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Title: Earthworms and Their Allies

Author: Frank E. Beddard

Release date: July 30, 2013 [eBook #43363]

Language: English

Credits: Produced by Chris Curnow, Mary Akers and the Online
        Distributed Proofreading Team at http://www.pgdp.net (This
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Produced by Chris Curnow, Mary Akers and the Online
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Transcriber's note:
    Spelling and punctuation inconsistencies have been harmonized.
    Obvious printer errors have been repaired. Italic text has been
    marked with _underscores_. The symbol for male has been
    rendered as [M] and for female as [F].




    The Cambridge Manuals of Science and
    Literature

    EARTHWORMS AND THEIR ALLIES




    CAMBRIDGE UNIVERSITY PRESS
    London: FETTER LANE, E.C.
    C. F. CLAY, MANAGER

  [Illustration]

    Edinburgh: 100, PRINCES STREET
    London: WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
    Berlin: A. ASHER AND CO.
    Leipzig: F. A. BROCKHAUS
    New York: G. P. PUTNAM'S SONS
    Bombay and Calcutta: MACMILLAN AND CO., LTD.

    _All rights reserved_




    EARTHWORMS AND
    THEIR ALLIES

    BY

    FRANK E. BEDDARD

    M.A. (Oxon.), F.R.S., F.R.S.E.

    Cambridge:
    at the University Press

    New York:
    G. P. Putnam's Sons

    1912


    Cambridge:
    PRINTED BY JOHN CLAY, M.A.
    AT THE UNIVERSITY PRESS

_With the exception of the coat of arms at the foot, the design on
the title page is a reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_.




PREFACE


The importance of earthworms in questions relating to geographical
distribution is so universally admitted that it seemed to me
convenient to embody in a short volume the principal facts.

It became necessary in order to accomplish this task in an adequate
fashion to preface the distributional facts with some anatomical and
zoological data. I have reduced this section of the book to a minimum
and I trust that the illustrations will enable the reader, who is
not specially acquainted with the structure of these animals, to
obtain an idea of their general features and variability in external
character and internal anatomy. While the use of technical terms is
inevitable in presenting such details, it will be found, I think,
that a reference to the figures will render them intelligible.

Since this volume mainly deals with the phenomena of distribution, I
have included in my survey nearly all of the usually admitted genera
of worms, particularly of the terrestrial forms, which are in the
light of our present knowledge the more important in considering this
subject.

    F. E. B.

    ZOOLOGICAL SOCIETY OF LONDON.
    _December_, 1911.




CONTENTS


    CHAP.                                                     PAGE

      I.  Structural and Systematic                              1

     II.  Mode of Life                                          43

     III. The external features of Earthworms and their
              relation to habit and environment                 59

     IV.  Sense Organs and Senses of Earthworms                 64

     V.   Relative frequency of Earthworms in different
              regions of the World                              70

     VI.  Peregrine forms                                       96

    VII . The Earthworms of Oceanic Islands                    109

    VIII. Movement and Migration among Earthworms              113

     IX.  The Geographical Distribution of Earthworms          129

          List of Literature referring to Earthworms           144

          Index                                                146




CHAPTER I

STRUCTURAL AND SYSTEMATIC


The group of segmented, bristle-bearing, worms, termed Oligochaeta by
zoologists, comprises what are popularly known as earthworms together
with certain forms, inhabitants of ponds, lakes and rivers, which are
not so well known as to have received a more distinctive name than
merely 'worms.' Their next allies are apparently the leeches and--a
little more remote--marine bristle-bearing worms termed Polychaeta;
the three groups, together with perhaps a certain number of other
forms belonging to smaller groups, constitute the Annelida which are
a distinct and separate assemblage of invertebrate animals.

