |
 |                    Uncites         |   |   |   |___|   |   |   |   |   |   |   |
 |                    Athyris         |   |   |___|___|___|___|___|   |   |   |   |
 |                    Merista         |   |   |___|___|   |   |   |   |   |   |   |
 |                    Retzia          |   |   |___|___|___|___|___|   |   |   |   |
 | Atrypidae          Atrypa          |   |___|___|___|___|___|___|   |   |   |   |
 |                    Dayia           |   |   |___|   |   |   |   |   |   |   |   |
 |                    Coelospira      |   |   |___|   |   |   |   |   |   |   |   |
 | Rhynchonellidae    Rhynchonella    |   |___|___|___|___|___|___|___|___|___|___|
 |                    Stenochisma     |   |   |   |___|___|___|   |   |   |   |   |
 |                    Stricklandia    |   |   |___|   |   |   |   |   |   |   |   |
 |                    Conchidium      |   |   |___|___|   |   |   |   |   |   |   |
 | Terebratulidae     Terebratula     |   |   |   |___|___|___|___|___|___|___|___|
 |                    Terebratulina   |   |   |   |   |   |   |   |___|___|___|___|
 |                    Waldheimia      |   |   |   |   |   |   |   |___|___|___|___|
 |                    Terebratella    |   |   |   |   |   |   |   |___|___|___|___|
 |                    Kingena         |   |   |   |   |   |   |   |___|___|   |   |
 |                    Magas           |   |   |   |   |   |   |   |   |___|   |   |
 |                    Centronella     |   |   |___|___|___|   |   |   |   |   |   |
 | Argiopidae         Argiope         |   |   |   |   |   |   |   |___|___|___|___|
 |                    Cistella        |   |   |   |   |   |   |   |___|___|___|___|
 | Stringocephalidae  Stringocephalus |   |   |   |___|   |   |   |   |   |   |   |
 | Thecidiidae        Thecidium       |   |   |   |   |   |   |___|___|___|___|___|
 |                    Oldhamina       |   |   |   |   |___|   |   |   |   |   |   |
 |                                    |   |   |   |   |   |   |   |   |   |   |   |
 +------------------------------------+---+---+---+---+---+---+---+---+---+---+---+


                        PHYLOGENY AND ONTOGENY

Wherever successive stages in the life history of an individual
resemble in important anatomical features the adult individuals of
other species occurring in successive members of a stratigraphical
series, the development of the individual may be regarded as an epitome
of the development of the species; it also generally throws light on
the origin and relationships of allied genera and families.

In the case of the fossil Brachiopoda comparatively little work has yet
been done in tracing their ontogeny or phylogeny, though the abundance,
variety, and excellent state of preservation of the extinct species
offer a promising field for investigation. It is to Dr. C. E. Beecher
and other recent American palaeontologists that we owe our advance in
this branch of the subject.

In the first place, in about forty genera, representing nearly all the
leading families of the group, the important fact has been established
of the presence of a common form of embryonic shell, termed the
“protegulum,” which is “semicircular or semielliptical in shape with a
straight or arcuate hinge line and no hinge area” (Beecher).[431] Its
minute size and delicate texture cause its preservation to be rare, but
its impression is not uncommonly left on the beak of the adult shell.

The main features of this embryonic shell are exhibited in the adult
Lower Cambrian Brachiopod _Obolus_ (_Kutorgina_) _labradoricus_
(Billings); the sub-equal semielliptical valves have lines of growth
running concentrically and parallel to the margin of the shell, and
ending abruptly against the straight hinge line; and this indicates
that there has been no change in the outline and proportions of the
shell during its stages of growth, but only a general increase in size.
It is very significant that we have here a mature type possessing the
common embryonic characters of a host of widely separated genera, and
we may therefore regard it as the most primitive form known.

Many genera pass through this so-called “Paterina” stage either in
the case of both their valves, or more generally in the case of the
dorsal valve only; but modifications in the form of the protegulum
arise, which are due to the influence of accelerated growth, by
which features belonging to later stages become impressed on the
early embryonic shell. The most variable and specialised valve--the
ventral or pedicle valve naturally exhibits the effect of this
influence first and to the greatest extent. The Palaeozoic adult forms
of many species represent various pre-adult stages of the Mesozoic,
Tertiary, and Recent species, as is especially well shown in the genera
_Orbiculoidea_ and _Discinisca_.

In the Strophomenoid shells the protegulum in the dorsal valve is
usually normal, but in the ventral valve abbreviation of the hinge
and curvature of the hinge line are produced by acceleration of the
“Discinoid stage” in which a pedicle notch is present.

No marked variation has yet been noticed in the spire-bearing, or
Terebratuloid, or Rhynchonelloid genera.

The form of the shell and the amount of difference in shape and size of
the valves seem to be largely due to the length of the pedicle and its
inclination to the axis of the body, as evidenced by the development
of _Terebratulina_. A series showing progressive dissimilarity of the
two valves arising from these causes can be traced from _Lingula_ to
_Crania_. The greater alteration that takes place in the ventral valve
appears to be due to its position as lower and attached valve. If the
pedicle is short a transversely-expanded shell with long hinge line
results when the plane of the valves is vertical or ascending, but
when the latter is horizontal a Discinoid form is found. This mode of
attachment is often accompanied by a more or less plainly developed
radial symmetry. Shells with long pedicles, on the other hand, are
usually longer than wide.

The character of the pedicle-opening is of great significance from
an evolutional and classificatory point of view, for the successive
stages through which it passes in embryonic growth are chronologically
paralleled by different genera, and are likewise accompanied by the
successive acquisition of other important anatomical characters, as
has been shown by Beecher and others. The first and simplest type of
pedicle opening is in shells with a posterior gaping of the valves,
where the pedicle protrudes freely between them in a line with the
axis, and the opening is shared by both valves, though generally
to a greater extent by the ventral valve. _Paterina_ (_= Obolus
labradoricus_) and _Lingula_ furnish examples of this type. In the
second type the pedicle opening is restricted to the ventral valve,
and the direction of the pedicle makes a right angle with the plane
of the valves; in the lower forms the pedicle lies in a slit or sinus
(_Trematidae_), but by further specialisation it becomes enclosed by
shell growth so as to lie within the periphery, and finally becomes
sub-central in some genera (_Discinidae_). The third type shows the
pedicle opening confined to the ventral valve and sub-marginal. A
pseudo-deltidium may preserve the original opening (_Clitambonites_);
or this shelly plate may become worn away or reabsorbed in the adult
so that the deltidial fissure through which the pedicle passes remains
quite open (_Orthidae_). In the fourth type the incipient stage marks
a return to the simple conditions of the first type; but ultimately
a pair of deltidial plates develop, and may completely limit the
pedicle opening below. Examples of this type are _Spirifera_ and
_Rhynchonella_. By means of these four types the Brachiopods have been
divided into four Orders: the _Atremata_ (type i.); the _Neotremata_
(type ii.); the _Protremata_ (type iii.); and the _Telotremata_ (type
iv.).

The _Telotremata_ were the last to appear, but the four types of
pedicle-opening with the various forms of calcareous brachial apparatus
were in existence in the Bala period of the Ordovician.

As _Paterina_ is the most primitive form of all, we may place it at
the root of the phylogenetic tree. From it sprang the _Atremata_,
which gave off the _Neotremata_ and _Protremata_; the most primitive
_Neotremata_ seem to be the _Trematidae_, while the connecting
link between the _Protremata_ and _Atremata_ is furnished by the
_Kutorginidae_. From the genus _Conchidium_ and its allies we may see
how the _Rhynchonellidae_ ushered in the _Telotremata_ as an offshoot
from the _Protremata_. The _Telotremata_ subsequently gave off two
main branches, which became specialised with the loop-bearing and
spire-bearing forms respectively.

The evolution and mutual relationships of genera have been indicated
with much probability by Hall, Clarke, and others. The Obolelloid
type may be connected with the Linguloid by means of _Lingulella_
and _Linyulepis_, while in _Lingula_ itself we find the point of
divergence for the ancestors of _Trimerella_, and for a line of
variation culminating in _Dignomia_. The Palaeozoic Rhynchonelloids
branched off at an early period from the same stock as _Orthis_, and
are connecting links between this genus and Mesozoic Rhynchonellae;
and a whole series of genera exhibit intermediate stages of structure
between the Rhynchonelloid and Pentameroid groups. The Terebratuloids
can be traced back to the primitive type _Renssoellaria_; and amongst
spire-bearing forms, the protean genus _Spirifera_ can be split up into
groups of species which diverge along lines tending to forms no longer
congeneric. When we come to deal with specific differences we find
frequently such a host of intermediate varieties that the separation of
many species, as in the case of Mesozoic Terebratulae, is to a large
extent arbitrary and artificial.




                                 INDEX

    References to figures are printed in thick type (=248=, =197=);
           to systematic position, in italics (_391_, _430_)


    _Abralia_, _391_

    Absorption of internal portions of shell, 259

    Abyssal Mollusca, 374

    _Acanthinula_, _441_

    _Acanthoceras_, _399_

    _Acanthochiton_, =403=, _403_

    _Acanthodoris_, _434_

    _Acanthopleura_, _403_;
      eyes, =188=

    _Acavus_, 303, =304=, 335, _441_

    _Acera_, 245, _430_

    _Achatina_, 278, 328–337, =333=, _442_, =443=;
      jaw, =211=;
      food, 33;
      size of egg, 124;
      _A. fulica_, 279

    _Achatinella_, 278, =326=, 327, _443_;
      radula, =234=;
      musical sounds, 51

    _Achatinelloides_, 332

    _Acicula_, 287, 296, _414_

    _Acmaea_, _405_;
      radula, 227

    _Acme_, _414_

    _Acmella_, 314, _415_

    _Acroptychia_, 336, _414_

    _Acrotreta_, _504_, 508

    _Actaeon_, 250, 427, =428=, _429_;
      radula, 217, 230;
      streptoneurous, 203 n.

    _Actaeonella_, _430_

    _Actaeonia_, _432_

    _Actaeonina_, 250, _429_

    _Actinoceras_, _394_

    _Actinodonta_, _447_

    _Acusta_, 306, 316, 318, _441_

    _Adacna_, =12=, 297, _455_

    _Adalaria_, _434_

    _Adamsiella_, _414_

    _Addisonia_, _412_

    _Adelphoceras_, _395_

    _Adeorbis_, _416_

    _Admete_, _426_

    _Aegires_, _434_

    _Aegista_, 305, 316, _441_

    _Aegoceras_, _398_

    _Aeolis_, =10=, =152=, _432_;
      radula, 217, =229=;
      stinging cells, 65;
      mimicked by _Sagartia_, 68;
      warning coloration, 72

    _Aerope_, 328, 333, _440_;
      radula, 215;
      habits, 54

    Aestivation, 25

    _Aetheria_, 328–336, _452_;
      variation, 92

    _Africarion_, 333, _440_

    _Agaronia_, _426_

    Age of snails, 39

    Aglossa, 7

    Agnatha, habits, 51

    _Akiodoris_, _434_

    _Alaba_, _415_

    _Alaria_, _418_

    _Alariopsis_, _420_

    _Albersia_, 320

    Albino varieties, 87

    _Alcadia_, 348–351, _410_

    _Alderia_, _432_

    _Alexia_, _439_

    _Alicia_, _459_

    _Allognathus_, _441_

    _Allopagus_, _452_

    Alloposidae, _384_

    _Alvania_, _415_

    _Alycaeus_, 266, 302 f., 309, 319, _414_

    _Amalia_, _440_

    _Amalthea_, 78

    _Amaltheus_, _398_

    _Amastra_, _443_

    _Amaura_, _411_

    _Amberleya_, _409_

    _Ambonychia_, _449_

    _Amicula_, _404_

    _Ammonites_, 247, =393=, =398=, _398_;
      sutures, =396=;
      aptychus, =397=

    Ammonoidea, _396_ f.

    _Amnicola_, 325, _415_

    _Amoria_, radula, 222

    _Ampelita_, 335, _442_

    _Amphibola_, 10, =18=, _439_;
      breathing, 151;
      radula, 236

    _Amphibulimus_, 352, _442_;
      radula, 233

    _Amphidoxa_, 358

    _Amphidromus_, 301, 305, 317, =310=, 359, _442_;
      radula, 233

    Amphineura, 8, _400_;
      breathing organs, 154, 168;
      nervous system, =203=;
      genitalia, 145

    _Amphipeplea_, _439_

    _Amphiperas_, _419_

    _Amphisphyra_, _430_

    _Amphissa_, _423_

    _Amphitretus_, =383=

    _Ampullaria_, 17, _416_;
      self-burial, 42;
      spawn, =125=;
      breathing organs, 151, =158=;
      jaws, =212=;
      shell, =249=, 263;
      operculum, =268=;
      distribution, 294, 320, 322, 343, 359

    _Ampullarina_, 302, _439_

    _Ampullina_, _411_

    _Amussium_, _450_

    _Amycla_, _423_

    _Anabathron_, _415_

    _Anachis_, _423_

    _Anadenus_, 24, _441_

    Anal glands, 241

    Anal siphon, 164, 173

    _Anastomopsis_, _442_

    _Anatina_, 274, 275, _459_

    Anatinacea, _458_;
      gills, 167

    _Anaulus_, _414_

    _Anchistoma_, 293, 296

    _Ancilla_, 267, _426_

    _Ancillina_, _426_

    _Ancistrochirus_, _391_

    _Ancistromesus_, _405_

    _Ancistroteuthis_, _391_

    _Ancula_, _434_;
      radula, 229, 230;
      warning coloration, 72

    _Anculotus_, _417_

    _Ancyloceras_, 247, _399_

    _Ancylus_, 19, _439_;
      breathing, 162;
      hibernating, 27;
      radula, =235=

    _Aneitea_, 325, _443_

    _Angitrema_, 340, _417_

    _Anisocardia_, _451_

    _Anodonta_, 259, 341, _452_;
      shower of, 47;
      variation, 92;
      _Glochidium_, =147=;
      gill, =167=;
      otocyst, =197=;
      nervous system, =206=;
      hinge, 274;
      _A. anatina_, 24;
      distribution, 282

    _Anodontopsis_, _451_

    _Anoglypta_, 325, _441_

    _Anomia_, =257=, _448_, 464;
      intestine, 241;
      byssus hole, =262=;
      hearing, 196

    Anomiacea, _448_

    _Anoplophora_, _451_

    _Anostoma_, =248=, 266, 356, 358, _442_;
      aperture, =63=

    _Anthracosia_, _451_

    _Anura_, _424_

    Anus, 209, 241

    _Apera_, 334, _440_

    _Aperostoma_, 344, _414_

    _Aphanotrochus_, _408_

    _Aphelodoris_, radula, 230

    _Apicalia_, _422_

    Aplacophora, 9, _404_;
      radula, 228

    _Aplecta_, 354, _439_

    _Aplustrum_, =245=, =428=, _430_;
      radula, 230

    _Aplysia_, 245, =428=, _431_;
      stomach, 239;
      purple fluid, 65

    _Aplysioidea_, _430_

    _Aporrhais_, _418_;
      radula, 215

    _Apricardia_, _455_

    Aptychus, =397=

    _Aptyxiella_, _417_

    _Aptyxis_, _424_

    Aral Sea, _Limnaea_ from near, =84=;
      _Cardium_ from, 91

    _Arca_, =14=, 171, =273=, _448_;
      eyes, =191=

    Arcacea, _448_

    Arcachon, oyster-parks at, 105

    _Arcestes_, _397_

    _Archidoris_, =434=, _434_;
      protective coloration, 73

    _Architeuthis_, 378, =390=, _390_;
      sucker, =381=

    _Arcomya_, _458_

    _Arconaia_, 307, _452_

    Arctic shells, colour of, 86

    _Arcuella_, _422_

    _Argiope_, =470=, 472, 479, _487_;
      parasite of, 485;
      distribution, 486;
      fossil, 501, _506_, 508

    Argiopidae, _506_, 508

    _Argobuccinum_, _420_

    _Argonauta_, =383=, _383_;
      egg-laying, 127;
      hectocotylised arm, 137;
      radula, 236

    _Arinia_, _413_

    _Ariolimax_, _441_, 341;
      radula, 233

    _Arion_, _440_;
      shell, 175, 245, 246;
      hardier than _Helix_, 24;
      voracity, 30 f.;
      egg-laying, 42 f.;
      protective coloration, 70;
      pulmonary orifice, 160;
      food, 179;
      smell, 193 f.;
      radula, 233;
      distribution, 285

    _Arionta_, =341=, 353, _441_

    _Ariophanta_, 301, =308=, 309, 316, _440_;
      protective coloration, 70

    Aristotle, on modified arm of polypus, 138

    _Artemis_, _454_

    _Arthuria_, _403_

    _Asaphis_, _456_

    _Ascoceras_, _394_

    Ascoglossa, 11 n., _431_

    Ashford, C., on pulsations of heart in _Helix_, 26;
      on homing of _Helix_, 35;
      on dart-sac, 143

    _Asolene_, _416_

    _Aspergillum_, 262, _459_

    _Aspidelus_, 329, _440_

    _Aspidoceras_, _399_

    _Assiminea_, _415_

    _Astarte_, _451_

    _Asthenothaerus_, _459_

    _Astralium_, _409_

    _Athoracophorus_, _443_--see _Janella_

    _Athyris_, 499, 500, _505_;
      stratigraphical distribution, 507, 508

    _Atilia_, _423_

    _Atlanta_, 421, _422_;
      foot, 200

    _Atopocochlis_, 330, _441_

    Atremata, 511

    _Atretia_, distribution, 486, _487_

    _Atrypa_, 501, =502=, _505_;
      stratigraphical distribution, 507, 508

    Atrypidae, 501, 505, 508

    _Aturia_, 393, _395_

    _Atys_, =428=, _430_

    Aucapitaine, H., on tenacity of life, 38

    _Aucella_, _449_

    _Aulopoma_, 157, 304, _414_;
      operculum, =269=

    _Aulosteges_, _504_;
      stratigraphical distribution, 507

    _Auricula_, =439=, _439_

    _Auriculella_, 327, _443_

    Auriculidae, 17, =18=, =260=, =439=, _439_;
      lung, 160;
      eyes, 186;
      radula, 235

    _Austenia_, 301, 304, _440_

    _Avellana_, _430_

    _Avicula_, 254, 258, =449=, _449_;
      eyes, 190;
      genital orifice, 242;
      _A. margaritifera_, 100

    _Aviculopecten_, _450_

    _Aviculopinna_, _449_

    _Axinus_, _452_

    _Azeca_, _442_

    Azygobranchiata, 155, 407


    _Babinka_, _447_

    _Bactrites_, _395_

    _Baculites_, _399_

    _Baikalia_, 290, _415_

    Baird, Mr., on the British Museum snail, 37

    _Balea_, _442_;
      _B. perversa_, 24, 41

    Baltic, fauna of the, 12, 83, 366

    _Bankivia_, _408_

    _Barbatia_, _448_

    _Barleeia_, _415_

    Barnacle, Rev. H. G., on musical sounds, produced by Mollusca, 51

    Barometers, snails as, 50

    _Bartlettia_, _452_

    _Basilissa_, 376, _408_

    Basommatophora, 11, 19, 181, _438_

    _Basterotia_, _451_

    Bateson, W., on variation in _Cardium_, 91;
      on hearing in _Anomia_, 196

    _Bathmoceras_, _395_

    _Bathydoris_, _433_

    _Bathyteuthis_, _390_

    _Batissa_, 320, _453_

    _Beddomea_, 304

    Beecher on phylogeny, 509

    Beetles, prey on Mollusca, 58

    _Bela_, _426_;
      radula, =219=

    _Belemnites_, 380

    Belemnitidae, _387_

    _Belemnosepia_, _390_

    _Bellerophon_, =266=, _407_

    _Belopetra_, 380

    Belopteridae, _388_

    _Belosepia_, 386, _388_

    _Beloteuthis_, _390_

    _Bembix_, 376, _408_

    _Benedictia_, 290, _415_

    _Benthobia_, 377

    _Benthodolium_, 377

    _Berendtia_, _441_

    Beudant, experiments on Mollusca, 12

    Bideford Bridge and mussels, 117

    Binney, Dr., on epiphragm, 28

    _Binneya_, 341, _441_

    _Biradiolites_, _456_

    Birds, devour Mollusca, 56 f.