The most interesting features about these Oligochaetous worms
are their very great anatomical variation and the facts of their
distribution over the globe. Their importance as geological agents
in levelling the ground was made known a long time ago by Darwin,
and that aspect of earthworms has remained in much the same position
as Darwin left it. We shall concern ourselves here only with the
structure, habits, and range of the earthworms and their immediate
allies, the aquatic Oligochaeta. These three aspects of the animals
dovetail into each other more thoroughly than is the case with some
other groups. This is due to the fact that they have of late years
been very thoroughly studied from the anatomical and distributional
side. So lately as 1889, M. Vaillant in a very comprehensive treatise
was only able to enumerate 369 species, of which a large number were
but incompletely differentiated, and some are no longer admitted.
There are at the moment of writing perhaps 1500 species, the vast
majority of which are well known owing to careful investigation.
Furthermore there are but few parts of the world, and these are
not of large area, from which earthworms at any rate have not been
gathered. Though there can be no doubt that a very considerable
number of species await discovery, it would seem that we are in
possession of information which is not likely to be seriously
affected by future researches.


THE ANATOMY OF EARTHWORMS.

Although it is not contemplated to make the present volume a guide
to the structure of this group of worms, it is necessary to give
some little anatomical sketch of the group in order first of all to
illustrate their diversity of structure, secondly to give reasons
for the classification of them, and thirdly to enable the reader to
realise certain structural details which it is absolutely necessary
to give some account of in order to explain other matters.

It is for example impossible to attempt any account of the fitness of
some of these animals for their terrestrial life and of others for an
aquatic life without treating of anatomy to some extent.

I shall take one particular species as a type and indicate later the
principal divergencies shown by other forms. According to the general
opinion among those who have studied the Oligochaeta I take as a
representative form a Megascolecid (this and the other families are
dealt with _seriatim_ on p. 14 et seq.), as this group is presumed to
be the oldest, and within that group a representative of the genus
_Notiodrilus_ which is with some reason held to be the most primitive
genus in the group. Finally I have no particular reason for selecting
the species _Notiodrilus tamajusi_ except that there happens to be a
longer and fuller description of it than of many.

  [Illustration: Fig. 1. _Notiodrilus tamajusi._ The worm shown
  from the ventral surface. About natural size. (After Eisen.)]

This earthworm is a native of Guatemala and is some six inches in
length with a diameter of perhaps a quarter of an inch. The front
part of the body is thicker than posteriorly. The body will be seen
to be divided into some 218 rings by circular furrows which run right
round the body. These divisions are termed segments or somites. At
the head the mouth is surrounded by the first of these, and on the
dorsal surface of that segment is a projection like an incomplete
segment which is known as the prostomium. From the XIIIth segment to
the middle of the XXth the body has a different appearance, and this
region is known as the clitellum. Each of the 218 segments of the
body except the first, and possibly one or two at the hinder end, is
furnished with eight minute projecting bristles, the setae; these are
disposed in pairs and all lie upon the ventral aspect of the worm.
The movement of these by special muscles aids in locomotion.

  [Illustration: Fig. 2. The same species; first two segments and
  prostomium shown from ventral surface, I prostomium, II first
  segment without any setae, III second segment with its four pairs
  of setae (the dorsalmost seta of each outer pair is not visible
  in this view).]

  [Illustration: Fig. 3. A section through the body of the same
  species showing the ventral position of the pairs of setae.
  (After Eisen.)]

An examination with even a hand lens shows a number of external pores
which are important. Anteriorly there is the mouth which is overhung
by the prostomium referred to above. At the extreme hind end--and
surrounded by the last segment of the body--is the vent. Along the
middle line of the back are a series of pores, one just at the very
anterior edge of each segment, through which, when the worm is dried
and then slightly pressed, liquid is seen to be ejected. These are
called the dorsal pores and they belong one to each segment with the
exception of the first seven, or--in some cases--more, segments.
In front of one or other of the pair of setae which is situated
most laterally, _i.e._ furthest from the ventral median line, is an
orifice on each side in all but the first one or two segments of the
body. These paired pores are the external outlets of the excretory
organs frequently termed on account of their regular repetition
with the segments 'segmental organs,' but more conveniently to be
named nephridia. In the clitellar region and in fact on each of the
segments XVII, XVIII, XIX are a pair of pores of which those on the
XVIIIth segment are the least conspicuous. The large pair of pores on
each of segments XVII and XIX occupy the position of the ventralmost
pair of setae, which are here absent, or rather replaced by a very
long curved and ornamented seta, which projects out of the orifice.
These two pairs of pores are the outlets of the prostatic glands as
they have been termed. The minute pair of pores on segment XVIII do
not take up the position of the ventral setae; for these are present
and to the inside of each pore. A groove, shaped something like a
reversed 3 or the Greek letter [Greek: S], connects the orifices of
each side of the body, the middle part of the groove, where the two
semicircular halves of which it is composed meet, coinciding with the
minute pores on segment XVIII which are the orifices of the sperm
ducts.

  [Illustration: Fig. 4. The same species. Ventral view of segments
  XVI-XX (numbered in the figure) which form the clitellum, the
  posterior boundary of which is shown by a curved line on segment
  XX. The figure will be understood from the annexed description.]

On segment XIV are a pair of very minute pores a little in front of
the ventralmost setae and thus very near together. These are the
openings of the oviducts. Finally, near to the anterior border-line
of segments VIII and IX and on a line with the ventral pair of setae
is a pore on each side through which the cavity of the spermathecae
reaches the exterior.

  [Illustration: Fig. 5. A longitudinal section through the middle
  of the first nineteen segments of the body of _Notiodrilus
  vasliti_ (a species very closely allied to that described in the
  text), the segments are numbered I, II, &c. _D.v._ dorsal blood
  vessel, _G_ gizzard, _H_ hearts, _oe_ oesophagus lying in front
  of gizzard behind which another tract of still narrow oesophagus
  is seen opening into _Int._ intestine. The whole alimentary canal
  is supported by the intersegmental septa (_Pr.c._) between which
  is the system of spaces forming the coelom, _ov.d._ pore of
  oviduct, _Splh._ orifices of spermathecae, [M] orifice of sperm
  duct, _o_ ovary, _t_ spermaries. (After Eisen.)]