    _Bithynella_, 289, 293, _415_

    _Bithynia_, 336, 342, _415_;
      stomach, 239;
      habitat, 25

    _Bittium_, _416_

    _Blaesospira_, 346, 351

    _Blandiella_, 16, _414_

    _Blanfordia_, _414_

    Blind Mollusca, 185

    Blood, 171

    Bodö, land Mollusca, 24

    Boeuf and French oysters, 107

    _Bolma_, _409_

    _Boltenia_, _346_

    _Boreofusus_, radula, 221

    _Bornella_, _433_;
      stomach, 239

    _Borsonia_, _426_

    _Borus_, 356–358, _441_

    _Bourcieria_, 357, _410_

    _Bourguetia_, _417_

    _Bourguignatia_, 332

    Bouvier--_see_ Fischer

    _Boysia_, 302, _442_

    Brachial apparatus, types of, 500

    Brachiopoda, fossil, limestone formed of, 492;
      shell, 493, 497;
      muscle scars on, 494, 501;
      platform, 495;
      synopsis of families, 503;
      stratigraphical distribution, 506;
      phylogeny and ontogeny, 509;
      Orders, 511

    Brachiopoda, recent, 463;
      historical account of, 464;
      shell, 465;
      body, 469;
      digestive system, 471;
      body cavity, 472;
      heart, 473;
      excretory organs, 474;
      muscles, 475;
      nervous system, 478;
      reproductive system, 478;
      embryology, 479;
      habits, 482;
      distribution, 484;
      classification, 487;
      affinities, 487

    _Brachytrema_, _417_

    Brackish-water species, 14

    Branchiae, 151, 153, 164

    Branchial siphon, 155, 164, 173

    Braun, on self-impregnation, 44

    Breathing organs--_see_ Respiration, Branchiae

    _Brechites_, _459_

    Breeding, periodicity in, 129

    _Broderipia_, _408_

    _Brotia_, 305

    _Brownia_, 133

    _Buccinanops_, _423_

    _Buccinopsis_, _424_;
      radula, 221, 222;
      egg-laying, 128

    _Buccinum_, =6=, _424_;
      radula, 217;
      monstrosity, =251=;
      breeding, 129;
      osphradium, =195=;
      spawn, =126=

    _Buliminus_, 24, 278, 285, =295= f., 316, 331, 339, _442_;
      protective habits, 70;
      _B. pallidior_, 38

    _Bulimulus_, 278, 334, 339–359, _442_;
      jaw, =211=, 233;
      radula, 233;
      variation, 87

    _Bulimus_, 278, 342–359, =355=, _441_;
      radula, 233;
      egg, =124=

    _Bulinus_--see _Isidora_

    _Bulla_, 428, _430_

    _Bullia_, =155=, _423_;
      habits, 192;
      foot, 198;
      radula, 221

    Bulloidea, _429_

    Burrowing Mollusca, 446

    Burying propensities of Mollusca, 27, 41

    _Busycon_, _424_;
      money made from, 97;
      egg-capsules, =125=--see _Fulgur_

    Butterell, Mr., on habits of _Testacella_, 52

    _Byssocardium_, _455_

    Byssus gland, 201


    _Cadlina_, _434_

    _Cadoceras_, =393=

    _Cadulus_, 376, _445_

    _Caecilianella_, _442_;
      habitat, 48;
      eyes, 186

    _Calcarella_, 133

    California, land Mollusca, 280

    _Calliostoma_, _408_;
      jaws, =212=

    _Callistochiton_, _403_

    _Callochiton_, _403_

    _Callogaza_, _408_

    _Callonia_, _442_

    _Callopoma_, _409_

    _Calma_, protective coloration, 74

    _Calybium_, _410_

    _Calycia_, 320, _442_

    _Calycidoris_, _434_

    _Calyptraea_, =248=, _412_

    _Camaena_, 305, 306, 315, =316=, _441_

    Cambrian, Mollusca of the, 2

    _Camitia_, _409_

    _Campaspe_, _433_

    _Camptoceras_, =302=

    _Camptonyx_, 278, =302=, _439_

    _Campylaea_, 285, 289 f., =293=, _441_

    Canal, 155

    _Cancellaria_, _426_

    _Canidia_, 16, 305, _423_

    Cannibalism in snails and slugs, 32, 33

    _Cantharidus_, _408_

    _Cantharus_, 275;
      radula, =222=

    _Caprina_, _456_

    _Caprotina_, _456_

    _Capulus_, _412_

    _Caracolus_, =347=-351, _441_

    _Carbonicola_, _451_

    Cardiacea, _454_

    _Cardiapoda_, _421_

    _Cardilia_, _454_

    Cardinal plate, 500

    Cardinal process, 497, 501

    _Cardinalia_, _408_

    _Cardinia_, _451_

    _Cardita_, =273=, _451_

    _Carditella_, _451_

    _Carditopsis_, _451_

    _Cardium_, =6=, =273=, =455=, _455_;
      _C. edule_, =12=, =164=;
      modifications, 12;
      variation, 84, =91=;
      nervous system, 207;
      distribution, 292, 297

    _Carelia_, 327, _443_

    _Carinaria_, =9=, =422=, _422_;
      foot, 200

    _Carinifex_, _439_

    _Carolia_, _448_

    _Cartusiana_, 296

    _Carychium_, 18, _439_

    _Caryodes_, 325, 359, _441_

    _Casella_, radula, 230

    _Caspia_, 12, 297

    Caspian Sea, fauna, 12, 297

    _Cassidaria_, _420_

    _Cassidula_, =18=, 278, =439=, _439_

    _Cassis_, 255, _420_;
      radula, =223=

    _Castalia_, 344, _452_

    _Cataulus_, 157, 266, 304, _414_

    Caterpillars mimicking _Clausilia_, 68

    _Cathaica_, 316, _441_

    _Catinella_, _443_

    _Cavolinia_, 158, _436_;
      eyes, 186

    _Cecina_, _414_

    _Cenia_, _432_;
      breathing, 152

    _Centrodoris_, _434_;
      radula, 230

    _Centronella_, 499, _506_, 508

    Cephalopoda, 378 f.;
      defined, 5;
      ink, 65;
      egg-laying, 127;
      embryo, =133=;
      branchiae, 168;
      osphradium, 195;
      foot, 200;
      nervous system, 206;
      jaws, 213;
      radula, 236

    _Cepolis_, 349–351, _441_

    _Cerastoma_, _423_

    _Cerastus_, 331, _441_

    Cerata of Nudibranchs, 71, 159

    _Ceratites_, _397_, =398=;
      suture, =396=

    _Ceratodes_, 357, _416_

    _Ceres_, =21=, 354, _410_

    _Ceritella_, _417_

    _Cerithidea_, 260, _417_;
      _C. obtusa_, breathing, 152

    _Cerithiopsis_, _417_

    _Cerithium_, =16=, _416_

    _Ceromya_, _458_

    _Chaetoderma_, =404=, _404_;
      breathing organs, =154=;
      nervous system, =203=;
      radula, 217, 228

    _Chaetopleura_, _403_

    _Chama_, 257, 272, 446, _455_

    _Chamostrea_, _458_

    Changes in environment, effect of, 83 f.

    Chank-shell, fishery of, 100

    _Charis_, 324, _442_

    _Charopa_, 319, 323–327, _441_

    _Chascax_, _424_

    _Chelinodura_, _430_

    _Chelotropis_, 133

    _Chenopus_, _418_

    Chilidium, 498

    _Chilina_, 19, _343_, 358

    Chilinidae, _439_;
      radula, 236

    _Chilotrema_, _441_

    China, use of shells in, 101

    _Chiropteron_, 133

    _Chiroteuthis_, 385, _391_

    _Chiton_, =8=, =153=, _403_;
      egg-laying, 126;
      breathing organs, 153 f.;
      eyes, =188=;
      osphradium, 195;
      radula, =228=;
      nervous system, =203=;
      valves, =401=, =402=;
      girdle, =403=

    _Chitonellus_, =404=, _404_;
      valves, =401=

    _Chittya_, 16, 348, 351, _414_

    _Chlamydephorus_, 333, _440_

    _Chlamydoconcha_, 175, 245, _453_

    _Chlamys_, _450_

    _Chloritis_, 306, 311, 319–324, _441_

    _Chlorostoma_, _408_

    _Chlorostracia_, 307

    _Choanomphalus_, 250, 290, _439_

    Chondrophora, _389_

    _Chondropoma_, 346–355, =348=, _414_

    _Chondrula_, 285, =295=, 296, _442_

    _Choneplax_, _404_

    _Chonetes_, _504_;
      stratigraphical distribution, 507, 508

    _Choristes_, _420_

    _Choristoceras_, _398_

    _Chorus_, _423_

    _Chromodoris_, _434_;
      jaws, =212=;
      radula, 230

    _Chrysallida_, _422_

    _Chrysodomus_, _423_

    _Chrysostoma_, _409_

    _Cingula_, _415_

    _Cingulina_, _422_

    _Cionella_, _442_

    _Circe_, _454_, =458=

    Circulatory system, 169

    _Circulus_, _408_

    Circumpolar species, 287

    _Cirrhoteuthis_, 381, =382=

    _Cistella_, 467, =470=, 472, 475, 476, 479, 480, _487_;
      larvae, =481=, 483;
      parasite of, 485;
      distribution, 486;
      fossil, _506_, 508

    _Cistopus_, _385_

    _Cistula_, 349, 351, 355, _414_

    Cladohepatica, _432_

    _Clanculus_, _408_

    Classification, 5, 8;
      of Gasteropoda, 8, 11

    _Clathurella_, _426_

    _Clausilia_, =442=, _442_;
      mimicked by caterpillars, 68;
      monstrosity, 251;
      distribution, 285 f., =294=, 305–318, 332, 339–356;
      _C. rugosa_, 24;
      _scalaris_, 278

    _Clavagella_, 262, _459_

    _Clavator_, 335, 359, _441_

    _Clavatula_, _426_

    _Clavella_, _424_

    _Claviger_, 329, _417_

    _Clea_, =16=, 305, _423_

    _Clementia_, _454_

    _Cleodora_, =436=, _436_

    _Cleopatra_, 294, 328, 331, 336, _416_

    Clessin, on duration of life, 39

    _Clessinia_, 12, 297

    _Clio_, =436=, _436_

    _Cliona_, enemy of oysters, 112

    _Clione_, 158, _438_

    _Clionopsis_, _437_

    _Clitambonites_, 498, _505_;
      stratigraphical distribution, 507, 508, 511

    _Clithon_, 327, _410_

    _Clydonites_, _398_

    _Clymenia_, _397_

    _Clypidella_, _406_

    _Cocculina_, _408_

    _Cochlicella acuta_, 278

    _Cochliolepas_, 77

    _Cochloceras_, _398_

    _Cochlodésma_, _459_

    _Cochlostyla_, 124, 278, =313=, 315, _441_

    Cockles, use of, 101, 118

    _Coecum_, 247, =260=, _417_, =418=

    _Coeliaxis_, 334, _442_;
      habitat, 49

    _Coelocentrum_, =353=, _442_

    _Coelospira_, _505_, 508

    Cold winter, effect on oysters, 112;
      on mussels, 116

    Collinge, W. E., on growth and burial of shells, 41

    _Collisella_, _405_

    _Collisellina_, _405_;
      radula, 227

    _Collonia_, _409_

    _Colobocephalus_, _430_

    Colour of arctic shells, 86

    _Colpodaspis_, _430_

    _Columbarium_, _426_

    _Columbella_, _423_;
      radula, =222=

    _Columbellaria_, _420_

    _Columbellina_, _420_

    _Columna_, 328, =330=, _443_

    _Cominella_, =16=, _424_

    Composition of shell, 252

    _Concha_, 463

    _Conchidium_, 497, =498=, 500, _505_;
      stratigraphical distribution, 507, 508, 511

    _Concholepas_, 267, _423_

    _Conidea_, _423_

    _Conocardium_, _455_

    _Conorbis_, _426_

    _Conus_, 247, 275, _426_;
      poisonous bite, 65;
      tooth, =66=;
      shell, =69=, =255=, 260;
      mimicked by _Strombus_, 69;
      prices given for rare, 121;
      spawn, =125=;
      radula, 218, =220=;
      operculum, =269=

    _Cookia_, _409_

    _Coptochilus_, 314, _414_

    _Coralliophaga_, _451_

    _Coralliophila_, 75, _423_

    Coralliophilidae, radula, 216

    _Corambe_, _434_

    _Corasia_, 311, 319–321

    _Corbicula_, 15, 288, 292 f., _453_

    _Corbis_, _452_

    _Corbula_, _456_

    _Corilla_, 303

    _Corona_, 27, _442_

    _Coronaria_, =297=

    _Coryda_, 346–351, _441_

    _Coryphella_, _432_

    _Cosmoceras_, _399_

    Cowry used as money, 96

    Coyote trapped by _Haliotis_, 57

    _Cranchia_, _391_

    Crania, 464, =467=, 468, 469, 471, 472, 473, 475, 476, =477=,
        _487_;
      distribution, 485;
      fossil, 493, 494, _504_;
      stratigraphical distribution, 506, 507, 508, 510

    Craniidae, _487_, 496, _504_, 508

    _Cranopsis_, =265=, _406_

    _Craspedochiton_, _403_

    _Craspedopoma_, 298, _414_

    _Craspedostoma_, _408_

    _Crassatella_, _451_

    _Cratena_, _432_

    Crawling of _Helix_, 45

    _Cremnoconchus_, 16, 302, _413_

    _Crenatula_, 75, _449_

    _Crenella_, _449_

    _Crenipecten_, _450_

    _Crepidula_, =248=, 257, =412=, _412_;
      parasitic, =78=

    _Crepipatella_, =248=, _412_

    _Creseis_, =436=, _436_;
      eyes, 186

    _Crimora_, _434_;
      radula, 229

    _Crioceras_, 247, =399=, _399_

    _Cristigibba_, 311, 319, 320, _441_

    _Crossostoma_, _408_

    _Crucibulum_, =248=, _412_

    _Cryptochiton_, 245, 371, 402, _404_

    _Cryptochorda_, _425_

    _Cryptoconchus_, _404_

    _Cryptophthalmus_, _430_

    _Cryptostracon_, 353, _441_

    Ctenidia, 151--_see_ Branchiae

    _Ctenopoma_, 346–351, _414_

    _Cucullaea_, 274, _448_

    _Cultellus_, _457_

    _Cuma_, _423_

    _Cumingia_, _453_

    _Cuspidaria_, _459_;
      branchiae, 168

    _Cuvierina_, =436=, _436_

    _Cyane_, _410_

    _Cyathopoma_, =247=, 268, 314, 338, _414_

    _Cyclas_, _453_;
      veliger, =132=;
      ova, 146;
      otocyst, =197=;
      _C. cornea_, thread-spinning, 29;
      distribution, 282

    _Cyclina_, _454_

    Cyclobranchiata, 156

    _Cyclocantha_, _409_

    _Cyclomorpha_, _414_

    _Cyclonassa_, _423_

    _Cyclonema_, _409_

    Cyclophoridae, origin, 21

    _Cyclophorus_, 302, =306=-319, 329–334, 344, 352–358, _414_;
      jaws, =212=;
      radula, =21=

    _Cyclostoma_, 328, 331–338, =414=, _414_;
      stomach, 239;
      vision, 184;
      osphradium, 195;
      nervous system, =205=;
      _C. elegans_, 287, 288

    Cyclostomatidae, origin, 21;
      radula, 224;
      gait, 199

    _Cyclostrema_, _408_

    _Cyclosurus_, =247=, 337, _414_

    _Cyclotopsis_, 338, _414_

    _Cyclotus_, 296, 319, 320, _414_

    _Cylichna_, =428=, _430_;
      radula, 215

    _Cylindrella_, =247=, =260=, 278, 343–355, =348=, _442_;
      monstrosity, 251, =252=

    Cylindrellidae, radula, 233, =234=

    _Cylindrites_, _430_

    _Cylindrobulla_, _430_

    _Cylindromitra_, _425_;
      radula, 222

    _Cymbium_, 255, 367, _425_;
      radula, =221=

    _Cymbulia_, _437_

    _Cymbuliopsis_, _437_

    _Cynodonta_, _424_

    _Cyphoma_, _419_

    _Cypraea_, =178=, _419_;
      prices given for rare, 122;
      mantle-lobes, 177, =178=;
      radula, =224=;
      shell, =255=, 260, =261=;
      _C. moneta_, 96

    _Cypraecassis_, _420_

    _Cypraedia_, _419_

    _Cypraeovula_, _419_

    _Cyprimeria_, _454_

    _Cyprina_, _451_

    _Cyrena_, 15, _453_;
      distribution, 285, 294

    _Cyrenella_, _453_

    _Cyrtia_, _505_;
      stratigraphical distribution, 507, 508

    _Cyrtoceras_, _394_

    _Cyrtodaria_, _457_

    _Cyrtodonta_, _452_

    _Cyrtolites_, _407_

    _Cyrtonotus_, _448_

    _Cyrtotoma_, _414_

    _Cysticopsis_, 346–351, _441_

    _Cystiscus_, _425_

    _Cystopelta_, 325, 326, _440_

    _Cytherea_, =454=, _454_


    _Dacrydium_, _449_

    _Daedalochila_, _441_

    Dall, W. H., quoted, 35;
      on branchiae, 164

    _Damayantia_, _440_

    _Daphnella_, _426_

    Darbyshire, R. D., on tenacity of life, 39

    _Dardania_, _415_

    Dart-sac, 142

    _Daudebardia_, =289=, 292 f., _440_

    _Davidsonia_, _505_, 508

    _Dawsonella_, _410_

    _Dayia_, _505_, 508

    Decapoda, _385_ f.

    Decollation, 260

    Deep-sea Mollusca, 374

    De Folin, experiment on _Cyclostoma_, 157

    _Deianira_, _410_

    Delage, experiments on otocysts, 197

    _Delphinula_, _409_

    Deltidium, 499

    _Dendronotus_, _433_;
      protective coloration, 72;
      habits, 51

    _Dentalium_, =6=, =444=, _445_;
      used as money, 97;
      veliger, =131=;
      radula, =228=

    _Dentellaria_, =350=-355, _441_;
      aperture, =63=

    Desert species, 25, 85

    _Deshayesia_, _411_

    _Desmoulea_, _423_

    Development of fertilised ovum, 130 f.

    _Dexiobranchaea_, _437_

    _Diadema_, _414_

    _Diala_, _415_

    _Dialeuca_, _441_

    _Diaphora_, 314

    _Diaphorostoma_, _412_

    _Diastema_, _418_

    _Diastoma_, _417_

    _Diaulula_, _434_

    _Dibaphus_, _425_

    Dibranchiata, _380_;
      eye, 183;
      nervous system, 207

    _Diceras_, 269, _455_

    _Didaena_, =12=, 297, _455_

    Differences of sex, 133

    _Dignomia_, 511

    Digonopora, 134, 144

    _Diloma_, _408_

    _Dimorphoptychia_, _410_

    _Dimya_, _450_

    _Dinobolus_, _504_, 508

    _Dinoplax_, _403_

    Ditocardia, 9, 170, _405_ f.

    _Diplodonta_, _452_

    _Diplommatina_, 302–327, _413_

    _Diplomphalus_, 322, 323, _440_

    _Diplopoma_, 346, 351, _414_

    _Dipsaccus_, _424_

    _Dipsas_, 307

    _Discina_, 464, 468, 471, 475, _487_;
      distribution, 485;
      fossil, 493, _504_;
      stratigraphical distribution, 506, 508

    Discinidae, _487_, 496, _504_, 508, 511

    _Discinisca_, _487_, 510;
      distribution, 485, 486

    _Discites_, _395_

    _Discodoris_, _434_

    _Discosorus_, _394_

    _Distortio_, 255--see _Persona_

    _Ditropis_, 312, 314, _414_

    Docoglossa, 227, _405_

    _Dolabella_, =428=, _431_

    _Dolabrifer_, _431_

    _Dolium_, _419_;
      acid secretion, 237

    _Donax_, 269, 446, _453_

    _Dondersia_, _404_

    _Dorcasia_, 333, _441_

    _Doridium_, _430_

    _Doridunculus_, _434_;
      radula, 229

    _Doriopsis_, _434_

    _Doris_, breathing organs, =159=;
      radula, 230

    _Dorsanum_, _423_

    _Dosidicus_, _390_

    _Dosinia_, _454_

    _Doto_, _433_;
      protective coloration, 71

    _Dreissensia_, =14=, 123, _452_;
      hibernation, 26;
      singular habitat, 48;
      veliger, =132=, 146;
      eyes, 192

    _Dreissensiomya_, _452_

    _Drepania_, _434_

    _Drillia_, _426_

    _Drymaeus_, 356, _442_

    _Dryptus_, 356, _441_

    _Durgella_, 301, 304, _440_

    Dwarf varieties, 88

    _Dybowskia_, 290


    _Eastonia_, _454_

    _Eburna_, =267=, _424_;
      radula, =220=

    Ecardines, 466;
      muscles, 476;
      fossil, 493;
      families, _487_, _503_, 508

    _Eccyliomphalus_, _413_

    _Echinospira_, 133

    _Edentulina_, 338

    Egg-laying of _Arion_, =42= f.;
      of Mollusca generally, 123

    _Eglisia_, _411_

    Eider-duck, shells used by, 102

    _Elaea_, 322, _440_

    _Elasmoneura_, _411_

    _Eledone_, =385=, _385_;
      radula, 236

    _Elizia_, _456_

    _Elysia_, _432_;
      protective coloration, 73;
      breathing, 152;
      radula, 217, =230=, =432=

    _Emarginula_, =265=, _406_

    _Embletonia_, 429

    _Emmericia_, _415_

    _Ena_, 296, _442_

    _Enaeta_, _425_

    _Endoceras_, _394_

    _Endodonta_, 325, 334, _441_

    _Engina_, _424_

    _Enida_, _408_

    _Ennea_, 298, 302, 306, 309, 314, 316, 328–337, =440=, _440_;
      habits, 54;
      _E. bicolor_, 279

    _Enoplochiton_, =403=, _403_

    _Enoploteuthis_, _391_

    _Ensis_, _457_

    _Entocolax_, 77, 79, 152

    _Entoconcha_, 77, 79, 152, 216

    _Entovalva_, 77, 82

    _Ephippodonta_, _453_;
      commensal, =81=

    _Epidromus_, _420_

    Epiphragm, 26, 27 f.

    Epipodia, 427

    _Erato_, _419_

    _Eremophila_, 294

    _Ergaea_, =248=, _412_

    _Erinna_, 327, _439_

    Erosion, 276

    _Ervilia_, _454_

    _Erycina_, _453_

    Escargotières, 119

    _Estria_, 329, _440_

    Estuarine species, 14

    _Ethalia_, _409_

    _Eucalodium_, 260, 353, _442_

    _Euchelus_, _408_

    _Euchrysallis_, _420_

    _Eudioptus_, _442_

    _Eudoxochiton_, _403_

    _Euhadra_, 316, 318, _441_

    Eulamellibranchiata, _451_;
      gill, =166=, 167

    _Eulima_, _422_;
      parasitic, 77, =79=

    _Eulimella_, 250, _422_

    _Eulota_, 296, _441_

    _Euomphalus_, 247, _413_

    _Euplecta_, _440_

    _Eupleura_, _423_

    _Euplocamus_, _434_

    _Eurybia_, _438_

    _Eurycampta_, 346–351

    _Eurycratera_, 349, 351, _441_

    _Eurystoma_, 304

    _Eurytus_, _442_

    _Euthria_, _424_

    Euthyneura, 203

    _Eutrochatella_, 347–351, =348=, _410_

    Exploring expeditions, 362

    Eye in Mollusca, 181 f.


    _Facelina_, _432_

    _Fasciolaria_, _424_;
      radula, =221=

    _Fastigiella_, _416_

    _Favorinus_, _432_

    _Fenella_, _415_

    Fertilised ovum, development, 130 f.

    _Ferussacia_, 291, 293, 297 f., _442_

    Fiji islanders, use of shells, 98

    Filibranchiata, _448_;
      gill, =166=

    _Fiona_, _432_;
      radula, 217

    _Firoloida_, _421_

    Fischer and Bouvier, on breathing of _Ampullaria_, 158

    _Fischeria_, 15, 328, _453_

    Fish devour Mollusca, 59

    _Fissurella_, =265=, _406_;
      breathing organs, =153=;
      apical hole, 156;
      nervous system, 204;
      radula, 227;
      growth, =261=

    _Fissurellidaea_, _406_

    _Fissuridea_, _406_

    _Fissurisepta_, _406_

    _Fistulana_, 262, _457_

    _Flabellina_, _432_

    _Fluminicola_, _415_

    _Folinia_, _415_

    Food of Mollusca, 30 f.;
      Mollusca as food, 102 f.

    Foot, 198;
      in classification, 5

    Forel, on deep-water _Limnaea_, 162

    Formation of shell, 255

    _Fortisia_, _429_

    _Fossarina_, _413_

    _Fossarulus_, 302, _415_

    _Fossarus_, _413_

    Fourth orifice in mantle, 174

    Fresh-water species living in sea, 12;
      frozen hard, 24

    Frogs and toads devour Mollusca, 58

    _Fruticicola_, 285, 290, 316, 318, _441_

    _Fruticocampylaea_, 296

    _Fryeria_, _434_

    _Fulgur_, =249=, _424_

    _Fusispira_, _420_

    _Fusus_, 262, _424_


    _Gadinia_, =152=, _431_;
      breathing, 18, 151;
      classification, 19;
      radula, 217, =230=

    Gain, W. A., quoted, 32, 33, 39;
      on taste of Mollusca, 179

    _Galatea_, 15, 328, 336, _453_

    _Galeomma_, 175, _453_

    _Galerus_, =248=, _412_;
      egg-capsules, 125

    Garstang, W., on protective and warning coloration, 73

    Gaskoin, on tenacity of life, 38;
      on egg-laying, 42

    Gassies, on hybrid union in snails, 130

    Gasteropoda, on classification, 8, 11, 400 f.

    _Gastrana_, _453_

    _Gastrochaena_, _457_;
      habits, 64

    _Gastrodonta_, _440_

    _Gastropteron_, 245, _430_

    _Gaza_, 376, _408_

    _Gena_, 246, _408_

    _Genea_, _424_

    _Genotia_, _426_

    _Geomalacus_, =160=, 288, 291, _441_;
      protective coloration, 70

    _Geomelania_, 16, 348, 351, _414_

    _Georgia_, 331, _414_

    _Georissa_, 318, _410_

    _Geostilbia_, 338, _442_

    _Gerontia_, _441_

    _Gerstfeldtia_, 290

    _Gibbula_, _408_

    _Gibbus_, 328–=338=, =440=, _440_

    _Gillia_, _415_

    Gills--_see_ Branchiae

    _Girasia_, 301, 304, _440_

    _Glandina_, =54=, =178=, 278, 292 f., 339–355, _440_;
      radula, 231, =232=;
      habits, 53

    Glands, germ, 134, 140;
      nidamental, 136

    _Glassia_, 501, _505_

    _Glaucomya_, 320, _454_

    _Glaucus_, 429, _432_

    _Gleba_, _437_

    _Glessula_, 301, 309, 310, 333, _442_

    Glochidium, =147=

    _Glomus_, _448_

    _Glossoceras_, _394_

    Glossophora, 7

    _Glottidia_, distribution, 485, _487_

    _Glycimeris_, _457_

    _Glyphis_, _406_

    _Glyptostoma_, 341, _441_

    _Gomphoceras_, _394_, =395=

    _Gonatus_, _391_

    _Goniatites_, _397_, =398=

    _Goniobasis_, 341, _417_

    _Goniodoris_, _434_;
      protective coloration, 73;
      radula, 229

    _Goniomya_, _458_

    _Gonostoma_, 291, 316, _441_

    _Goniostomus_, _442_

    _Grammysia_, _459_

    _Grateloupia_, _454_

    Great _Eastern_ and mussels, 116

    Greenhouses, slugs in, 35

    Green oysters, 108

    _Gresslya_, _458_

    Growth of shell, 40, 257

    _Guesteria_, _440_

    _Guildfordia_, _409_

    _Guivillea_, 186, 376, _425_

    Gulls and Mollusca, 56

    _Gundlachia_, 19, 325, 345, 352, 359, _439_

    Gymnoglossa, 216, 225, _422_

    Gymnosomata, _437_

    _Gyroceras_, 247, _395_

    _Gyrotoma_, _417_


    _Hadra_, 306, 315, 319–325, =322=, _441_

    _Hadriania_, _423_

    Haemoglobin, 171

    _Hainesia_, 336, _414_

    _Halia_, 366, _426_

    _Haliotinella_, _431_

    _Haliotis_, =266=, _407_;
      and coyote, 57;
      holes of, 156;
      osphradium, 195;
      epipodium, 199;
      nervous system, 204;
      radula, 215, 226

    _Halopsyche_, 159, =438=, _438_

    _Haminea_, =428=, _430_;
      protective coloration, 73

    _Hamites_, _399_

    _Hamulina_, _399_

    _Hanleyia_, _403_

    _Hapalus_, 331, _442_

    _Harpa_, radula, _425_, 216, 221;
      self-mutilation, 45

    _Harpagodes_, _418_

    _Harpoceras_, _399_

    _Harvella_, _454_

    Hatching of eggs, 43

    Hazay, on duration of life, 39;
      on variation in _Limnaea_, 93

    Hearing powers of Mollusca, 196

    Heart, in classification, 9;
      action during hibernation, 26;
      and branchiae, =169=

    Hectocotylus arm, 137 f.

    _Helcion_, _405_;
      protective coloration, 69

    _Helcioniscus_, _405_

    Hele, F. M., on _Hyalinia_, 33;
      on _Stenogyra_, 34

    _Helicarion_, 309, 316, 325, 332, _440_;
      radula, 232;
      habits, 45, 67

    Helicidae, radula, 232, =234=

    _Helicina_, 305, 306, 316–327, 338–358, _410_;
      origin, =21=;
      exterminated by cold, 24

    _Helicophanta_, 335, =336=, =441=, _441_

    _Heligmus_, _449_

    _Helix_, _441_;
      toothed aperture, 63;
      protective coloration, 70;
      variation, 87;
      carbonic acid, 163;
      eye, =181=, =183=;
      food, 179;
      smell, 194;
      jaw, =211=;
      distribution, 285;
      tenacity of life, 37;
      breeding, 129

    _Helix alternata_, =340=;
      _angulata_, =350=;
      _aperta_, 38, 39, 51, =293=;
      _arbustorum_, bathing, 23;
      _caperata_, variation, 89;
      _cereolus_, =340=;
      _cicatricosa_, =316=;
      _crenilabris_, 45;
      _delphinuloides_, =297=;
      _desertorum_, 37, 38, 70, 294;
      _fidelis_, =341=;
      _haemastoma_, habits, 70;
      _harpa_, 287;
      _hortensis_, =10=, 279;
        pulsations, 26;
        epiphragm, 28;
        rock-boring, 49;
        dart, =143=;
      _imperator_, =347=;
        habits, 45;
      _laciniosa_, =297=;
      _lactea_, 25, 38, 42, 279;
      _lima_, =350=;
      _muscarum_, =347=;
      _nemoralis_, 38, =180=;
      _niciensis_, =292=;
      _nux denticulata_, =350=;
      _palliata_, =340=;
      _pisana_, 25;
        habits, 33;
      _pomatia_, 25, 34, 40;
        eye, =181=;
      _pomum_, =322=;
      _pulchella_, 279;
      _richmondiana_, =322=;
      _rosacea_, =259=;
      _rostrata_, =347=;
      _rota_, =314=;
      _rufescens_, pulsations, 26;
      _similaris_, 279;
      _souverbiana_, =336=, =441=;
      _strigata_, =293=;
      _tristis_, habits, 49;
      _turricula_, =297=;
      _Veatchii_, 38;
      _Waltoni_, =304=;
      _Wollastoni_, =297=;
      _zonata_, =293=

    _Helix aspersa_, homing, 35;
      smell, 36;
      duration of life, 39;
      growth, 40;
      strength, 45;
      boring rock, 50;
      variation, 87, 89;
      eaten, 119;
      hybrid union, 130;
      generative organs, 140 f., =141=;
      dart-sac, =143=;
      pulmonary chamber, 160;
      radula, 217;
      alimentary canal, =237=;
      monstrosities, 251, =252=;
      growth, 258;
      distribution, 279, 289

    _Hemiarthrum_, _403_

    _Hemicardium_, _455_

    _Hemidonax_, _453_

    _Hemifusus_, _424_

    _Hemipecten_, _450_

    _Hemiplecta_, 310, 316, 319, 321, _440_

    _Hemisepius_, _389_

    _Hemisinus_, 357, _417_

    _Hemitoma_, =265=

    _Hemitrichia_, 314

    _Hemitrochus_, 346–351, _441_

    _Hemphillia_, 245, 341, _441_

    _Hercoceras_, _395_

    Herdman, Prof. W. A., on cerata of Nudibranchs, 71 f.;
      experiments on taste of Nudibranchs, 72;
      on _Littorina rudis_, 151 n.

    _Hermaea_, _432_;
      protective coloration, 73

    Hermaphrodite Mollusca, 134, 140, 145

    Hermit-crabs, shells used by, 102

    _Hero_, _432_

    _Heterocardia_, _454_

    _Heterodiceras_, _455_

    Heteropoda, 9, _420_ f.;
      radula, 228;
      foot, 200

    _Heudeia_, 316, _410_

    _Hexabranchus_, _434_

    Hibernation, 25, 163

    High altitudes, Mollusca living at, 24

    _Himella_, 15

    _Hindsia_, _424_

    _Hindsiella_, _453_

    Hinge area, 493, 498

    Hinge, in bivalves, 272

    _Hinnites_, 257, _450_

    _Hipponyx_, 248, _412_

    _Hippopus_, _455_

    _Hippurites_, =455=, _456_

    _Histiopsis_, _391_

    _Histioteuthis_, _391_

    _Holcostoma_, _417_

    _Holohepatica_, _433_

    _Holopella_, _411_

    _Holospira_, 339, 353, _442_

    Holostomata, 156

    _Homalogyra_, _413_;
      radula, 223

    _Homalonyx_, 245, =343=-358, _443_

    Homing powers of Mollusca, 34

    _Homorus_, 330–337, _443_

    _Hoplites_, _399_

    _Hoplopteron_, _422_

    _Horea_, 332

    _Horiostoma_, _409_

    Hot springs, Mollusca living in, 25

    _Huronia_, _394_

    _Hyalaea_, =10=, _436_

    _Hyalimax_, 245, 305, 306, 338, _443_

    Hyaline stylet, 240

    _Hyalinia_, _440_;
      pulsations, 26;
      food, 33;
      smell, 194;
      dart, =143=;
      radula, 232, =234=;
      distribution, 287 f., 318, 340–357;
      _H. alliaria_, 279;
      smell, 194;
      _cellaria_, 279;
      _Draparnaldi_, 33

    _Hyalocylix_, _437_

    _Hyalosagda_, 352

    _Hybocystis_, =305=, 309, _414_

    Hybridism, 129

    _Hydatina_, _430_;
      radula, =231=

    _Hydrobia_, 325, 332, _415_;
      _H. ulvae_, egg-laying, 128

    _Hydrocena_, 298, _410_;
      radula, 226

    Hymenoptera build in dead shells, 102

    _Hypobranchaea_, _434_;
      radula, 230

    _Hypotrema_, _448_

    _Hypselostoma_, 248, =302=, 305, 314, _442_

    _Hyria_, 344, _452_

    _Hystricella_, 297


    _Ianthina_, 360, =126=, _411_;
      egg-capsules, 125;
      eyes, 186;
      radula, =224=

    _Iapetella_, _385_

    _Iberus_, 285–=293=, =297=, _441_

    _Ichthyosarcolites_, _456_

    _Idalia_, =179=, 429, _434_;
      radula, 229, 230

    _Idas_, _449_

    _Idiosepion_, _389_

    _Illex_, _390_

    _Imbricaria_, _425_;
      radula, =221=

    _Imperator_, _409_

    Indians of America, use of shells, 100

    _Infundibulum_, _408_

    _Inioteuthis_, _389_

    Ink-sac, =241=

    _Inoceramus_, _449_

    Insects eaten by Mollusca, 32

    _Insularia_, 319, 320

    Intestine, 241

    _Io_, =16=, 340, _417_

    _Iopas_, _423_

    _Iphigenia_, 15, _453_

    _Iravadia_, 305, _415_

    _Iridina_, 294

    _Irus_, 297

    _Isanda_, _409_

    _Ischnochiton_, _403_

    _Isidora_, 298, 320–327, 333, 336, 359, _439_

    _Ismenia_, _404_

    _Isocardia_, 269, =451=, _451_

    _Isodonta_, _453_

    _Isomeria_, 343, 356, _441_

    _Issa_, _434_


    _Jamaicia_, _414_

    _Janella_, =161=, _443_;
      pulmonary orifice, =161=

    Janellidae, radula, 234;
      distribution, 321–326

    _Janus_, _432_

    _Japonia_, 318

    Jaws, 210

    _Jeanerettia_, 346–351, _441_

    Jeffreys, Dr., on _Limnaea_, 34;
      on _Neptunea_, 193

    _Jeffreysia_, _415_;
      radula, 223

    _Jorunna_, protective coloration, 73

    _Jouannettia_, _457_

    _Jullienia_, 307, _415_

    _Jumala_, _424_


    _Kaliella_, 301, 304, 310, 314–317, 335, _440_

    _Kalinga_, _434_

    Kashmir, land Mollusca, 280

    _Katherina_, _403_

    _Kelletia_, _424_

    _Kellia_, _453_

    _Kellyella_, _452_

    Kidneys, 242

    King, R. L., on smell in bivalves, 195

    _Kingena_, _506_, 508

    Kitchen-middens, 104

    _Koninckella_, _505_;
      stratigraphical distribution, 507, 508

    _Koninckina_, _505_;
      stratigraphical distribution, 507, 508

    Koninckinidae, 501, _505_, 508

    _Kutorgina_, _504_;
      stratigraphical distribution, 506, 508;
      embryonic shell, 509


    Labial palps, 210

    _Labyrinthus_, 342, 353–357, _441_;
      aperture, =63=

    Lacaze-Duthiers on _Testacella_, 52 f.;
      on smell in _Helix_, 194

    _Lacuna_, _413_

    _Lacunopsis_, 332

    _Lagena_, _424_

    _Lagochilus_, 309, 316–319, _414_

    _Lamellaria_, 245, _411_;
      habits and protective coloration, 74;
      parasitic, 78;
      radula, 223

    _Lamellidoris_, _434_;
      radula, 229, 230, =231=

    _Lampania_, _417_

    Land Mollusca, origin, 11 f.

    _Lanistes_, 249, 294, 328, 331, _416_

    Lankester, Prof. E. Ray, on shell-gland, 132;
      on haemoglobin, 171

    _Lantzia_, 278, 338, _439_

    _Laoma_, _441_

    _Larina_, 302, _417_

    Larvae of Pelecypoda, 7;
      of insects resembling Mollusca, 67 f.

    _Lasaea_, _453_

    _Latia_, 19, 326, _439_

    _Latiaxis_, _423_

    _Latirus_, _424_

    Latter, O. H., on _Glochidium_, 147

    Layard, E. L., on self-burying Mollusca, 41;
      on sudden appearance of _Stenogyra_, 47;
      on _Coeliaxis_, 49;
      on _Rhytida_ and _Aerope_, 54

    _Leda_, _447_

    _Leia_, 348–351, _442_

    _Leila_, 344, _452_

    _Leonia_, _414_

    _Lepeta_, _405_

    _Lepetella_, _405_

    Lepetidae, radula, 227

    _Lepidomenia_, _404_;
      radula, 229

    _Leptachatina_, 327

    _Leptaena_, 500, 501, 502, =503=, _505_;
      stratigraphical distribution, 507, 508

    _Leptaxis_, _441_

    _Leptinaria_, 357, 358, _442_

    _Leptochiton_, _403_

    _Leptoconchus_, 75, _423_

    _Leptoloma_, 348, 351

    _Lepton_, _453_;
      parasitic, 77;
      commensal, 80;
      mantle-edge, 175, 178

    _Leptoplax_, _403_

    _Leptopoma_, 316, 319, 338, _414_

    _Leptoteuthis_, _390_

    _Leptothyra_, _409_

    _Leroya_, 331

    _Leucochila_, _442_

    _Leucochloridium_, =61=

    _Leucochroa_, =292=, 295, _441_

    _Leuconia_, _439_

    _Leucotaenia_, 335, 359, _441_

    _Leucozonia_, 64, =424=, _424_

    _Levantina_, 295

    _Libania_, 295

    _Libera_, 327, _441_;
      egg-laying, 128

    _Libitina_, _451_

    _Licina_, _414_

    Life, duration of, in snails, 39

    Ligament, 271

    _Liguus_, 349, 351, _442_

    _Lima_, 178, =179=, _450_;
      habits, 63

    Limacidae, radula, 232

    _Limacina_, 59, 249, =436=, _436_

    _Limapontia_, 429, _432_;
      breathing, 152

    _Limax_, 245, _440_;
      food, 31, 179;
      variation, 86;
      pulmonary orifice, 160;
      shell, 175;
      jaw, =211=;
      radula, 217;
      distribution, 285, 324;
      _L. agrestis_, eats May flies, 31;
      _arborum_, slime, 30;
      food, 31;
      _flavus_, food, 33, 36;
      habits, 35, 36;
      _gagates_, 279, 358;
      _maximus_, =32=, =161=;
      eats raw beef, 32;
      cannibalism, 32;
      sexual union, 128;
      smell, 193 f.