So much then for the external characters of our worm. We next turn
to the internal anatomy. When the worm is opened by a longitudinal
section from end to end, and the two flaps of skin are turned
outwards and pinned down, the internal structure is almost completely
revealed. Running from end to end is seen the alimentary canal;
the general cavity of the body (coelom) in which it lies, as do of
course the other organs to be enumerated, is seen to be divided by
cross divisions, the intersegmental septa, into a series of chambers
which correspond with the external division into segments. The
septa are in fact inserted on to the body-wall along the furrows
which mark the divisions between adjacent segments. Anteriorly
the large pharynx is responsible for confusing the arrangement of
the septa, which become subdivided and fused or are prolonged a
greater way backwards and thus present a less obviously segmental
disposition. Certain of the more anteriorly placed of these septa
are much thicker than the rest. This is the case with the septa
which separate segments V to XII. The alimentary canal is perfectly
straight and runs in the middle line, being supported by the septa
which it perforates. The mouth leads into a buccal cavity which later
becomes the pharynx, a portion of the tube which is much thickened
by muscular walls dorsally. Then follows a very short section of
the oesophagus and in the fifth segment this becomes the gizzard,
a very characteristic organ with thick muscular walls quite smooth
and with a very thick lining of structureless membrane. After this
is a narrower tube, the rest of the oesophagus. Into this open in
each of segments VII, VIII, IX a pair of calciferous glands; these
are diverticula of the gut with much folded walls, the cells of
which secrete carbonate of lime. In the XIIth segment or so, the
oesophagus suddenly widens out to form the intestine which runs as
such to the end of the body. This wider tube has a ridge running
along its dorsal side, the typhlosole. Along the dorsal surface of
the intestine and the oesophagus is seen a red tube, contractile
during the life of the worm, which is the dorsal blood vessel and
whose contained blood is coloured red, as is the blood of vertebrated
animals, by haemoglobin. But in the earthworm the colouring matter
is not situated in corpuscles as in the vertebrate. The dorsal
vessel is connected by a few pairs of equally contractile transverse
trunks with a ventral vessel which is not contractile. There are
other branches of these main longitudinal trunks and some minor
longitudinal trunks which we shall not stop to describe further.
The nervous system of the worm consists of a pair of ganglia which
lie above the gut in the third segment; they are connected by a
commissure running round the gut with a chain of closely fused pairs
of ganglia, one for each segment to the very end of the body. In each
of the segments, except the first two or three, there are a pair of
excretory organs known as nephridia; these are essentially coiled
glandular tubes opening on to the exterior by the regularly placed
pores already referred to in considering the external characters. The
tube ends in a funnel-shaped, and therefore dilated, mouth, which
opens into the segment in front of that which contains the rest of
the organ; a nephridium therefore lies in two segments. The only
other important organs which are left for consideration are those
devoted to the reproduction of the species. The essential organs are
the spermaries and the ovaries. Of the former there are two pairs
of minute whitish bodies which lie in segments X and XI on either
side of the nerve cord attached to the anterior septal wall of their
segments. The ovaries are not in the following, but in the XIIIth,
segment, and occupy an identical position in that segment. A short
tube with a funnel or trumpet-shaped and wide orifice opens into the
cavity of the XIIIth segment opposite to each, and, perforating
the septum, opens on to the exterior on the XIVth segment. A similar
but larger and more folded pair of trumpet-shaped funnels opens in
the same way opposite to each spermary. But in this case the two
tubes of the sperm ducts run backwards for some way and those of
each side after joining open on to the XVIIIth segment by the pores
already mentioned. On the XVIIth and XIXth segments open two glands
which are long and tubular in form and much coiled. These are the
spermiducal glands and each opens in common with a muscular sac
containing the long and ornamented seta referred to in describing
the various external orifices. It will be noticed that the sperm
duct has no direct connection with these glands but only indirectly
through the external gutter which connects the three male orifices
of each side of the body. Segments IX-XII inclusive contain certain
sacs which depend from, and are formed as outgrowths of, the septa
of those segments. These are the sperm sacs in which the male germ
cells undergo their development. A corresponding body (but very much
smaller) is sometimes found in relation to the ovary but has not
been actually described in the particular species dealt with here.
Finally, in segments VIII and IX are a pair (that is four altogether)
of roundish sacs, with two or three minute diverticula, known as the
spermathecae. In the diverticula of these sacs are stored the sperm
derived from another individual.

  [Illustration: Fig. 6. Genital region of _Maoridrilus dissimilis_
  (in which species the different organs are essentially as in
  _Notiodrilus tamajusi_). The oesophagus is cut away between the
  Xth and XVIIIth segments to display the various organs. _n_
  external orifices of nephridia which alternate in different
  segments; the one lettered opens in front of dorsalmost pair of
  setae, that of segment XVI opens in front of ventralmost pair,
  _p_ coiled glands opening on to XVIth and XIXth segments, _sc._
  sacs containing long seta, associated with these glands, _t_
  spermary of segment; another pair in an exactly similar position
  in segment XI. Behind spermaries are funnel-shaped openings
  of sperm ducts which are seen running along the body to their
  external orifice on XVIIIth segment. _v.s._ sperm sacs behind the
  posterior of these and in segment XIII is seen the large ovary
  corresponding in position to the spermary and opposite to each
  ovary the oviduct. (x 3.)]

  [Illustration: Fig. 7. A spermatheca of _Notiodrilus tamajusi_.
  The external orifice is shown at _o_; above this are the
  diverticula, _s_ is a portion of an intersegmental septum. (After
  Eisen.)]

This completes the general sketch of the structure of _Notiodrilus
tamajusi_ which we have selected as a type. In this same genus are a
large number of species which differ from that selected in various
small structural points. Thus in _N. annectens_ (Beddard), a species
from New Zealand, the spermaries and ovaries are attached to the
posterior, instead of to the anterior, wall of their segments, and
there are neither calciferous glands nor modified setae upon segments
XVII and XIX. In all essentials however the two types agree and are
thus to be looked upon as referable to the same genus. Starting from
the structure of these types we may now sketch in quite a brief way
the main divergencies of structure shown in the group of Oligochaeta.