    _Limea_, _450_

    _Limicolaria_, 329–332, _443_

    _Limnaea_, _439_;
      self-impregnation, 44;
      development and variation, 84, 92, 93;
      size affected by volume of water, 94;
      eggs, 124;
      sexual union, 134;
      jaw, 211;
      radula, 217, =235=;
      _L. auricularia_, 24;
      _glutinosa_, sudden appearance, 46;
      _Hookeri_, 25;
      _involuta_, 82, 278, 287;
      _peregra_, =10=, =180=;
      burial, 27;
      food, 34, 37;
      variation, =85=;
      distribution, 282;
      _palustris_, distribution, 282;
      _stagnalis_, food, 34, 37;
      variation, =85=, =95=;
      circum-oral lobes, 131;
      generative organs, =414=;
      breathing, 161;
      nervous system, =204=;
      distribution, 282;
      _truncatula_, parasite, 61;
      distribution, 282

    _Limnocardium_, _455_

    _Limnotrochus_, 332, _415_

    _Limopsis_, _448_

    Limpet-shaped shells, 244

    Limpets as food for birds, 56;
      rats, 57;
      birds and rats caught by, 57;
      as bait, 118

    _Lingula_, 464, 467, 468, 471, =472=, 473, 475, 477, 478, _487_;
      habits, =483=, 484;
      distribution, 485;
      fossil, 493, =494=, _503_;
      stratigraphical distribution, 506, 508, 510, 511

    _Lingulella_, 493, _503_;
      stratigraphical distribution, 506, 508, 511

    _Lingulepis_, _503_, 511

    Lingulidae, 485, 487, 496, _503_, 508

    _Linnarssonia_, _504_;
      stratigraphical distribution, 506, 508

    _Lintricula_, _426_

    _Liobaikalia_, 290

    _Liomesus_, _424_

    _Lioplax_, 340, _416_

    _Liostoma_, _424_

    _Liostracus_, _442_

    _Liotia_, _408_

    _Liparus_, 324, 359, _441_

    _Lissoceras_, _399_

    _Lithasia_, 340, _417_

    _Lithidion_, _414_

    _Lithocardium_, _455_

    _Lithodomus_, _449_

    _Lithoglyphus_, 294, 296, 297, _415_

    _Lithopoma_, _409_

    _Lithotis_, 302, _443_

    _Litiopa_, 30, 361, _415_

    _Littorina_, _413_;
      living out of water, 20;
      radula, =20=, 215;
      habits, 50;
      protective coloration, 69;
      egg-laying, 126;
      hybrid union, 130;
      monstrosity, =251=, =252=;
      operculum, =269=;
      erosion, 276;
      _L. littorea_, in America, 374;
      _obtusata_, generative organs, =135=;
      _rudis_, 150;
      Prof. Herdman’s experiments on, 151 n.

    _Littorinida_, _415_

    _Lituites_, 247, _395_

    Liver, 239;
      liver-fluke, 61

    _Livinhacea_, 333, 359, _441_

    _Livona_, _408_;
      radula, 226;
      operculum, =268=

    Lloyd, W. A., on _Nassa_, 193

    _Lobiger_, _432_

    _Lobites_, _397_

    _Loligo_, 378–_389_;
      glands, 136;
      modified arm, 139;
      eye, =183=;
      radula, 236;
      club, =381=;
      _L. punctata_, egg-laying, 127;
      _vulgaris_, larva, 133

    _Loligopsis_, _391_

    _Loliguncula_, _390_

    _Loliolus_, _390_

    _Lomanotus_, _433_

    _Lophocercus_, _432_

    _Lorica_, _403_

    Lowe, E. J., on growth of shell, 40

    _Loxonema_, _417_

    _Lucapina_, _406_

    _Lucapinella_, _406_

    _Lucerna_, _441_

    _Lucidella_, 348–351, _410_

    _Lucina_, =270=, _452_

    _Lucinopsis_, _454_

    Lung, 151, 160

    _Lunulicardium_, _455_

    _Lutetia_, _452_

    _Lutraria_, 446, _456_

    _Lychnus_, _442_

    _Lyonsia_, _458_

    _Lyonsiella_, _458_;
      branchiae, 168

    _Lyra_, stratigraphical distribution, 507

    _Lyria_, _425_

    _Lyrodesma_, _447_

    _Lysinoe_, _441_

    _Lytoceras_, _398_


    _Maackia_, 290

    _Macgillivrayia_, 133

    _Machomya_, _458_

    _Maclurea_, _410_

    _Macroceramus_, 343–353, _442_

    _Macroceras_, _440_

    _Macrochilus_, _417_

    _Macrochlamys_, 296, 299, 301 f., 310, 316–322, _440_

    _Macrocyclis_, 358, =359=, _442_

    _Macron_, _424_

    _Macroön_, _441_

    _Macroscaphites_, 247, =399=, _399_

    _Macroschisma_, =265=, _406_

    _Mactra_, 271, 446, _454_

    _Macularia_, 285, 291, =292= f., _441_

    _Magas_, _506_;
      stratigraphical distribution, 507, 508

    _Magellania_, 500

    _Magilus_, =75=, _423_

    _Mainwaringia_, 302

    _Malaptera_, _418_

    _Malea_, _419_

    _Malletia_, _447_

    _Malleus_, _449_

    _Mangilia_, _426_

    Mantle, 172 f., =173=;
      lobes of, 177

    _Margarita_, _408_;
      radula, =225=

    _Marginella_, _425_;
      radula, 221

    _Mariaella_, 314, 338, _440_

    _Marionia_, _433_

    _Marmorostoma_, _409_

    Marrat, F. P., views on variation, 82

    _Marsenia_, 133

    _Marsenina_, _411_

    _Martesia_, 305, _457_

    _Mastigoteuthis_, _390_

    _Mastus_, 296, _442_

    _Matheronia_, _455_

    _Mathilda_, 250, _417_

    _Maugeria_, =403=

    _Mazzalina_, _424_

    _Megalatractus_, _424_

    Megalodontidae, _451_

    _Megalomastoma_, 344, _414_

    _Megalomphalus_, _416_

    _Megaspira_, 358, _442_

    _Megatebennus_, _406_

    _Megerlia_, distribution, 486, _487_

    _Meladomus_, 249, 328, 331, _416_

    _Melampus_, =18=, 199, 250, =439=, _439_

    _Melanatria_, 336

    _Melania_, =276=, =417=, _417_;
      distribution, 285, 292 f., 316 f., 324, 336

    _Melaniella_, _442_

    Melaniidae, origin, 17

    Melanism in Mollusca, 85

    _Melanopsis_, _417_;
      distribution, 285, 291, 292 f., 323, 326

    _Melantho_, 340, _416_

    _Melapium_, _424_

    _Meleagrina_, _449_

    _Melia_, 348

    _Melibe_, _432_

    _Melongena_, _424_;
      radula, =220=;
      stomach, =238=

    _Merica_, _426_

    _Merista_, _505_, 508

    _Meroe_, _454_

    _Merope_, 327

    _Mesalia_, _417_

    _Mesembrinus_, 356, _442_

    _Mesodesma_, _454_

    _Mesodon_, =340=, _441_

    _Mesomphix_, 340, _440_

    _Mesorhytis_, 377

    _Meta_, _423_

    _Metula_, _424_

    _Meyeria_, _424_

    _Miamira_, _434_

    _Microcystis_, 323, 324, 327, 338, _440_

    _Microgaza_, _408_

    _Micromelania_, 12, 297

    _Microphysa_, protective habits, 70

    _Microplax_, _403_

    _Micropyrgus_, _415_

    _Microvoluta_, _425_

    _Middendorffia_, _403_

    _Milneria_, _451_

    Mimicry, 66

    _Minolia_, _408_

    _Mitra_, _425_;
      radula, 221

    _Mitrella_, _423_

    _Mitreola_, _425_

    _Mitrularia_, =248=, _412_

    _Modiola_, 446, _449_;
      habits, 64;
      genital orifice, 242

    _Modiolarca_, _449_

    _Modiolaria_, _449_;
      habits, 78

    _Modiolopsis_, _452_

    _Modulus_, _417_

    _Monilia_, _408_

    Monkey devouring oysters, 59

    _Monoceros_, _423_

    _Monocondylaea_, _452_

    _Monodacna_, 12, 297, _455_

    _Monodonta_, _408_, =408=;
      tentaculae, =178=

    _Monogonopora_, 134, 140

    _Monomerella_, 496, _504_

    _Monopleura_, _456_

    _Monotis_, _449_

    Monotocardia, 9, 170, _411_

    Monstrosities, 250

    _Montacuta_, _452_;
      _M. ferruginosa_, commensal, 80;
      _substriata_, parasitic, 77

    _Mopalia_, _403_

    Moquin-Tandon, on breathing of Limnaeidae, 162;
      on smell, 193 f.

    _Moreletia_, _440_

    _Morio_, _420_

    _Mormus_, 356, _442_

    Moseley, H. N., on eyes of _Chiton_, 187 f.

    _Moussonia_, 327

    Mouth, 209

    _Mucronalia_, _422_

    Mucus, use of, 63

    _Mulinia_, =272=

    _Mülleria_, 344, _452_

    _Mumiola_, _422_

    _Murchisonia_, 265, _407_

    _Murchisoniella_, _422_

    _Murex_, _423_;
      attacks _Arca_, 60;
      use of spines, 64;
      egg-capsules, 124;
      eye, =182=;
      radula, =220=;
      shell, =256=

    Musical sounds, 50

    Mussels, cultivation of, 115;
      as bait, 116;
      poisonous, 117;
      on _Great Eastern_, 116

    _Mutela_, 294, 328, 331, 336, _452_

    _Mutyca_, _425_

    _Mya_, 271, 275, =446=, _456_;
      stylet, 240;
      _M. arenaria_, variation, 84

    Myacea, _456_

    _Myalina_, _449_

    _Mycetopus_, 307, 316, 344, _452_

    _Myochama_, =458=

    _Myodora_, _458_

    _Myophoria_, _448_

    Myopsidae, _389_

    _Myrina_, _449_

    _Myristica_, _424_

    Mytilacea, _448_

    _Mytilimeria_, _458_

    _Mytilops_, _452_

    _Mytilopsis_, 14

    _Mytilus_, 258, _449_;
      gill filaments, =166=, 285;
      _M. edulis_, =14=, =165=;
      attached to crabs, 48, 78;
      pierced by _Purpura_, =60=;
      Bideford Bridge and, 117;
      rate of growth, 258;
      stylet, 240

    _Myxostoma_, _414_


    _Nacella_, _405_

    _Naiadina_, _449_

    _Nanina_, 278, 300 f., 335, _440_;
      radula, 217, 232

    _Napaeus_, 296–299, 316, _442_

    _Naranio_, _454_

    _Narica_, _412_

    _Nassa_, _423_;
      egg-capsules, =126=;
      sense of smell, 193

    _Nassodonta_, _423_

    _Nassopsis_, 332

    _Natica_, =246=, 263, _411_;
      spawn, =126=;
      operculum, =268=

    _Naticopsis_, _409_

    ‘Native’ oysters, 106

    _Nausitora_, 15

    Nautiloidea, _393_

    _Nautilus_, 254, =392=, _395_;
      modified arms, 140;
      eye, =183=;
      nervous system, 206;
      radula, 236;
      kidneys, 242

    _Navicella_, 267, 268, 324, 327, _410_;
      origin, =17=

    _Navicula_, =358=, _442_

    _Navicula_ (Diatom), cause of greening in oysters, 108

    _Nectoteuthis_, _389_

    _Neda_, _431_

    _Nematurella_, 12, 297

    _Nembrotha_, _434_

    _Neobolus_, _504_

    _Neobuccinum_, _424_

    _Neocyclotus_, 357, 358

    _Neomenia_, =8=, 133, 216, 228, =404=, _404_;
      breathing organs, =154=;
      nervous system, =203=

    _Neothauma_, 332

    Neotremata, 511

    _Neptunea_, 252, 262, _423_;
      egg-capsules, =126=;
      capture, 193;
      monstrosity, =251=

    _Nerinea_, _417_

    _Nerita_, =17=, _410_;
      _N. polita_ used as money, 97

    Neritidae, 260, _410_;
      radula, =226=

    _Neritina_, =256=, _410_;
      origin, 16, =17=, =21=;
      egg-laying, 128;
      eye, 181;
      distribution, 285, 291 f., 324, 327;
      _N. fluviatilis_, habitat, 12, 25

    _Neritoma_, _410_

    _Neritopsis_, _409_;
      radula, 226;
      operculum, =269=

    Nervous system, 201 f.

    _Nesiotis_, 357, _442_

    New Zealanders, use of shells, 99

    _Nicida_, _413_

    _Ninella_, _409_

    _Niphonia_, _408_

    _Niso_, _422_

    _Nitidella_, _423_

    _Nodulus_, _415_

    _Notarchus_, _431_

    _Nothus_, 358, _442_

    _Notobranchaea_, _438_

    _Notodoris_, _434_

    _Notoplax_, _403_

    _Novaculina_, 305

    _Nucula_, 254, 269, =273=, _447_

    Nuculidae, otocyst, 197;
      foot, 201

    _Nuculina_, _448_

    Nudibranchiata, _432_;
      defined, 10;
      protective and warning colours, 71 f.;
      breathing organs, 159

    _Nummulina_, 295

    _Nuttallina_, _403_


    _Obba_, 311, 315, _441_

    _Obbina_, 306, 311, 312, =314=, 319

    _Obeliscus_, _442_

    _Obolella_, 496, _504_;
      stratigraphical distribution, 506, 508

    Obolidae, 496, _504_, 508

    _Obolus_, _504_, 508;
      embryonic shell, 509

    _Ocinebra_, _423_

    Octopodidae, hectocotylised arm, 137, 139, =140=

    _Octopus_, =379=-_386_;
      egg-capsules, =127=;
      vision, 184;
      radula, =236=;
      crop, 238

    _Ocythoe_, _384_;
      hectocotylus, =138=

    _Odontomaria_, _407_

    _Odontostomus_, =358=, =442=

    _Odostomia_, 250, _422_;
      parasitic, 78

    Oesophagus, 237

    _Ohola_, _434_

    Oigopsidae, _390_

    _Oldhamina_, _506_, 508

    _Oleacina_, habits, 55

    _Oliva_, =199=, =255=, 275, =425=, _426_

    _Olivancillaria_, _426_

    _Olivella_, 260, 267, _426_;
      _O. biplicata_ as money, 97

    _Olivia_, _408_

    _Omalaxis_, _413_

    _Omalonyx_, habitat, 23

    _Ommastrephes_, =6=, 378, _390_

    Ommatophores, 180, 187

    _Omphalotropis_, 306, 309, 316, 324, 327, 338, _414_

    _Onchidiella_, _443_

    Onchidiidae, 245;
      radula, 234;
      anus, 241

    _Onchidiopsis_, _411_

    _Onchidium_, _443_;
      breathing, 163;
      eyes, 187

    _Onchidoris_, radula, 230

    _Oniscia_, _420_

    _Onoba_, _415_

    _Onychia_, _390_

    _Onychoteuthis_, _390_;
      club, =386=

    _Oocorys_, _420_

    _Oopelta_, 329, _440_

    _Opeas_, _442_

    Operculum, 267 f.

    _Ophidioceras_, 247, _395_

    _Ophileta_, _413_

    _Opis_, _451_

    Opisthobranchiata, _427_;
      defined, 9;
      warning, etc., colours, 71 f.;
      generative organs, 144;
      breathing organs, 158;
      organs of touch, 178;
      parapodia, 199;
      nervous system, 203;
      radula, 229

    _Opisthoporus_, =266=, 300, 314–316, _414_

    _Opisthostoma_, 248, =309=, _413_

    _Oppelia_, _399_

    _Orbicula_, =464=

    Orbiculoidea, _504_, 510

    Orders of Mollusca, 5–7

    Organs of sense, 177

    Origin of land Mollusca, 11 f.

    _Ornithochiton_, _403_

    _Orphnus_, 356, _441_

    _Orpiella_, _440_

    _Orthalicus_, 342–358, =355=, _442_;
      habits, 27;
      variation, 87;
      jaw, =211=;
      radula, 233, =234=

    _Orthis_, _505_;
      stratigraphical distribution, 506, 507, 511

    _Orthoceras_, =394=, _394_

    _Orthonota_, _457_

    _Orthothetes_, _505_;
      stratigraphical distribution, 507, 508

    _Orygoceras_, 247

    Osphradium, 194 f.

    _Ostodes_, 327

    _Ostracotheres_, 62

    _Ostrea_, 252, 258, 446, _449_;
      intestine, 241

    _Otina_, 18, _439_

    _Otoconcha_, 326, _440_

    Otocysts, 196 f., =197=

    _Otopleura_, _422_

    _Otopoma_, 331, 338, _414_

    _Otostomus_, 353, _442_

    Ovary, 135

    Ovoviviparous genera, 123

    _Ovula_, _419_;
      protective coloration, 70, 75;
      radula, 80, 224;
      used as money, 97

    Ovum, development of fertilised, 130

    _Oxychona_, 358

    _Oxygyrus_, _422_;
      foot, 200

    _Oxynoe_, _432_;
      radula, 230

    Oyster-catchers, shells used by, 102

    Oyster, cultivation, 104–109;
      living out of water, 110;
      enemies, 110 f.;
      reproduction, 112 f.;
      growth, 114;
      cookery, 114;
      poisonous oysters, 114;
      vision, 190


    _Pachnodus_, 329–335, _441_, _442_

    _Pachybathron_, _425_

    _Pachychilus_, _354_

    _Pachydesma crassatelloides_, money made from, 97

    Pachydomidae, _451_

    _Pachydrobia_, 307, _415_

    _Pachylabra_, _416_

    _Pachyotus_, 334, =336=, =355=, 358, _441_

    _Pachypoma_, _409_

    _Pachystyla_, 337, _440_

    _Pachytypus_, _451_

    _Padollus_, _407_

    Palaearctic region, 284 f.

    _Palaeoneilo_, _447_

    _Palaeosolen_, _457_

    _Palaina_, 327, _413_

    _Palio_, _434_

    Pallial line and sinus, 270

    _Pallifera_, 340, _440_

    Palliobranchiata, 464

    _Paludina_, _416_;
      penis, 136;
      eye, 181;
      vision, 184;
      _P. vivipara_, 24--see also _Vivipara_

    _Paludomus_, 332, 336, 338, _417_

    Panama, Mollusca of, 3

    _Panda_, 322, 325, 335

    _Pandora_, _458_

    Papuans, use of shells, 99

    _Papuina_, 309, 319–324, _441_

    _Paramelania_, 332

    _Paramenia_, _404_

    Parasitic worms, 60 f.;
      Mollusca, 74 f.

    _Parastarte_, _451_

    _Parkinsonia_, =398=

    _Parmacella_, 245, 291, 294 f., 438 n., _440_;
      radula, 232;
      shell, 175

    _Parmacochlea_, 322, 326, _440_

    _Parmarion_, 309, _440_

    _Parmella_, 326, _440_

    _Parmophorus_, _406_

    _Parthena_, 349–352, =350=, _441_

    Parts of univalve shell, 262;
      bivalve, 269

    _Partula_, 319–327, =326=, _442_;
      radula, 233

    _Paryphanta_, 321, 325, _440_

    _Paryphostoma_, _415_

    _Passamaiella_, 332

    _Patella_, _405_, 464;
      as food, 56 f.;
      eye, =182=;
      radula, =214=, 215, =227=;
      crop, 238;
      anus, 241;
      kidneys, 242;
      shell, 262;
      _P. vulgata_, veliger, =132=;
      breathing organs, etc., 156, =157=

    Patelliform shell in various genera, 19

    _Paterina_, 509, 510, 511

    _Patinella_, radula, 227

    _Patula_, 297, 298, 318–338, =340=, _441_

    _Paxillus_, _413_

    Pearl oysters, 100

    _Pecten_, 446, =450=, _450_;
      organs of touch, 178;
      ocelli, =191=;
      flight, 192;
      nervous system, =206=;
      genital orifice, 242;
      ligament, 271

    _Pectinodonta_, _405_;
      radula, 227

    _Pectunculus_, _448_

    _Pedicularia_, 75, _419_;
      radula, 224

    _Pedinogyra_, 319, 322, _442_

    _Pedipes_, =18=, 199, =439=, _439_

    _Pedum_, _450_

    Pelagic Mollusca, 360

    Pelecypoda, _7_, _445_;
      development, 145;
      generative organs, 145;
      branchiae, =166–169=;
      organs of touch, 178;
      eyes, 189 f.;
      foot, 201;
      nervous system, 205

    _Pella_, 333

    _Pellicula_, 352, _442_

    _Peltoceras_, _399_

    _Pentadactylus_, _423_

    _Peraclis_, _436_

    _Pereiraea_, _418_

    _Perideris_, 328–330, _443_

    Periodicity in breeding, 129

    _Periophthalmus_, 187

    Periostracum, 275

    _Periploma_, _459_

    _Perisphinctes_, _399_

    _Perissodonta_, _418_

    _Perissolax_, _424_

    _Peristernia_, _424_

    _Perna_, _449_;
      ligament, 271

    _Pernostrea_, _449_

    _Peronaeus_, 358, _442_

    _Peronia_, _443_

    _Perrieria_, 319, _442_

    _Perrinia_, _408_

    _Persicula_, _425_

    _Persona_ (= _Distortio_), _420_

    _Petenia_, 353, _440_

    _Petersia_, _420_

    _Petraeus_, 295, 331, _442_

    _Petricola_, _454_

    _Phacellopleura_, _403_

    _Phanerophthalmus_, _430_

    _Phaneta_, _408_

    _Phania_, 312, _441_

    _Pharella_, _457_

    _Pharus_, _457_

    Pharynx, 210

    _Phasianella_, _409_

    _Phasis_, 333

    Phenomena of distribution, 362

    _Philine_, 245, =428=, _430_;
      protective coloration, 73;
      radula, 229, 230

    _Philomycus_, 245, 318, _440_

    _Philonexis_, =138=

    _Philopotamis_, 304, _417_

    _Phoenicobius_, 315, _441_

    Pholadacea, _457_

    _Pholadidea_, _457_

    _Pholadomya_, _459_

    _Pholas_, 245, 274, 447, _457_;
      in fresh water, 15

    _Phos_, _424_

    _Photinula_, _408_

    Phragmophora, _386_

    _Phyllidia_, _434_;
      breathing organs, 159

    _Phyllirrhoe_, 360, 428, _433_

    _Phyllobranchus_, _432_

    _Phylloceras_, =398=, _398_;
      suture, =396=

    _Phylloteuthis_, _390_

    _Physa_, _439_;
      aestivating out of water, 27;
      spinning threads, 29;
      sudden appearance, 46;
      osphradium, 195;
      nervous system, =205=;
      radula, =235=;
      _P. hypnorum_, 23, 27

    _Pileolus_, _410_

    _Pileopsis_, 76

    _Piloceras_, _394_

    _Pinaxia_, _423_

    _Pineria_, _442_

    _Pinna_, _449_;
      shell, =254=

    _Pinnoctopus_, _385_

    _Pinnotheres_, 62

    _Pinoceras_, _398_

    _Pirena_, _417_

    _Pirenella_, _416_

    _Piropsis_, _424_

    _Pirula_--see _Pyrula_

    _Pisania_, _424_

    _Pisidium_, _453_;
      smell, 195;
      ova, 146;
      _P. pusillum_, distribution, 282

    _Pitys_, 327

    _Placobranchus_, _432_

    _Placostylus_, 322, =323=-325, 359, _442_;
      radula, 233

    _Placuna_, _448_;
      _P. placenta_ used for windows, 101

    _Placunanomia_, _448_

    _Placunopsis_, _448_

    _Plagioptycha_, 347–351, _441_

    _Plagioptychus_, _456_

    _Planaxis_, _417_

    _Planispira_, 311, 312, 319, _441_

    _Planorbis_, 27, 247, _439_;
      monstrosity, 93;
      eye, 181;
      _P. albus_, distribution, 282

    _Platyceras_, 76, _412_

    _Platydoris_, _434_

    Platypoda, _411_

    _Platyschisma_, _413_

    _Plaxiphora_, _403_

    _Plecochilus_, _442_

    _Plecotrema_, _439_

    _Plectambonites_, _505_

    _Plectomya_, _459_

    _Plectopylis_, 303, 305, 314, 316;
      aperture, =63=

    _Plectostylus_, 358, _442_

    _Plectotropis_, 305, 306, 310, 311, 314–318, _441_

    _Plectrophorus_, 298

    _Plesiastarte_, _451_

    _Plesiotriton_, _420_

    _Pleurobranchaea_, 431;
      jaws, =212=

    _Pleurobranchoidea_, _431_

    _Pleurobranchus_, 245, =428=, _431_;
      warning coloration, 73;
      jaws, =212=;
      radula, 230

    _Pleurocera_, 340, _417_

    Pleuroceridae, origin, 17

    _Pleurodonta_, 348;
      aperture, =63=

    _Pleuroleura_, _433_

    _Pleuromya_, _458_

    _Pleurophorus_, _451_

    _Pleurophyllidia_, _433_;
      breathing organs, =159=;
      radula, 230

    _Pleuropyrgus_, _357_

    _Pleurotoma_, =426=, _426_;
      slit, =263=, 265

    _Pleurotomaria_, =266=, 373, 376, =407=, _407_;
      prices given for recent, 122;
      slit, 156;
      radula, 226

    _Plicatula_, _450_

    Pliny the elder, on use of snails, 118, 120

    _Plocamopherus_, _434_

    _Plochelaea_, _425_

    _Plutonia_, 298, _440_

    _Pneumoderma_, 158, _437_, =438=

    _Poecilozonites_, 352, _440_

    Poisonous bite of _Conus_, 65;
      poisonous oysters, 114;
      mussels, 117

    _Polycera_, _434_;
      radula, 230

    _Polycerella_, _434_

    _Polyconites_, _456_

    _Polydontes_, 346–351, =347=, _441_

    _Polygona_, _424_

    _Polygyra_, =340=, 345–353, _441_;
      aperture, =63=

    _Polygyratia_, =246=, 263, 357, _442_

    _Polymita_, 346–351, =347=, _441_

    Polyplacophora, 9, _401_ f.;
      radula, 228

    _Polytremaria_, =266=, _407_

    _Pomatia_, 285, =293=, 295, _441_

    _Pomatias_, 288, 289, 292 f., 302, _413_

    _Pomatiopsis_, _415_

    _Pomaulax_, _409_

    _Pompholyx_, 250, 341, _439_

    _Ponsonbya_, 332

    _Poromya_, _459_;
      branchiae, 168

    _Porphyrobaphe_, 27, 356, _442_

    Position of Mollusca in Animal Kingdom, 4

    _Potamides_, =16=, _416_

    _Potamomya_, _15_

    _Potamopyrgus_, 325, 326, _415_

    Poterioceratidae, _394_

    _Praecardium_, _459_

    _Prasina_, _449_

    Prices given for rare shells, 121

    Primitive mollusc, form of, 245;
      types of, 7

    _Prisogaster_, _409_

    _Pristiloma_, 341, _440_

    _Proboscidella_, 497, _504_

    Productidae, 497, 500, _504_, 508

    _Productus_, 492, 501, =502=, _504_;
      stratigraphical distribution, 508

    _Promachoteuthis_, _389_

    _Proneomenia_, _404_;
      breathing organs, 154;
      nervous system, =203=;
      radula, 229

    _Prophysaon_, 341, _441_;
      habits, 44

    _Propilidium_, _405_

    _Proserpina_, =21=, 355, _410_

    _Proserpinella_, 354, _410_

    Proserpinidae, relationships, 21

    Prosobranchiata, _9_, _404_ f.;
      breathing organs, 154

    _Prosocoelus_, _451_

    Protective coloration, 69 f.;
      in snails, 70;
      in Nudibranchs, 71 f.;
      in other Mollusca, 74

    Protegulum, 509

    Protobranchiata, _447_;
      branchiae, =166=

    _Protoma_, _417_

    Protremata, 511

    _Provocator_, 376, _425_

    _Psammobia_, _456_

    _Pseudachatina_, 328–330, _443_

    _Pseudedmondia_, _452_

    _Pseudobalea_, 350

    Pseudo-deltidium, 498, 511

    _Pseudodon_, 295, 307, _452_

    Pseudolamellibranchiata, 167, _449_

    _Pseudoliva_, _424_

    _Pseudomelania_, _417_

    _Pseudomilax_, 296, _440_

    _Pseudomurex_, _423_

    _Pseudopartula_, 323

    _Pseudosubulina_, _440_

    Ptenoglossa, 224, _411_

    _Pterinaea_, _449_

    _Pteroceras_, 256, 262, _418_

    _Pteroctopus_, _384_

    _Pterocyclus_, =266=, 267, 300, 316, _414_;
      tube, 157

    _Pterodonta_, _418_

    Pteropoda, _7_, _434_;
      breathing organs, 158;
      foot, 200;
      radula, 230

    _Pterotrachaea_, _421_;
      foot, 200;
      radula, =227=

    _Ptychatractus_, _424_

    _Ptychoceras_, _399_

    _Ptychodesma_, _452_

    _Pugilina_, _424_

    Pulmonata, 10, 22, 151, 185, _438_;
      origin, 17, 19;
      breathing organs, 160;
      nervous system, 203

    _Pulsellum_, _444_

    _Punctum_, _441_

    _Puncturella_, =265=, _406_

    _Pupa_, 289, 296, 325–357, _442_;
      _P. cinerea_, hybrid union, 129

    Pupidae, radula, 233

    _Pupilla_, _442_

    _Pupillaea_, _406_

    _Pupina_, 157, 266, 309, 318–327, _414_

    _Pupinella_, 318, _414_

    _Purpura_, _423_;
      operculum, =269=;
      erosion, 276;
      _P. coronata_, 367;
      _lapillus_, feeding on _Mytilus_, 60;
      on oysters, 111;
      protective coloration, 69;
      variation, =90=;
      egg-capsules, 124;
      time of breeding, 129;
      distribution, 363 n.

    _Purpuroidea_, _423_

    _Pusionella_, _426_

    _Pygocardia_, _451_

    _Pygope_, 497

    _Pyramidella_, _422_

    Pyramidellidae, 262

    _Pyrazus_, 50, _416_

    _Pyrgina_, 330

    _Pyrgula_, _415_

    _Pyrochilus_, _441_

    _Pyrolofusus_, _423_

    _Pyrula_ (= _Pirula_), _419_, =420=;
      spawn, =125=;
      operculum, =269=

    _Pythina_, _453_


    _Quenstedtia_, _456_

    _Quoyia_, 260, _417_


    Rachiglossa, 220, _422_;
      eggs, 124

    _Rachis_, 329–335, 441, _442_

    _Radiolites_, _456_

    _Radius_, _419_

    _Radsia_, =403=

    Radula, 213 f.;
      of _Littorina_, =20=;
      of _Cyclophorus_, =21=;
      of parasitic Mollusca, 79

    _Raëta_, _454_

    _Ranella_, 256, _420_

    Range of distribution, 362 f.

    _Rangia_, 15, _453_

    _Ranularia_, _420_

    _Rapa_, _423_

    _Rapana_, _423_

    _Raphaulus_, 305, 309

    _Rathouisia_, 316, _440_

    Rats devouring Mollusca, 57

    _Realia_, 316, 327, _414_

    _Recluzia_, _411_

    Rectum, 241

    _Registoma_, _414_

    Relationship of Mollusca to other groups, 5

    _Renssoellaria_, 512

    Reproductive activity of oyster, 112;
      system in Mollusca, 123, 134 f.

    _Requienia_, 269, =455=, _455_

    Respiration, 150 f.

    _Retzia_, 508

    _Revoilia_, 331, _414_

    _Reymondia_, 332

    _Rhabdoceras_, _398_

    _Rhagada_, 311, 324

    _Rhenea_, 325, _440_

    _Rhinobolus_, _504_

    _Rhiostoma_, 247, =266=, 309, _414_

    Rhipidoglossa, 225, _405_

    _Rhizochilus_, 75, _423_

    _Rhodea_, 356, _441_

    _Rhodina_, 307, 310, _442_

    _Rhynchonella_, 466, 470, 471, 472, 474, 483, _487_;
      distribution, 487;
      fossil, 492, 497, 499, _505_;
      stratigraphical distribution, 506, 507, 508, 511

    Rhynchonellidae, _487_, 501, _505_;
      stratigraphical distribution, 507, 508, 511

    _Rhysota_, 67, 310, 314, 316, 319, _440_

    _Rhytida_, 319–326, 333, 359, _440_;
      habits, 54;
      radula, =232=

    _Rillya_, _442_

    _Rimella_, _418_

    _Rimula_, 265, _406_

    _Ringicula_, _430_;
      radula, 230

    _Risella_, _413_

    _Rissoa_, _415_

    _Rissoina_, _415_

    _Robillardia_, 77

    _Rochebrunia_, 331, _414_

    Rock-boring snails, 49

    _Rolleia_, 349

    _Rossia_, _389_

    _Rostellaria_, _418_

    Rudistae, _456_

    _Rumina_, 260, _442_

    _Runcina_, _431_;
      protective coloration, 73


    _Sabatia_, _430_

    _Sactoceras_, _394_

    _Sagda_, =348=-351, _441_

    _Sageceras_, _398_

    _Salasiella_, 353, _440_

    Salivary glands, 237

    Sandford, on strength of _Helix_, 45

    Sandwich islanders, use of shells, 99

    _Sanguinolaria_, _456_

    _Sarepta_, _447_

    _Sarmaticus_, _409_

    _Satsuma_, 314, 316, _441_

    _Saxicava_, 447, _457_

    _Saxidomus arata_, money made from, 97

    _Scalaria_, 247, 263, _411_;
      radula, 224

    _Scaldia_, _452_

    _Scalenostoma_, _422_

    _Scaliola_, _415_

    _Scaphander_, =428=, 429, _430_;
      radula, =231=;
      gizzard, =238=

    _Scaphites_, =399=, _399_

    Scaphopoda, _444_;
      defined, 6;
      breathing organs, 160;
      nervous system, 205;
      radula, 236

    _Scaphula_, =14=, 305, _448_

    _Scarabus_, =18=, 278, =439=, _439_

    Scharff, R., on food of slugs, 31;
      on protective coloration in slugs, 70

    _Schasicheila_, 347, 351, 354, _410_

    _Schismope_, =266=, _407_

    _Schizochiton_, 187, 402, _403_

    _Schizodus_, _448_

    _Schizoglossa_, 325, _440_

    _Schizoplax_, _403_

    _Schizostoma_, _413_

    _Schloenbacia_, _398_

    _Scintilla_, 175, _453_

    _Scissurella_, 265, _407_;
      radula, 226

    _Sclerochiton_, _403_

    _Scrobicularia_, 15, =164=, _453_;
      siphons, =164=

    _Sculptaria_, 333

    _Scurria_, _405_

    _Scutalus_, 356, _442_

    _Scutellastra_, _405_

    _Scutus_, 245, =406=, _406_

    _Scyllaea_, _433_;
      jaws, =212=;
      stomach, 239

    _Segmentina_, 320

    _Selenites_, 339, 341, _440_

    Selenitidae, radula, 231

    _Selenochlamys_, 296

    Self-fertilisation, 42–44

    _Semele_, _453_

    _Semicassis_, _420_

    Semper, K., on habits of _Limnaea_, 34;
      of _Helicarion_, 45, 67;
      on mimicry, 67;
      on parasitic _Eulima_, 79;
      on development of _Limnaea_, 84, 94;
      on sexual maturity in snails, 129;
      on _Onchidium_, 187

    _Sepia_, 381, 385–=387=, _389_;
      egg-capsules, =127=;
      glands, 136;
      jaws, =214=;
      radula, 236;
      alimentary canal, =238=;
      ink-sac, =241=;
      hectocotylus, =389=

    _Sepiadarium_, _389_

    _Sepiella_, _389_

    _Sepiola_, _389_;
      glands, 136;
      radula, 236

    _Sepioloidea_, _389_

    Sepiophora, _388_

    _Sepioteuthis_, _390_;
      hectocotylus, 139

    _Septaria_, 337, 338, _410_

    Septibranchiata, 145, 167, _459_;
      branchiae, =166=

    _Septifer_, 274, _449_

    _Sequenzia_, _420_

    Sergius Orata, 104

    _Serrifusus_, _424_

    _Sesara_, 305, _440_

    Sex, differences of, 133

    Shell, 244 f.;
      internal, 174;
      shape of bivalve, 445

    Shell-gland, primitive, 132

    Shells as money, 96 f.;
      as ornament, etc., 98 f.;
      various uses of, 98 f.;
      prices given for rare, 121;
      sinistral, 249

    Shores of N. Asia, no littoral fauna, 2

    Showers of shells, 47

    _Sigaretus_, =186=, =245=, 267, _411_;
      foot, =198=

    Sight, 180

    _Silenia_, _459_;
      branchiae, 168

    _Silia_, _425_

    _Siliqua_, 274, _457_

    _Siliquaria_, =248=, _418_

    _Simnia_, _419_

    _Simpulopsis_, 345, 350, _442_

    _Simpulum_, _420_

    Simroth, on recent forms of _Helix_, 22;
      on food of slugs, 31;
      on crawling of _Helix_, 45

    Singular habitat, 48

    Sinistral shells, 249

    _Sinistralia_, _424_

    _Sinusigera_, 133

    _Sipho_, _424_

    _Siphonalia_, _424_

    _Siphonaria_, 18, _431_;
      classification, 19;
      breathing organs, 151, =152=

    Siphonarioidea, _431_

    _Siphonodentalium_, _444_

    Siphonostomata, 156

    _Siphonotreta_, 493, 496, _504_;
      stratigraphical distribution, 507, 508

    Siphons, 173;
      in burrowing genera, 165;
      branchial, 155

    _Sistrum_, 75, _423_;
      radula of _S. spectrum_, 79, =222=

    _Sitala_, 301, 304, 310, 314–319, 333, _440_

    Skärgard, Mollusca of the, 13

    _Skenea_, _415_

    _Skenidium_, _505_, 508

    Slit, in Gasteropoda, 265, 406

    Slugs, habits and food of, 30 f.;
      bite hand of captor, 33;
      in bee-hives, 36;
      in greenhouses, 36;
      protective coloration, 70;
      eaten in England, 120

    _Smaragdia_, 21

    _Smaragdinella_, _430_

    Smell, sense of, 192

    Smith, W. Anderson, quoted, 98, 111, 114, 191

    Snails as barometers, 50;
      plants fertilised by, 102;
      cultivation for food, 118 f.;
      used for cream, 119;
      as medicine, 120;
      banned by the Church, 121

    _Solariella_, _408_;
      radula, 225

    _Solarium_, =264=, _412_, =413=;
      radula, 224

    _Solaropsis_, 343, 353–357, _442_

    _Solecurtus_, =165=, _457_

    _Solen_, 171, 446, _457_;
      vision, 190;
      habits, 45

    _Solenaia_, _452_

    _Solenomya_, 275, _448_

    _Solenotellina_, _456_

    Solomon islanders, use of shells, 98

    _Somatogyrus_, _415_

    _Sophina_, 305

    Spallanzani, experiments on _Helix_, 163

    Spat, fall of, 113

    _Spatha_, 294, 331, 336, _452_

    _Spekia_, =333=

    Spermatophore, in Cephalopoda, 137;
      in _Helix_, 142

    Spermatozoa, forms of, 136

    _Sphaerium_, _453_

    _Sphenia_, _456_

    _Sphenodiscus_, _398_

    _Sphyradium_, _442_

    Spines, use of, 64

    _Spiraculum_, 266, _414_

    _Spiraxis_, _442_

    _Spirialis_, 249

    _Spirifera_, 468, =501=, _505_;
      stratigraphical distribution, 507, 508, 511, 512

    Spiriferidae, 501, _505_, 508

    _Spiriferina_, stratigraphical distribution, 507, 508

    Spirobranchiata, 464

    _Spirotropis_, _426_;
      radula, 218, =219=

    _Spirula_, 247, 386, _387_, =388=

    _Spirulirostra_, 380, 386, _388_

    Spondylium, 500

    _Spondylus_, 257, 446, =450=, _450_;
      ocelli, 191;
      genital orifice, 242

    _Spongiobranchaea_, _437_

    _Spongiochiton_, _403_

    _Sportella_, _453_

    Starfish eat oysters, 110

    Stearns, R. E. C., on tenacity of life, 38

    _Stegodera_, 306

    _Stenochisma_, _505_;
      stratigraphical distribution, 507, 508

    _Stenogyra_, 324, _442_;
      _S. decollata_, 279;
      food, 34;
      smell, 194;
      _Goodallii_, 279;
      _octona_, sudden appearance, 47