We shall naturally begin with the family Megascolecidae of which a
type has just been described.

Within the limits of the same sub-family as that which contains
_Notiodrilus_, _i.e._ the Acanthodrilinae, the changes of structure
affect all the principal organs of the body except the nervous
system, but are not very large and vary from genus to genus. They
are mainly perhaps in the direction of reduction and simplification.
Thus in _Chilota_, _Maheina_ and _Yagansia_ the spermaries are
reduced to one pair in either the Xth or XIth segment, while in
_Yagansia_ one pair of spermathecae and of spermiducal glands have
also disappeared. In _Microscolex_ the spermaries remain normal, but
one pair of spermathecae and of spermiducal glands have disappeared,
the remaining organs of these series being in the IXth and XVIIth
segments respectively. In _Microscolex_, _Chilota_ and _Yagansia_,
moreover, there is a further degeneration in the disappearance of the
calciferous glands. These glands are often absent and sometimes less
developed in the New Zealand _Maoridrilus_, which is otherwise not a
degenerate form and differs characteristically from _Notiodrilus_
by the fact that the paired nephridia alternate in position in
successive segments, being now in front of the dorsal, and in other
segments in front of the ventral, pairs of setae. While these genera
are somewhat degenerate, the New Zealand _Plagiochaeta_ has undergone
specialisation in an upward direction. For the setae of each segment
are increased to a large number much exceeding eight.

It is not a long step to the sub-families Diplocardiinae and
Trigastrinae. In the first of these, an American race confined to the
northern and central parts of that continent, the male pore shows
a tendency to move backwards, being situated on any of segments
XVIII-XXI. The two spermiducal glands follow it, but are always
placed one pair in front and one behind, as in _Notiodrilus_. In this
group we get a new feature of specialisation in the duplication or
triplication of the gizzard.

So too with the Trigastrinae where there are either two or three
gizzards; but in this sub-family another modification has become
apparent. The paired nephridia have disappeared and their place
is taken by several, often quite numerous, pairs of much smaller
nephridia called on that account 'micronephridia' instead of
'meganephridia.' To this sub-family belong the especially African but
also American and Malayan _Dichogaster_, whose name is derived from
the important fact that it possesses two gizzards.

Not far off is to be placed another sub-family, that of the
Octochaetinae, which is New Zealand and Indian in range, the
intermediate countries being, strange to say, not populated by this
race of Oligochaeta. The group contains several genera of which
_Octochaetus_, _Eutyphoeus_, and _Dinodrilus_ are the best known.
All these worms agree in the main features of their anatomy with
_Notiodrilus_; but they have diverged in different directions. Thus
in _Octochaetus_ the typical two pairs of gonads and glands belonging
to the generative system have been retained, while the nephridial
system consists of micronephridia; in _Eutyphoeus_, one pair of
spermiducal glands has disappeared, and as a general rule the species
of this genus have only one pair of spermaries and the corresponding
pair of sperm ducts. They are close to _Octochaetus_. The third
genus mentioned, _Dinodrilus_, is a New Zealand form specialised in
possessing 12 setae in each segment. Otherwise it is not far removed
from _Octochaetus_.

  [Illustration: Fig. 8. Ventral view of _Eutyphoeus masoni_. _p_
  papillae, [M] male pores, [F] oviduct pores. (x 3.)]

A fifth sub-family is also easily referable to the type whose
structure has been dealt with as a preliminary to the present survey.
That sub-family is the Ocnerodrilinae which is American and African
in range. These worms are somewhat degenerate in comparison with
their allies. Thus the calciferous glands are reduced to a single
pair or to a single gland in the IXth segment, the nephridia though
regular and paired have no covering plexus of blood vessels, and
the worms themselves are slender and delicate, being indeed often
aquatic in habit. The spermiducal glands, which are as in the former
sub-families independent of the sperm ducts though sometimes opening
in common with them into a short pocket-like ingrowth of the skin,
are reduced in their minute structure and much simpler than in the
other types.