    Stenogyridae, radula, 234

    _Stenopus_, _440_;
      habits, 45

    _Stenothyra_, _415_

    _Stenotis_, _416_

    _Stenotrema_, 340, _441_

    _Stephanoceras_, _399_

    _Stepsanoda_, 358

    _Stilifer_, 76, 77, 79, _422_

    _Stiliferina_, 76, _422_

    _Stiliger_, _432_

    _Stilina_, 76

    _Stoastoma_, 348–351, _410_

    _Stoloteuthis_, _389_

    Stomach, 239

    _Stomatella_, _408_

    _Stomatia_, _408_

    _Stomatodon_, 302, _417_

    _Strebelia_, 353, _440_

    Strength of _Helix_, 45

    _Strephobasis_, _417_

    _Strepsidura_, _424_

    _Streptaulus_, _414_

    _Streptaxis_, =302=, 306, 309, 314–331, 343, 357–359, _440_;
      variation, 87

    Streptoneura, 203, 404

    _Streptosiphon_, _424_

    _Streptostele_, 329, 338, _440_

    _Streptostyla_, 343–355, =353=, _440_

    _Stricklandia_, _505_;
      stratigraphical distribution, 507, 508

    _Strigatella_, _425_

    Stringocephalidae, _506_, 508

    _Stringocephalus_, 492, 497, =498=, 500, 501, _506_;
      stratigraphical distribution, 507, 508

    _Strobila_, 340, 345–353

    _Strobilops_, _442_

    Strombidae, habits, 64;
      penis, 136

    _Strombina_, _423_

    _Strombus_, =69=, =200=, 252, _418_;
      mimicking _Conus_, 69;
      operculum, =78=, =269=;
      pearls from, 101;
      metapodium, 199;
      stomach, 239

    _Strophalosia_, _504_;
      stratigraphical distribution, 507, 508

    _Stropheodonta_, 497, _505_, 508

    _Strophia_, 343–355, _442_;
      _S. nana_, 278

    _Strophochilus_, 358, _441_

    _Strophomena_, 499, _505_;
      stratigraphical distribution, 507, 508

    Strophomenidae, 500, _505_, 508

    _Strophostoma_, 248, _414_

    Structure of shell, 252

    _Struthiolaria_, 99, _418_;
      radula, 216

    _Styliola_, _437_

    _Stylodonta_, 339, _441_

    Stylommatophora, 11, 181, _439_;
      origin, 19

    _Subemarginula_, _406_

    Submytilacea, _451_

    _Subularia_, _422_

    _Subulina_, 332, 352, _442_

    _Subulites_, _420_

    _Succinea_, 325, 327, 358, _433_;
      jaw, =211=;
      _S. putris_, parasite of, 61

    Succineidae, _443_;
      radula, 234

    Sudden appearance of Mollusca, 46

    _Suessia_, stratigraphical distribution, 507

    Sulphuric acid, 237

    _Surcula_, _426_

    _Sycotypus_, _424_

    _Synaptocochlea_, _408_

    _Syndosmya_, _453_

    _Syringothyris_, 500, 508

    _Syrnola_, _422_

    _Syrnolopis_, 332, =333=

    _Systrophia_, 356, 357


    _Tachea_, _441_

    Taenioglossa, 223, _411_

    _Taheitia_, _414_

    _Talona_, _457_

    _Tanalia_, 304, _417_

    _Tancredia_, _453_

    _Tanganyicia_, 332, _415_

    Tanganyika, L., fauna of, 12

    _Tanysiphon_, _454_

    _Taonius_, =391=, _391_

    _Tapes_, _454_

    Taste, 179

    _Tebennophorus_, 143, 340, _440_

    _Tectarius_, _413_

    Tectibranchiata, 10, _429_

    _Tectura_, 305, _405_

    _Tectus_, _408_

    Teeth in aperture of the shell, 63

    _Teinostoma_, 247, _408_

    _Teinotis_, _407_

    _Telescopium_, 252, _416_

    _Tellina_, 440, =453=, _453_;
      _T. balthica_, variation, 84

    Tellinacea, _453_

    Telotremata, 511

    Tenacity of life, 37

    Tenison-Woods, on red blood, 171;
      on shell-eyes, 189

    Tennent, Sir J. E., on musical sounds produced by Mollusca, 50

    _Tennentia_, 304, 314, 338, _440_

    _Terebellum_, _418_;
      jumping powers, 64

    _Terebra_, =246=, 263, =426=, _426_;
      radula, =219=

    _Terebratella_, 468, _487_;
      distribution, 486;
      fossil, _506_;
      stratigraphical distribution, 508

    _Terebratula_, 467, 468, _487_;
      size, 484;
      distribution, 485, 486;
      fossil, 492, =499=, _506_;
      stratigraphical distribution, 506, 507, 508

    Terebratulidae, _487_;
      fossil, 500, _505_, _506_;
      stratigraphical distribution, 507, 508

    _Terebratulina_, =466=, 479, _487_;
      larva, =482=;
      distribution, 486;
      fossil, _506_;
      stratigraphical distribution, 508;
      form of shell, 510

    _Teredina_, _457_

    _Teredo_, 262, =457=, _458_;
      nervous system, =206=;
      intestine, =241=

    _Tergipes_, _432_

    _Terquemia_, _450_

    _Testacella_, 22, =52=, _440_;
      habits, etc., 49, 51 f.;
      pulmonary orifice, 160;
      eyes, 186;
      radula, 231;
      anus, 241

    Testicardines, 466, _487_;
      muscles, 476;
      fossil, 497, _504_;
      external characters, 497;
      internal characters, 499;
      attachment of muscles, 501;
      stratigraphical distribution, 508

    Testis, 135

    Tethyidae, 216

    _Tethys_, _432_

    Tetrabranchiata, _391_ f.

    _Thala_, _425_

    _Thalassia_, 319

    _Thalotia_, _408_

    _Thapsia_, 329

    _Thaumasia_, 349, _442_

    _Thaumastus_, 356, _442_

    _Thecacera_, _434_;
      radula, 229

    Thecidiidae, _487_;
      fossil, 501, _506_, 508

    _Thecidium_, 475, 479, =480=, 483, _487_;
      fossil, _506_, 508

    Thecosomata, _435_

    _Thelidomus_, 346–351, =350=, _441_

    _Theora_, _453_

    _Therasia_, _441_

    _Thersites_ (Helicidae), 322, 325

    _Thersites_ (Fasciolariidae), _424_

    _Thetis_, _454_

    _Thracia_, 245, _459_

    Thread-spinning, 29

    _Thridachia_, _432_

    _Thyca_, 76, 79

    _Thyrophorella_, 330, _440_

    _Thysanoteuthis_, _390_

    _Tiedemannia_, veliger, =132=

    _Tiphobia_, 332, =333=, _417_

    Titicaca, L., Mollusca of, 25

    _Todarodes_, _390_

    _Tomichia_, _414_

    _Tomigerus_, 334, 356, 358, _442_

    _Tomocyclus_, 354

    _Tomostele_, 330, _440_

    _Tonicella_, _403_

    _Tonicia_, _403_;
      eyes, 188

    _Torellia_, _411_

    _Torinia_, _413_;
      radula, 224;
      operculum, =269=

    _Tornatellina_, 278, 319, 323–327, 338, 358, _443_

    _Tornatina_, 250, _430_

    _Torquilla_, _442_

    _Toucasia_, _455_

    Touch, sense of, 177

    Toxoglossa, 218, _426_

    _Trachia_, 314

    _Trachyceras_, _397_

    _Trachydermon_, _403_

    _Trachyteuthis_, _389_

    _Tralia_, _439_

    _Transovula_, _419_

    _Trematis_, 492, 493, _504_;
      stratigraphical distribution, 507, 508

    _Trematonotus_, _407_

    _Tremoctopus_, _384_;
      radula, 236;
      hectocotylus, 137

    _Trevelyana_, _434_

    _Trichia_, 316

    _Trichotropis_, 275, _411_

    _Tricula_, 302

    _Tridacna_, =273=, _455_

    _Triforis_, _416_;
      radula, 224

    _Trigonellites_, =397=

    _Trigonia_, =15=, 254, 269, _448_;
      jumping powers, 65;
      distribution, 370

    _Trigonochlamys_, 296, _440_

    _Trigonostoma_, _426_

    _Trimerella_, =495=, _504_, 508, 511

    Trimerellidae, 493, 494, 496, _504_;
      stratigraphical distribution, 507, 508

    _Trinacria_, _448_

    _Triodopsis_, 340, _441_

    _Triopa_, _434_

    _Triopella_, _434_

    _Triopha_, _434_

    _Tritaxeopus_, _385_

    _Triton_, 256, =275=, _420_;
      jaws, =212=

    _Tritonia_, _433_;
      protective coloration, 71

    _Tritonidea_, _424_

    _Trivia_, _419_

    Trochidae, egg-capsules, 125

    _Trochiscus_, _408_

    _Trochita_, 248, _412_

    _Trochoceras_, _395_

    _Trocholites_, _395_

    _Trochomorpha_, 306, 321, 324, 327, 333, _441_

    _Trochonanina_, 331, _440_

    Trochosphere, 5, 130

    _Trochotoma_, 266, _407_

    _Trochus_, 263, _408_;
      eye, 182;
      stomach, 239

    _Trophon_, _423_

    Tropical beach, Mollusca of a, 3

    _Tropidophora_, _414_

    _Tropites_, _397_

    _Troschelia_, _424_

    _Truncaria_, _423_

    _Truncatella_, 260, _414_

    _Tryblidium_, _405_

    _Trypanostoma_, 340

    Trypho of Lampsacus, prayer against snails, 121

    Tubed operculates, 157, 266, 300, 307, 309

    _Tudicla_, _424_

    _Tudora_, 291, 349, 351, _414_

    _Tugonia_, _456_

    _Tulotoma_, 340, _416_

    _Turbinella_, 100, 262, =264=, =424=, _424_

    _Turbo_, _409_;
      eye, =182=;
      osphradium, 195;
      operculum, =268=

    _Turbonilla_, 250, 332, _422_

    _Turcica_, _408_

    _Turricula_, _425_;
      radula, 221

    _Turrilites_, =399=, _399_

    _Turritella_, 252, _417_;
      radula, 215, 224

    _Tyleria_, _459_

    _Tylodina_, _431_

    _Tylopoma_, _416_

    _Tympanotonus_, _416_

    _Tyndaria_, _447_

    _Typhis_, _423_


    Ultra-dextral shells, 250

    _Umbonella_, _409_

    _Umbonium_, _409_

    _Umbrella_, =10=, _431_;
      radula, 217, 230

    _Uncites_, _505_;
      stratigraphical distribution, 507, 508

    Underground snails, 48

    _Ungulina_, _452_

    _Unicardium_, _452_

    _Unio_, _452_;
      shell, =254=, 259, =273=, 341;
      variation, 92

    Union of _Limax_, 128

    Unionidae, origin of, 15;
      eaten by rats, 57;
      larvae, 146

    _Urocyclus_, =331=, _440_

    _Urosalpinx_, _423_

    _Utriculus_, _430_

    _Uvanilla_, _409_


    _Vaginula_, 245, 319, 343, 352, _443_

    Vaginulidae, radula, 234;
      anus, 241

    _Valletia_, _456_

    _Vallonia_, _441_

    _Valvata_, 133, _416_;
      branchia, =159=

    Valves of Chitonidae, 401 f.

    _Vanganella_, _454_

    Variation, 82 f.

    _Varicella_, 346, 348

    _Velates_, 260, _410_

    _Velifera_, 353, _440_

    Veliger stage, 131;
      mistaken for perfect form, 133

    _Velorita_, 302, _453_

    Velum, 131

    _Velutina_, 275, _411_;
      radula, 223

    Veneracea, _454_

    _Venericardia_, _451_

    _Venerupis_, _454_

    _Veniella_, _451_

    _Venilicardia_, _451_

    _Venus_, =270=, =271=, 446, _454_;
      _V. mercenaria_, 97, 374

    _Verania_, _391_

    _Vermetus_, 247, _418_;
      radula, =223=

    _Veronicella_, _443_

    _Verticordia_, _458_

    _Vertigo_, 327, _442_;
      _V. arctica_, 287

    _Vexilla_, _423_

    _Vibex_, _417_

    _Vitrella_, 289

    _Vitrina_, 22, 296 f., 332, _440_;
      hardy habits, 24;
      jumping powers, 65;
      shell, 175;
      radula, 217

    _Vitrinella_, _408_

    _Vitriniconus_, 314, _440_

    _Vitrinoidea_, 314, _440_

    _Vitrinozonites_, 340, _440_

    _Vitularia_, _423_

    _Vivipara_, 324, 343, _416_

    Volume of water, effect in producing variation, 94

    _Voluta_, 267, =425=, _425_;
      spawn, =125=;
      radula, 217, 221;
      distribution, 370;
      prices given for rare, 122

    _Volutaxis_, 348

    _Volutharpa_, 267, _424_

    _Volutolithes_, _425_

    _Volutolyria_, _425_;
      radula, 222

    _Volutomitra_, _425_;
      radula, 221

    _Volutopsis_, _423_

    _Volvaria_, _429_

    _Volvatella_, _430_

    _Volvula_, _430_

    _Vulsella_, 75, =446=, _449_


    _Waldheimia_, 464, 467, 468, 473, 474, _487_;
      size, 484;
      distribution, 486;
      fossil, =500=, 501, 502, _506_, 508

    Walton and mussel cultivation, 115

    Wampum, 97

    Warner, R., quoted, 37

    Warning coloration, 71 f.

    West Coast, South America, melanism of shells occurring on, 85

    Whelks, use of, 118

    _Whitneya_, _424_

    Whitstable, oyster-parks at, 106, 112

    Willem, V., on vision of Mollusca, 185

    Wollaston, T. V., quoted, 32

    Wood, Rev. J. G., on starfish eating oysters, 111

    _Woodia_, _451_

    Woodward, S. P., on tenacity of life, 38;
      Dr., on the same, 38

    Wotton, F. W., on egg-laying of _Arion_, 42

    Wright, Bryce, on tenacity of life, 38


    _Xenophora_, _412_;
      habits, =64=

    _Xenopoma_, 346, 351

    _Xerophila_, 285, 296, _441_

    _Xesta_, 310, 319, 321, _440_;
      mimicry by, 66 f.

    _Xylophaga_, _457_


    _Yetus_, _425_

    _Yoldia_, _447_;
      genital orifice, 242


    _Zagrabica_, 297

    _Zebrina_, 285, 296, _442_

    _Zeidora_, _406_

    _Zidona_, _425_

    _Zittelia_, _420_

    Zones of depth, 361

    _Zonites_, 275, _440_;
      food, 33;
      radula, 232;
      distribution, 294, 296, 340

    _Zospeum_, 187, _442_

    Zygobranchiata, 154, _406_


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FOOTNOTES:

[1] See especially Moseley, _Nature_, 1885, p. 417.

[2] _Quart. Journ. Conch._ i. p. 371.

[3] _Manuel de Conchyliologie et de Paléontologie Conchyliologique._
Dr. P. Fischer. Paris, 1887.

[4] κεφαλή, head; γαστήρ, stomach; σκάπτειν, to dig; πέλεκυς, an axe;
πούς, ποδός, a foot.

[5] Also known as _Lamellibranchiata_, _Conchifera_, and
_Acephala_.

[6] πτερόν, wing.

[7] γλῶσσα, tongue; φέρειν, to carry.

[8] λείπειν, to be wanting.

[9] ἀμφί, on both sides; νεὕρον, nerve, vessel. Some authorities regard
the Amphineura as a distinct Order.

[10] πολύς, many; πλάξ, plate.

[11] πρόσω, in front. Often alluded to in the sequel as ‘operculate
Gasteropoda.’

[12] κτενίδιον, a little comb.

[13] δὐω, two; mόnos, single; ὦτα, auricles; καρδία, heart.

[14] ὄπισθεν, behind.

[15] _Pulmo_, a lung.

[16] στὕλος, pillar; ὄμματα, eyes.

[17] The _Ascoglossa_ are dealt with below (chap. xv.).

[18] Beudant, by very gradually changing the water, accustomed marine
species to live in fresh, and fresh-water species to live in salt water.

[19] Braun, _Arch. f. Naturk. Liv._ (2), x. p. 102 f.

[20] Lindström, _Oef. K. Vet. Förh. Stockh._, 1855, p. 49.

[21] Mendthal, _Schr. Ges. Königsb._, xxx. p. 27.

[22] _SB. K. Akad. Wiss. Wien_, 1889, p. 4, but the view is not
universally accepted.

[23] Not to _Nassa_, as has been generally held. The shape of the
operculum, and particularly the teeth of the radula, show a much closer
connexion with _Cominella_.

[24] _E.g._ Bouvier, _Le Natural_, 1889, p. 242.

[25] Köhler, _Zool. Jahrb._ vii. 1893, p. 1 f; Haller, _Arb.
Zool. Inst. Wien_, x. p. 71.

[26] Plate, _SB. kön. Preuss. Ak. Wiss. Berl._ 1893. p. 959.

[27] _E.g._ Pelseneer, _Bull. Sc. France Belg._ xxiv. p. 347
f.

[28] _E.g._ Bergh, _Zool. Jahrb._ v. p. 1 f.

[29] Calkins, _Amer. Nat._ xi. p. 687.

[30] One step even further (or perhaps it should be termed a branch
derivative) is seen in the genus _Smaragdia_, which is probably a
_Neritina_ which has resumed a purely marine habit of life.

[31] _SB. Naturf. Gesell. Leipz._ 1886–87, pp. 40–48.

[32] _L. and F. W. Moll. of India_, iv. p. 167.

[33] T. Scott, _Journ. of Conch._ v. p. 230.

[34] J. S. Gibbons, _ibid._ ii. p. 129.

[35] _Bull. Soc. Linn. Nord_, Abbeville, 1840, p. 150.

[36] Joly, _Comptes Rendus_, 1842, p. 460; compare W. A. Gain,
_Science Gossip_, xxvii. p. 118.

[37] Von Martens, _SB. Nat. Fr. Berl._ 1881, p. 34.

[38] Moquin-Tandon, _Moll. de France_, i. p. 116.

[39] _Journ. of Conch._ iii. p. 321 f.; iv. p. 13; _Science
Goss._ 1866, p. 158.

[40] Reichel, _Zool. Anz._ x. p. 488.

[41] Schumann, _Schr. Ges. Danz._ (2) vi. p. 159.

[42] Fischer and Crosse, _Mexico_, p. 437.

[43] _Journ. de Conch._ iv. p. 397, but the species observed is
not mentioned.

[44] _Bull. Mus. C. Z. Harv._ iv. p. 378.

[45] W. Harte, _Proc. Dubl. N. H. Soc._ iv. p. 182.

[46] See on the whole subject of threads G. S. Tye, _Journ. of
Conch._ i. p. 401.

[47] _Zoologist_, ii. p. 296; iii. p. 833; iv. p. 1216; iii. p.
1036; iv. p. 1216; iii. p. 1037.

[48] _Ann. Nat. Hist._ ii. 1838, p. 310.

[49] H. W. Kew, _Naturalist_, 1889, p. 103.

[50] _Zeit. wiss. Zool._ xlii. p. 203 f.

[51] _Sci. Trans. R. Dubl. Soc._ (2) iv. p. 520.

[52] _Zoologist_, iv. p. 1504; iii. p. 1038; iii. p. 943.

[53] H. W. Kew, _l. c._

[54] _Zoologist_, xix. p. 7819.

[55] _Naturalist_, 1889, p. 55.

[56] H. W. Kew, _l. c._

[57] W. G. Binney, _Bull. Mus. C. Z. Harv_. iv. p. 144.

[58] _Naturalist_, _l. c._

[59] _Science Gossip_, 1885, p. 154.

[60] R. Standen, _Journ. of Conch._ vii. p. 197.

[61] _Journ. of Conch._ v. p. 43.

[62] A. Paladilhe in MS. letter.

[63] J. S. Gibbons, _Quart. Journ. Conch._ ii. p. 143.

[64] _Bull. Mus. C. Z. Harv._ iv. p. 193.

[65] _l. c._ p. 362.

[66] _Animal Life_, p. 59.

[67] _Zoologist_, 1861, p. 7400; _Brit. Conch._ i. p. 108.

[68] H. Ullyett, _Science Gossip_, xxii. (1886), p. 214.

[69] _Descent of Man_, i. p. 325, ed. 1.

[70] _Amer. Nat._ xv. 1881, p. 976.

[71] W. A. Gain, quoted by H. W. Kew in _Naturalist_, 1890, p.
307, an article to which I am much indebted.

[72] _Ann. Mag. Nat. Hist._ (5) xvi. p. 519.

[73] _Science Gossip_, 1882, pp. 237, 262.

[74] H. W. Kew, _Naturalist_, 1893, p. 149, another most valuable
article.

[75] _Garden_, v. p. 201, quoted by Kew, _ut sup._

[76] Kew, _ut sup._

[77] _Science Gossip_, 1883, p. 163.

[78] T. D. A. Cockerell, _Science Gossip_, 1885, p. 211.

[79] _Ann. Mag. Nat. Hist._ (2) vi. (1850) p. 68.

[80] _Ann. Mag. Nat. Hist._ (2) vi. p. 489.

[81] _Ibid._ (3) iii. p. 448.

[82] _Amer. Nat._ xi. (1877) p. 100; _Proc. Calif. Ac._ iii.
p. 329.

[83] _Gaz. Med. Alger._ 1865, 5th Jan. p. 9.

[84] _Science Gossip_, 1867, p. 40.

[85] _Ann. Mag. Nat. Hist._ (2) ix. p. 498.

[86] _Journ. of Conch._ vi. p. 101.

[87] _Naturalist_, 1889, p. 55.

[88] _Malak. Blätt._ (2) iv. pp. 43 and 221.

[89] _Phil. Trans._ 1854 (1856), p. 8.

[90] _Naturalist_, 1891, p. 75 f.; _Conchologist_, ii. 1892,
p. 29.

[91] Taylor, _Journ. of Conch._ 1888, p. 299.

[92] See Tennent’s _Ceylon_, i. p. 221, ed. 5.

[93] W. A. Gain, _Naturalist_, 1889, p. 55; Brockmeier, _Nachr.
Deutsch. Malak. Gesell._ xx. p. 113.

[94] _Ann. Mag. Nat. Hist._ (2) ix. p. 498.

[95] _Journ. Conch._ vii. 1893, p. 158 f.

[96] I succeeded in hatching out eggs of _Helix aspersa_, during
the very warm summer of 1893, in 17 days.

[97] _Nachr. Deutsch. Malak. Gesell._ xx. p. 146.

[98] Raymond, _Nautilus_, iv. p. 6.

[99] Quoted by Oehlert, _Rév. Sc._ xxxviii. p. 701.

[100] _Animal Life, Intern. Scientif. Ser._ ed. 1, p. 395.

[101] _Zoologist_, 1886, p. 491.

[102] Thomas, quoted by Jeffreys, _Brit. Conch._ i. p. 30.

[103] _Journ. of Conch._ iv. p. 117.

[104] Rev. L. Jenyns, _Observations in Nat. Hist._ p. 318.

[105] _Id. ib._ p. 319.

[106] Further detailed examples will be found in Kew, _The dispersal
of Shells_, pp. 5–26.

[107] _P. Z. S._ 1888, p. 358.

[108] W. A. Gain, _Naturalist_, 1889, p. 58.

[109] _Das Wetter_, Dec. 1892. Another case is recorded in
_Amer. Nat._ iii. p. 556.

[110] _Zoologist_, x. p. 3430.

[111] _Science Gossip_, 1888, p. 281.

[112] Lecoq, _Journ. de Conch._ ii. p. 146.

[113] Bouchard-Chantereaux, _Ann. Sci. Nat. Zool._ (4) xvi. (1861)
p. 197.

[114] Forel, _Ann. Sci. Nat._ (3) xx. p. 576; Bretonnière,
_Comptes Rendus_, cvii. p. 566.

[115] Brit. Mus. Collection.

[116] Thomas, quoted by Récluz in _Journ. de Conch._ vii. 1858, p.
178.

[117] _Nat. Hist. of Ceylon_, p. 382. See also T. L. Taylor,
_Rep. Brit. Ass._ for 1848, p. 82.

[118] Dr. R. E. Grant, _Edinb. Phil. Journ._ xiv. p. 188.

[119] _Rep. Brit. Ass._ for 1848, p. 80. The statement is
confirmed by Rossmässler.

[120] _Journ. of Conch._ iv. p. 118.

[121] _Zoologist_, 1887, p. 29.

[122] _Arch. Zool. Exp. Gén._ (2) v. p. 459 f.

[123] _Journ. of Conch._ iii. p. 277; compare W. M. Webb,
_Zoologist_, 1893, p. 281.

[124] _Bull. Mus. Comp. Zool. Harv._ iv. p. 85.

[125] Erjavec, _Nachr. Deutsch. Malak. Gesell._ 1885, p. 88.

[126] Crosse, _Journ. de Conch._ (3) xiv. (1874) p. 223.

[127] C. Wright, _Zoologist_, 1869, p. 1700.

[128] W. V. Legge, _Zoologist_, 1866, p. 190.

[129] Blackwall, _Researches_, p. 139.

[130] Barrow, _Travels in South Africa_, ii. p. 67.

[131] _Loch Creran_, p. 102.

[132] Cordeaux, _Zoologist_, 1873, p. 3396.

[133] _Amer. Nat._ xii. p. 695; _Science Gossip_, 1865, p. 79.

[134] _Journ. Trent. N. H. Soc._ 1887, p. 58.

[135] _Ann. Nat. Hist._ iii. 1893, pp. 238, 239.

[136] _Rev. Nat. Sc. Ouest_, 1891, p. 261.

[137] Petit de la Saussaye, _Journ. de Conch._ iii. p. 97 f.

[138] J. W. Williams, _Science Gossip_, 1889, p. 280.

[139] Noack, _Zool. JB._ ii. p. 254.

[140] _La Nature_, xv. (2) p. 46.

[141] François, _Arch. Zool. Exp. Gén._ (2) ix. p. 240.

[142] A. Lang, _Ber. Naturf. Ges. Freib._ vi. 1892, p. 81.

[143] A. P. Thomas, _Q. J. Micr. Sc._ N. S. xxiii. (1883) p. 99.

[144] H. Woodward, _P. Z. S._ 1886, p. 176.

[145] W. E. Collinge, _Zoologist_, 1890, p. 467.

[146] _Proc. Linn. Soc. N. S. Wales_, ix. p. 944.

[147] _Zoologist_, xviii. (1860) p. 7136.

[148] A. Adams, _Samarang_, vol. ii. _Zoology_, p. 357.

[149] In Thomson’s _British New Guinea_, p. 283.

[150] _Animal Life_, p. 395. It should be mentioned that Von
Möllendorff (_Ber. Senck. Ges._ 1890, p. 198) ridicules the whole
theory.

[151] Von Martens, _SB. Nat. Fr. Berl._ 1891, p. 83.

[152] Von Martens, _ibid._ 1887, p. 183.

[153] _SB. Nat. Gesell. Leipz._ xiii.-xiv. p. 45.

[154] Garstang, _Journ. Mar. Biol. Ass._ N. S. i. p. 432; Giard,
_Bull. Sci. Fr. Belg._ 1888, p. 502 f.

[155] _Nautilus_, vi. 1892, p. 90.

[156] R. F. Scharff, _Sci. Trans. R. Dubl. Soc._ (2) iv. p. 553 f.

[157] _Q. Journ. Micr. Sci._ N. S. xxxi. (1890) p. 41 f.

[158] A detailed account is given in _Proc. Liverp. Biol. Soc._
iv. (1890) pp. 150–163.

[159] _Journ. Mar. Biol. Ass._ N. S. i. p. 418 f.

[160] Garstang, _Conchologist_, ii. p. 49.

[161] Hecht, _Comptes Rendus_, cxv. p. 746.

[162] _Conchologist_, ii. p. 130.

[163] Described as a _Cypraea_, but no doubt an _Ovula_ or
_Pedicularia_: _CB. Bakt. Par._ v. p. 543.

[164] Von Graff, _Z. wiss. Zool_ xxv. p. 124.

[165] _Proc. Amer. Phil. Soc._ xxv. p. 231.

[166] _Ergeb. naturw. Forsch. Ceylon_, abstr. in _Journ. Roy.
Micr. Soc._ (2) vi. p. 412.

[167] _Voyage of the Samarang_, Moll. p. 69, Pl. xi. f. 1; p. 47,
Pl. xvii. f. 5.

[168] E. A. Smith, _Ann. Mag. Nat. Hist._ (6) iii. p. 270.

[169] _Journ. de Conch._ (3) xxix. p. 101.

[170] _Zool. Jahrb. Abth. f. Syst._ v. p. 619.

[171] See especially Semper, _Animal Life_, Ed. 1, p. 351.

[172] Gould, _Moll. of U.S. expl. exped._ 1852, p. 207 (_St.
acicula_, from Fiji).

[173] Stimpson, _Proc. Bost. Soc. N. H._ vi. 1858, p. 308.

[174] Pidgeon, _Nature_, xxxix. p. 127.

[175] W. Anderson Smith, _Loch Creran_, p. 46.

[176] Smart, _Journal of Conch._ v. p. 152.

[177] _Animal Life_, p. 351.

[178] _Journ. of Conch._ vi. 1891, p. 399.

[179] _Ann. Mag. N. H._ (6) vii. p. 276.

[180] Stimpson, quoted by Jeffrey’s _Brit. Conch._ ii. 194.

[181] Stimpson, _Journ. Bost. Soc. N. H._ vi. 1857, p. 48.

[182] E. H. Matthews, _Conchologist_, ii. p. 144.

[183] Thus _Limnaea involuta_, which is almost universally
regarded as a good and distinct species, has been held to be no
more than a variety of _L. peregra_ produced by locality; see
_Zoologist_, 1889, p. 154.

[184] J. W. Taylor, _Journ. of Conch._ v. p. 289, an interesting
article, with many useful references.

[185] Möbius, _Report on ‘Pommerania’ Exped._ pp. 138–141.

[186] _Journ. de Conchyl._ xxiii. 1875, p. 105.

[187] J. W. Taylor _ut sup._ p. 300.

[188] _Sci. Trans. R. Dubl. Soc._ (2) iv. p. 555.

[189] J. S. Gibbons, _Journ. of Conch._ ii. p. 129.

[190] C. H. Morris, _ibid._ vii. p. 191.

[191] F. M. Hele, _ibid._ iv. p. 93.

[192] T. D. A. Cockerell, _Science Gossip_, 1887, p. 67.

[193] J. G. Jeffreys, _British Conchology_, vol. i. p. 214.

[194] _Journ. of Conch._ vi. p. 123.

[195] _Phil. Trans._ 1889, vol. 180 B, p. 207. A somewhat similar
case (the celebrated Steinheim series of _Planorbis_) is dealt
with by Hilgendorf, _MB. Akad. Berl._ 1866, p. 474; and Hyatt,
_Proc. Amer. Ass. Sc._ xxix. p. 527.

[196] J. B. Bridgman, _Quart. Journ. Conch._ i. p. 70.

[197] W. C. Hey, _Journ. of Conch._ iii. p. 268.

[198] _Zool. Anz._ xiii. p. 662.

[199] J. Madison, _Journ. of Conch._ v. p. 260.

[200] _Quart. Journ. Conch._ i. 339.

[201] Whitfield, _Bull. Amer. Mus. N. H._ i. p. 29.

[202] _Amer. Nat._ xiv. p. 51.

[203] _Animal Life_, Ed. 1, p. 160 f.

[204] _Conch. Syst._ ii. p. 262 _n._

[205] P. L. Simmonds, _Commercial Products of the Sea_, p. 278.

[206] _Benderloch_, p. 118.

[207] C. Hedley in J. P. Thomson, _Brit. New Guinea_, p. 283.

[208] Most of the above facts are derived from a study of a collection
of native implements, weapons, ornaments, etc., in the Antiquarian
Museum at Cambridge.

[209] Thurston, _Notes on the Pearl and Chank Fisheries_, Madras,
1890.

[210] See in particular, P. L. Simmonds, _The Commercial Products of
the Sea_.

[211] H. Friend, _Field Club_, iv. 1893, p. 100.

[212] _Nature_, xxxi. 1885, p. 492.

[213] W. Anderson Smith, _Benderloch_, p. 173.

[214] Dominique, _Feuill. Nat._ xviii. p. 22.

[215] SB. _Nat. Fr. Berl._ 1889, p. 197.

[216] A. Adams, _Voyage of the ‘Samarang,’_ ii. p. 308.

[217] Much information has been derived, on this subject, from
Bertram’s _Harvest of the Sea_, Simmonds’ _Commercial Products
of the Sea_, the publications of the Fisheries Exhibition,
especially vol. xi. (Anson and Willett); see also Philpots, _Oysters
and all about them_.

[218] Juvenal, _Sat._ iv. 140–142.

[219] _Hist. Nat._ ix. 79.

[220] _Vol. Max._ ix. 1.

[221] _Quart. Journ. Micr. Sc._ xxvi. p. 71.

[222] See G. H. Lewes, _Sea-side Studies_, p. 339.

[223] _Bull. U.S. Fish. Comm._ v. p. 161.

[224] W. Anderson Smith, _Loch Creran_, p. 228.

[225] _Longmans’ Magazine_, June 1889.

[226] _St. James’s Gazette_, 6th January 1893.

[227] Also at Arcachon (W. A. Herdman, _Nature_, 1893, p. 269).

[228] See especially Hoek, _Tijdschr. Ned. Dierk. Vereen_, Suppl.
Deel, i. 1883.

[229] _Benderloch_, p. 136.

[230] This is the view of E. Ray Lankester, _Quart. Journ. Micr.
Sc._ xxvi. 80.

[231] De Quatrefages, _Rambles of a Naturalist_.

[232] Quoted by Jeffreys, _Brit. Conch._, ii. p. 109.

[233] M. S. Lovell, _Edible Mollusks_, p. 49.

[234] _Science_, vii. p. 175.

[235] _Hist. Nat._ ix. 82.

[236] _De re rustica_, iii. 14.

[237] _Epistles_, i. 15.

[238] Hor. _Sat._ II., iv. 58, tr. Conington.

[239] Roberts, _Zoologist_, 1885, p. 425.

[240] _Hist. Nat._ xxx. 15, 19.

[241] _Science Gossip_, 1891, p. 166.

[242] Jeffreys, _Brit. Conch._ iii. p. 355.

[243] W. Clark, _Mag. Nat. Hist._ xvi. p. 466.

[244] Examples will be found in _Journ. Linn. Soc. Zool._ xi. p.
90; _Ann. Sc. Nat._ xx. p. 472; _Zeit. wiss. Zool._ xxiv. p.
419.

[245] Herdman, _Proc. Liverp. Biol. Soc._ iii. p. 30.

[246] Garrett, _Journ. Ac. Nat. Sc. Phil._ viii. (1880).

[247] J. Bladon, _Zoologist_, xvi. p. 6272.

[248] Lo Bianco, _MT. Zool. Stat. Neap._ viii. p. 414.

[249] _Animal Life_, pp. 126, 135.

[250] R. Rimmer, _Land and Fresh-Water Shells_, p. 119.

[251] _Journ. de Conch._ ii. p. 245.

[252] _Journ. de Conchyl._ iii. p. 107.

[253] Jeffreys, _Brit. Conch._ iii. p. 359; Sauvage, _Journ. de
Conchyl._ xxi. p. 122.

[254] Hermaphroditism seems to occur in (_a_) whole families, _e.g._
_Anatinidae_ and the _Septibranchia_; (_b_) genera, _e.g._ _Cyclas_,
_Pisidium_; (_c_) single species, _e.g._ in the generally dioecious
genera _Ostrea_, _Pecten_, _Cardium_.

[255] δὐω, two; μόνος, single; γόνος, semen; πόρος, passage.

[256] Von Brunn, _Arch. Mikr. Anat._ xxiii. p. 413.

[257] _Hist. Anim._ v. 6 and 12, iv. 1, ed. Bekker, 1837.

[258] ‘On pourra constater si ce ne seraient pas des parties détachées
de quelque céphalopode dans le but de servir à le fécondation,’
_Hist. Nat. Helminthes_, 1845, p. 482.

[259] Steenstrup, _Ann. Mag. Nat. Hist._ (2), xx. p. 81 f.

[260] C. Ashford, _Journ. of Conch._ iii. p. 239, iv. pp. 69, 108.

[261] W. E. Collinge, _Zoologist_, 1890, p. 276.

[262] Pelseneer, _Comptes Rendus_, cx. p. 1081.

[263] _Kon. Vet. Akad. Handl._ 1848, pp. 329–435.

[264] _P. Z. S._ 1891, p. 52 f.

[265] The result of some experiments by Professor Herdman upon
_Littorina rudis_, tends to show that it can live much better in
air than in water, and goes far to support the view that the species
may be undergoing, as we know many species must have undergone (see p.
20), a transition from a marine to a terrestrial life. It was found
that marked specimens upon the rocks did not move their position for
thirty-one successive days (_Proc. Liverp. Biol. Soc._ iv. 1890,
p. 50).

[266] Diminutive of κτείς, a comb.

[267] Stoliezka, quoted in _Journ. de Conch._ xviii. p. 452.

[268] ζύγος, a yoke, from the symmetrical position of the branchiae.

[269] Pelseneer, _‘Challenger’ Reports_, vol. xxiii. part lxvi.

[270] _Zoologist_, xii. p. 4248.

[271] _Mollusques de France_, i. p. 81.

[272] _N. Denk. Schw. Ges._ xxix. (2) p. 196 f.

[273] Bergh, _Morph. Jahrb._ x. p. 172.

[274] P. Fischer, _Journ. de Conch._ ix. p. 101.

[275] _Bull. Mus. C. Z. Harv._ xviii. p. 434.

[276] Pelseneer, _Comptes Rendus_, cvi. p. 1029.

[277] _E.g._ Kollmann, _Zeit. wiss. Zool._ xxvi. p. 87.

[278] _Proc. Roy. Soc._ 1873, p. 70.

[279] Griesbach (_Arch. mikr. Anat._ xxxvii. p. 22) finds haemoglobin
in several bivalves, _e.g._ _Poromya granulata_, _Tellinata planata_,
_Arca Noae_, and _Pectunculus glycimeris_.

[280] _Trans. Roy. Soc. N. S. Wales_, xxii. p. 106.

[281] Pelseneer, _Comptes Rendus_, cx. p. 154.

[282] _Science_, iv. p. 50.

[283] P. Fischer, _Journ. de Conchyl._ (3) xxvii. p. 201.

[284] _Journ. of Conch._ vi. p. 349 ff.

[285] _Quart. Journ. Micr. Sc._ N.S. xv. p. 37.

[286] _Ann. Mag. Nat. Hist._ (2), xx. p. 336.

[287] V. Willem (_Arch. Biol._ ut infr.) denies this, and declares
that _Cyclostoma_ is only very sensitive to movements. The present
writer has often approached, with the greatest care, a crawling
_Cyclostoma_, but it always withdrew into its shell or fell to the
ground when approached within about 10 or 12 inches.

[288] _Arch. Biol._ xii. 1892, p. 57.

[289] _‘Challenger’ Reports_, Zoology, vol. xxvii. part lxxiv. p.
3.

[290] _Animal Life_, p. 372 f.

[291] Bergh, _Morph. Jahrb._ x. p. 172.

[292] _Ann. Mag. Nat. Hist._ (5) xiv. p. 141.

[293] The nature of the grouping of the eyes into rows varies
considerably in different species. As a rule, the rows radiate from the
beak, but occasionally they run parallel to the girdle. In _Tonicia
lineolata_ Fremb., they are grouped, as it were, under the shelter
of strongly marked longitudinal wavy lines.

[294] =Shell-Eyes in other Mollusca.=--The Rev. J. E. Tenison-Woods
(_Trans. Linn. Soc. N. S. Wales_, xxii. p. 106) is of opinion that
‘shell-eyes’ are by no means confined to the Chitonidae, but that,
in fact, multiplicity of eyes of this kind is the rule rather than
the exception among the Mollusca. He finds (1) exceedingly minute and
numerous ‘eyes’ on the outer surface of the shell in both univalves and
bivalves; (2) large and solitary ‘eyes’ in the shell substance; (3)
eyes on the mantle lobes in both univalves and bivalves; (4) eyes on
the opercula.

[295] _Mitth. Stat. Zool. Neap._ v. p. 447 ff.

[296] W. Patten, _Mitth. Zool. Stat. Neap._ vi. (1886) pp. 546,
605 f.

[297] _Benderloch_, p. 136.

[298] _Quart. Journ. Micr. Soc._ xx. p. 443.

[299] _Quart. Journ. of Conch._ i. p. 368.

[300] _British Conchology_, i. p. xxviii.

[301] _Science Gossip_, 1865, p. 259.

[302] _Mollusques de France_, i. p. 130.

[303] _E.g._ Sochaczewer, _Zeits. wiss. Zool._ xxxv. p. 30.

[304] _Zool. Anz._ 1882, p. 472.

[305] _Zoologist_, iv. p. 1266.

[306] _Journ. Mar. Biol. Ass._ N.S. i. p. 217.

[307] Moquin-Tandon, _Moll. de France_, i. p. 133.

[308] _Zool. Jahrb. Anat._ iv. (1890) p. 501.

[309] Baudon, _Rév. Mag. Zool._ 1852, p. 575.

[310] _Arch. Zool. Exp. Gén._ (2) v. 1887, p. 2; compare also C.
H. Hurst, _Natural Science_, ii. pp. 360, 421.

[311] Compare Pelseneer, _Bull. Sci. Fr. Belg._ (3) xix. pp. 107,
182.

[312] Pelseneer, _Arch. Biol._ viii. p. 723.

[313] Also known as _labial_ and _supra-oesophageal_ ganglia.

[314] Wivén, however (_K. Sv. Vet. Ak. Handl._ xxiv. 1892, No.
12), describes transverse connectives in _Chaetoderma_.

[315] στρεπτός, twisted; εὐθύς, straight.

[316] With the exception of _Actaeon_, which is streptoneurous
(Bouvier, _Comptes Rendus_, cxvi. p. 68).

[317] This fusion of the cerebral and pleural ganglia and the
consequent union of the cerebro-pedal and pleuro-pedal commissures
can be recognised by sections of the mass (Pelseneer, _Comptes
Rendus_, cxi. p. 245).

[318] There is practically no pharynx in the Pelecypoda, the mouth
opening directly into the oesophagus.

[319] _Radere_, to scrape; ὸδούς, tooth; φέρειν, to carry.

[320] The mechanism of the radula has been dealt with by Geddes,
_Trans. Zool. Soc._ x. p. 485. Rücker has observed (_Ber.
Oberhess. Gesell. Nat. Heilk._ xxii. p. 207) that the radula in
_Helix pomatia_ is the product of five rows of cells; the use
of the first row is uncertain, the second forms the membrane of the
radula, while rows three to five originate the teeth.

[321] _Jahrb. Deut. Malak. Gesell._ iii. p. 193.

[322] The whole of the radulae and jaws figured in this work are
taken from the original specimens in the collection of the Rev. Prof.
H. M. Gwatkin, who has always been ready to give me the run of his
cabinets, which probably contain the finest series of radulae in the
world. To his kindness I owe the following description of the process
of mounting: “The first step is to obtain the radula. Dissection is
easy in species of a reasonable size. On opening the head from above,
so as to lay open the floor of the mouth, the radula itself is seen in
most of the marine species, though in others it is contained in a sort
of proboscis; and in the Pulmonata and others the student will find
the buccal mass, with commonly a brown mandible at its front end, and
the lingual ribbon in its hinder part. The teeth may be recognised by
their silvery whiteness, except in a few cases like _Patella_ and
_Chiton_, where they are of a deep brown colour. When obtained,
the radula may be cleaned by boiling in a solution of caustic potash.
There is no risk of injury if the solution is not too strong.

“Smaller species may be treated more summarily. The proboscis, the
buccal mass, or even the whole animal may be thrown into the potash
solution and boiled till scarcely anything is left but the cleaned
radula. Remains of animals dried inside the shell may be similarly
dealt with, after soaking in clean water. With a little care, this
process will answer for shells down to the size of _Ancylus_ or
_Rissoa_. The very smallest (_Carychium_, _Tornatellina_, _Skenea_,
etc.) must be crushed on the slide and boiled on it, after removing as
much as possible of the broken shell. The radula can then be searched
for under the microscope, and washed and mounted on the slide.

“The student must be warned that though the general process is simple,
there are difficulties in particular cases. In the Pulmonata, for
example, membranes on both sides of the radula need careful removal.
_Murex_, _Purpura_, and most of the _Taenioglossa_ have the side teeth
folded down over the central, so that the arrangement is not well seen
till they have been brushed back. The Cones, again, have no basal
membrane at all, so that if the potash is not used with great care,
the single teeth will fall asunder and be lost. Perhaps the worst case
is where a large animal has a radula as small as that of a _Rissoa_,
like _Turritella_, _Harpa_, or _Struthiolaria_, or where the radula is
almost filmy in its transparency, like those of _Actaeon_ and the small
_Scalaria_.

“When once the radula is laid out, the mounting is commonly easy.
Canada balsam makes it too transparent. Fluids may be used, and are
almost necessary for thick radulae like those of large _Chitons_;
but the best general medium is glycerine jelly. It runs under the
cover glass by capillary attraction, and may be boiled (though only
for a moment) to get rid of air bubbles. It should then be left
unfinished for several weeks. If cracks appear, the reason is either
that the jelly is a bad sample, or that it has been boiled too long,
or (commonly) that the object is too thick; and there is not often
any difficulty in remounting. I have no serious complaint of want
of permanence against the medium, if I may speak from a pretty wide
experience during the last twenty years.”

[323] The substance both of the jaw and radula is neither crystalline
nor cellular, but laminated. Chitin is the substance which forms
the ligament in bivalves, the ‘pen’ in certain Cephalopoda, and the
operculum in many univalves. Neither silica nor keratine enter into the
composition of the radula.

[324] τόξον, arrow; ῥάχις, ridge, sharp edge; ταινία, ribbon; πτηνός,
winged; γυμνὀς, bare; ῥιπίς, fan; δοκός, beam.

[325] _V. concinna_, according to Schacko (_Conch. Mitth._ i.
p. 126, Pl. xxiv. f. 5); the lateral is large, strong, unicuspid on a
broad base.

[326] In some cases (_e.g._ _Hyalinia inornata_) the laterals
are very few, while in _Zonites laevigatus_ the first side tooth
is more of a marginal than a lateral.

[327] Semon, _Biol. Centralbl._ ix. p. 80.

[328] According to Moquin-Tandon (_Moll. de France_, i. p. 44)
this process in _Bithynia_ is attached by one end to the wall of
the stomach. _Vivipara_, with two jaw pieces, does not possess
this stylet; _Bithynia_, which does possess it, has no jaw.

[329] J. H. Vanstone, _Journ. Linn. Soc._ xxiv. p. 369.

[330] _Biol. Centralbl._ vii. p. 683; _SB. Ges. Nat. Fr._
1890, p. 42; _Mag. Nat. Hist._ (2) v. 1850, p. 14.

[331] νεφρός, kidney.

[332] _Ann. Mag. Nat. Hist._ (2) xvi. p. 298.

[333] See, for instance, _Quart. Journ. Conch._ i. p. 340
(_Cyl. Raveni_): _Jahrb. Deut. Malak. Gesell._ 1879, p. 98
(_Clausilia dubia_).

[334] Cailliaud, _Journ. de Conchyl._ vii. p. 231; Gassies,
_ibid._ p. 44.

[335] _Arch. Naturgesch._ xlii. p. 209.

[336] Dr. W. B. Carpenter, _Rep. Brit. Ass._ xiii. p. 71; xiv. p.
1; xvii. p. 93; J. S. Bowerbank, _Trans. Micr. Soc._ i. p. 123;
Ehrenbaum, _Zeit. wiss. Zool._ xli. p. 1.

[337] See also p. 258.

[338] J. E. Gray, _Phil. Trans._ 1833, p. 774 f.

[339] J. E. Gray, _Phil. Trans._ 1833, p. 774 f.

[340] _Journ. de Conchyl._ iv. p. 424.

[341] _Journ. de Conchyl._ xii. p. 3.

[342] T. Scott, _Journ. of Conch._, 1887, p. 230.

[343] M. de Villepoix, _Comptes Rendus_, cxiii. p. 317.

[344] _Proc. Ac. Nat. Sc. Phil._, 1892, p. 350.

[345] Mr. B. B. Woodward has recently pointed out (P. Z. S. 1892,
p. 528) a very remarkable method of shell absorption and growth in
_Velates_ and certain other Neritidae.

[346] The only exception appears to be _Pedipes_, while in _Cassidula_
and _Scarabus_ the absorption is partial (Crosse and Fischer, _Journ.
de Conch._ xxx. p. 177 f.).

[347] _Strombus_ and _Pteroceras_ (see Fig. 99, p. 200) exceptionally
develop a siphonal notch which is distinct from the anterior canal.

[348] The _columella_, as distinct from the _columella lip_, is the
solid pillar of shell round which the whorls are coiled (Fig. 177), the
lower, or anterior portion of which alone is usually visible.

[349] J. E. Gray, _Phil. Trans._ 1833, p. 812.

[350] W. H. Dall, _Amer. Journ. Sc._ xxxviii. p. 445 f.

[351] The term _epidermis_, as distinct from _periostracum_,
is properly restricted to the outer layer of the skin of the
_mantle_ and body generally.

[352] J. Lewis, _Proc. Bost. Soc._ vi. p. 149.

[353] _Journ. of Conch._ v. p. 66.

[354] _The Dispersal of Shells_, pp. 182–195.

[355] E. A. Smith, _P. Z. S._ 1892, p. 259.

[356] C. T. Musson, _Proc. Linn. Soc. N. S. Wales_ (2), v. p. 883.

[357] _Scient. Results Sec. Yarkand Exped._ “Mollusca,” pp. 1–16.

[358] Mr. H. W. Kew, _The Dispersal of Shells_, has brought
together a very large series.

[359] _The Naturalist in Nicaragua_, p. 334 f.

[360] Morelet, _Journal de Conch._ 1875, p. 194.

[361] Pollonera, _Boll. Mus. Zool. Torino_, v. 1890, No. 87.

[362] South and south-western France, however, belong to the
Mediterranean Sub-region.

[363] The coast-line of north-east China, including Corea and Japan to
north Niphon, is much more definitely tropical than the adjacent inland
districts. The coast-line, therefore, must be placed in the Oriental
Region, while the inland districts belong to the Palaearctic Region.

[364] _Biol. Centralbl._ ii. p. 208.

[365] Craven, _Journ. de Conchyl._ (3) xxviii. p. 101.

[366] _Jahrb. Deutsch. Malak. Gesell._ viii. p. 278.

[367] Netchayeff, _Kazan Soc. Nat._ xvii. fasc. 5.

[368] _Fauna der Congerien-Schichten_, p. 142.

[369] _Streptaxis_ is a remarkable instance of a _mainland_
genus. Although abundant in the Oriental, Ethiopian, and Neotropical
regions, it never seems to occur on any of the adjacent islands,
except in the case of Trinidad (1 sp.), which is practically mainland.
_Omphalotropis_, on the other hand, is the exact reverse of
_Streptaxis_ in this respect, occurring all over Polynesia and the
Malay Is., as far west as Borneo, as well as on the Mascarenes, but
never, save in a doubtful case from China, on the mainland of Asia,
Australia, or Africa.

[370] The Amboyna group has been much the better explored. Common to
both groups are one sp. each of _Kaliella_, _Trochomorpha_,
_Opeas_, _Leptopoma_, _Cyclotus_, _Helicina_.

[371] A. H. Cooke, _P. Z. S._ 1892, pp. 447–469.

[372] Mysol, with 2 _Chloritis_, 1 _Insularia_, 1 _Cristigibba_, is
decidedly Papuan.

[373] See especially C. Hedley, Note on the Relation of the Land
Mollusca of Tasmania and New Zealand, _Ann. Mag. Nat. Hist._ (6)
xiii. p. 442.

[374] Hedley and Suter, _Proc. Linn. Soc. N. S. Wales_ (2), vii.
p. 613. Twenty-one species are “introduced.”

[375] Nine species have been introduced: 6 from Europe, 2 from the West
Indies, 1 from the Western Isles.

[376] It is by no means implied that _unbroken_ land communication
between India and Madagascar, across the Indian Ocean, ever existed.
A series of great islands, whose remains are attested by the Chagos
and other banks, would be quite sufficient to account for the results,
as we find them. See especially Medlicott and Blanford, _Geology of
India_, vol. i. p. lxviii.

[377] _Journ. Cinc. Soc. Nat. Hist._ iii. p. 317. The number is
doubtless susceptible of very considerable reduction, say by one-half
at least.

[378] Simpson, _Amer. Nat._ xxvii. 1893, p. 354.

[379] Compare von Martens, _Malak. Blätt._ 1868, p. 169; von
Ihering, _Nachr. Deutsch. Malak. Gesell._ 1891, p. 93.

[380] The distribution of some Pteropoda has been worked out by Munthe,
_Bih. Svensk. Ak. Handl._ XII. iv. 2, by Pelseneer _“Challenger”
Rep._, Zool. xxiii., and by Boas, _Spolia Atlantica_.

[381] _Bull. Mus. C. Z. Harv._ xiv. p. 202; xxiii. p. 34 f.

[382] See papers in _P. Z. S._ 1878–85.

[383] A break in this uniformity may be found underneath the course
of a great oceanic current like the Gulf Stream, which rains upon
the bottom a large amount of food. A. Agassiz (_Bull. Mus. C. Z.
Harv._ xxi. p. 185 f.) explains in this way the richness of the
fauna of the Gulf of Mexico as compared with that of the west coast of
tropical America.

[384] On the western coasts of Europe and America, where the change
in surface temperature is very gradual, _Purpura lapillus_ (the
west American ‘species’ are at best only derivatives) is able to creep
as far south as lat. 32° (Mogador) in the former case, and lat. 24°
(Margarita Bay) in the latter, the mean annual temperature of the
surface water being 66° off Mogador, with an extreme range of only 8°,
and that of Margarita Bay 73°, with an extreme range of only 5°. On
the eastern coasts, where the Pacific and Atlantic gulf-streams cause
a sudden change of temperature, the _Purpura_ is barred back at
points many degrees farther north, _viz._ at lat. 41° (Hakodadi),
surface temperature 52°, extreme range 25°; and at lat. 42° (Newhaven),
surface temperature 52°, extreme range 30°.

[385] E. A. Smith, _P. Z. S._ 1890, pp. 247, 317.

[386] A. H. Cook, _Ann. Mag. Nat. Hist._ (5) xviii. (1886) p. 380
f; E. A. Smith, _P. Z. S._ 1891, p. 391 f.

[387] C. Keller, _Neue denksch. Schw. Gesell._ xxviii. 1883, pt. 3.

[388] According to Tate (_Trans. Roy. Soc. S. Austr._ 1887–88,
p. 70), ‘Australian’ species predominate at Freemantle (32°), but
Tenison-Woods (_J. Roy. Soc. N. S. Wales_, xxii. p. 106) holds
that the tropical fauna extends as far south as Cape Leeuwin (34°), and
that the Australian forms are not predominant until the extreme south.
Tenison-Woods regards Cape Byron (31°) as the limit of the tropical
fauna on the east coast, while some characteristic tropical genera
reach Port Jackson, and a few (_e.g._ _Cypraea annulus_)
Tasmania.

[389] A full account of the distribution of _Voluta_ is given by
Crosse, _Journ. de Conchyl._ (3) xix. p. 263.

[390] Usually known as ‘Patagonian,’ but since the Magellanic
Sub-region includes a considerable part of Patagonia, and since the
greater part of sub-region (6) lies out of Patagonia, it has been
thought advisable to change the name.

[391] _Amer. Nat._ xx. p. 931.

[392] W. H. Dall, _Proc. Biol. Soc. Washington_, v. p. 1 f.

[393] _Trans. Connect. Acad._ v. p. 177; _Zoologist_, 1875,
p. 4502.

[394] _Rep. Scotch Fish._ iii. 1885, App. F, p. 67.

[395] _Nautilus_, vi. 1892, p. 82.

[396] _Journ. Mar. Zool._ i. pp. 3, 9.

[397] _Rep. Brit. Assoc._ 1844, Transactions, p. 74; _P. Z.
S._ 1839, p. 35.

[398] It is convenient, but not morphologically correct, to apply the
terms ‘ventral’ and ‘dorsal’ in this sense.

[399] φραγμός, partition; σήπιον, cuttle-bone; χόνδρος, long cartilage.

[400] μυέω, close the eyes; ὕψις, sight; contrasted with Oigopsidae
(οἰγω, open).

[401] The classification is that of Foord, _Catal. Fossil Cephal.
Brit. Mus._, 1888.

[402] Saville Kent, _Proc. Roy. Soc. Queensland_, vi. p. 229.

[403] J. Power, _Ann. Mag. N. H_. (2) xx. p. 334; _P. Z. S._
1836, p. 113; _Arch. Zool. Exp. Gén._ (3) i. 1893, p. 105.

[404] In deference to Bergh’s high authority, the position of a
sub-order is here given to the Ascoglossa. It may be doubted whether
that position will stand the test of further investigation, and
whether the families concerned will not be added to the Cladohepatic
Nudibranchs.

[405] This family has also been classified with the Bulloidea and with
the Aplysioidea.

[406] It appears more convenient to treat the whole group together,
rather than deal with the two sections separately.

[407] An operculum is said to exist in the young forms of
_Auricula_ and _Parmacella_.

[408] _Proc. Ac. Philad._ 1892, p. 390.

[409] Compare Jackson, _Amer. Nat._ xxv. p. 11 f.

[410] “A Monograph of the British Fossil Brachiopoda,”
_Palaeontographical Society_, London, vols. i.-v. 1851–84.

[411] _Ibid._ vol. vi. 1886.

[412] “Contributions to the Anatomy of the Brachiopoda,” _Proc. Roy.
Soc._, vol. vii.

[413] “Untersuchungen über den anatomischen u. histologischen Bau der
Brachiopoda Testicardinia,” _Jenaische Zeitschrift_, vol. xvi.,
1883.

[414] “On a living Spinose _Rhynchonella_ from Japan,” _Ann.
Mag. Nat. Hist._, 5th ser., vol. xvii., 1886

[415] _Loc. cit._ p. 465.

[416] Shipley, “On the Structure and Development of Argiope,” _Mitt.
aus d. Zool. Stat. zu Neap._ Bd. iv. 1883.

[417] Schulgin, “Argiope Kowalevskii,” _Zeit. f. wiss. Zool._ Bd.
41, 1885.

[418] _American Jour. of Sci. and Arts_, 3rd series, vol. xvii.
1879.

[419] _Loc. cit._ p. 470.

[420] “Recherches sur l’Anat. des Brachiopodes Inarticules,” _Arch.
Zool. Exp._ (2), Tome iv., 1886.

[421] “Untersuchungen über den Bau der Brachiopoden,” Jena, 1892.

[422] “Vorläufige Mittheilungen über Brachiopoden,” _Zool. Anz._
Bd. viii. 1885.

[423] Hancock’s nomenclature is here used. The corresponding names used
by King and Brooks are placed in brackets. Their nomenclature is used
by many palaeontologists, and is adopted in Fig. 322.

[424] _Development of the Brachiopoda_, 1873 (Russian).

[425] “Histoire de la Thécidie,” _Ann. d. Sci. Nat._, Sér. 4, vol.
xv., 1861.

[426] “On the Early Stages of Terebratulina septentrionalis,” _Mem.
Boston Soc. Nat. Hist._, vol. ii., 1869. “On the Development of
Terebratulina,” _Ibid._ vol. iii., 1873.

[427] “Choses de Nouméa,” _Arch. d. Zool. exp. et gen._, 2nd ser.,
vol. ix., 1891.

[428] J. Barrande, _Syst. Silur. Bohème_, vol. v., 1879. Hall and
Clarke, _Introd. Palaeozoic. Brach._ (_Palaeont. of New York_,
1892–1894). Davidson, _Monogr._ _Brit. Foss. Brach._ (_Palaeont. Soc._,
1851–1884). Waagen, _Salt Range Fossils_ (_Mem. Geol. Surv. India_,
1879–1885).

[429] The results of the investigations of King (_Ann. Mag. Nat.
Hist._, 4th ser., vol. xii., 1873) and of Brooks (_Chesapeake Zool.
Laboratory, Scientific Results_, p. 35, 1879), and the simple
nomenclature of these authors are here followed in preference to those
of others, owing to the difference of opinion amongst anatomists of the
functions and homologies of the muscles. The lateral muscles enable the
valves to move backwards and forwards on each other; the centrals close
the shell; the umbonals open it; and the transmedians allow a sliding
sideways movement of one valve across the other (see also p. 477).

[430] Davidson and King, _Quart. Jour. Geol. Soc._, xxx. (1874),
p. 124.

[431] _Amer. Jour. Science_, 1890–1893.


Transcriber’s Notes:

1. Obvious printers’, punctuation and spelling errors have been
corrected silently.

2. Where hyphenation is in doubt, it has been retained as in the
original.

3. Some hyphenated and non-hyphenated versions of the same words have
been retained as in the original.

4. Superscripts are represented using the caret character, e.g. D^r. or
X^{xx}.

5. Italics are shown as _xxx_.

6. Bold print is shown as =xxx=.