The genus _Kerria_ is the least reduced perhaps. It has the male
pores on segment XVIII with a pair of spermiducal glands on the
segments preceding and following this in the typical Acanthodriline
fashion. There are two pairs of spermathecae in VIII and IX,
but the spermaries are reduced to one pair in X. The gizzard is
present. OCNERODRILUS is a little further reduced from this last.
The gizzard has gone; there is but one pair of spermiducal glands
(as a rule) opening in common with the extremity of the male duct
on to segment XVII; the spermathecae also are reduced to one pair,
but there are two pairs of spermaries. The African _Nannodrilus_ is
more robustly built. There are two gizzards, the male duct opens
into a muscular pouch, into which also open one of the two or three
pairs of spermiducal glands. There are two or three other genera
and sub-genera not showing any great divergencies from the range of
structure indicated in briefly defining those enumerated above.

Finally, we have the largest of all the sub-families of this family,
viz. that of the Megascolecinae. These worms are mainly tropical in
range and also mainly found in the Indo-Australian portion of the
world. In them the sperm ducts open in common with the usually single
pair of spermiducal glands and prevalently upon the XVIIIth segment.
The glands moreover have not always, though they often have, the
tubular form shown in all the sub-families hitherto considered. In
many forms they are branched and lobate glands, and if there are two
pairs one may be of one type and the other of the second and derived
type, as for instance in _Megascolex ceylonicus_. Furthermore, it is
much commoner among the genera of this sub-family for the setae to
become numerous and to spread right round the segment; this condition
is seen in the genera _Pheretima_, _Megascolex_, _Diporochaeta_,
_Perionyx_, _Plionogaster_. The spermathecae also are commonly more
than the typical two pairs of the forms already considered, and in
certain species (for example _Pheretima hexatheca_) there are as many
as six pairs of those organs which are moreover--and in this they
resemble the majority of species of the last sub-families--nearly
always furnished with a diverticulum or diverticula. The nephridia
are either paired or numerous and these various characters allow of
the sub-family being split up into sixteen genera or thereabouts.

As an example of another type of organisation and as contrasting with
_Notiodrilus_ we may now briefly describe the structure of the genus
_Pontoscolex_ (better known as _Urochaeta_), a member of the American
and African family Geoscolecidae.

The worm is some four inches long and composed of as many as 212
segments. Each of these except the first has eight setae which for
the first few segments of their occurrence are disposed in four pairs
in the usual fashion. Behind this point however the setae cease to
present this symmetrical arrangement and are irregularly disposed
so that a given seta is not in the same line with the corresponding
seta of the segments in front or behind. There is thus no region of
the body which has not a seta implanted upon it; and the effect is
therefore comparable to the condition obtaining in those worms, such
as _Pheretima_, where circles of numerous setae are met with. There
are however only eight in a given segment. The clitellum extends
from segment XV to XXII or XXIII and is developed only dorsally and
laterally. It has setae like the rest of the body; but those upon
the clitellum are longer and more markedly ornamented than are those
of the body generally. The latter are not only sculptured with
fine ridges but are bifid at their free extremity. The prostomium
is often apparently completely absent. It is however really present
but is retractile. As to the pores which are visible externally the
dorsal pores are completely absent. The pores of the nephridia lie in
front of the dorsal pair of setae or in a line corresponding to the
position of those setae where the arrangement has become irregular.
The spermathecal pores are three pairs and are in the very front
of segments VII, VIII, IX. The male pores, very inconspicuous, lie
on the ventral side of segment XXI just within the clitellum. The
oviducal pores are on segment XIV.

As to internal anatomy the general plan of the segmentation shows
no great differences. Certain septa only show a difference, _i.e._
those separating segments VI-XI which are specially thickened. In the
alimentary canal a gizzard in segment VI is to be noted and three
pairs of calciferous glands in segments VII-IX. The nephridia are
paired structures and commence early. The first two or three segments
are occupied by a pair of large glands opening into the buccal
cavity which are apparently a slightly modified pair of nephridia
and are generally termed 'peptonephridia' since they are associated,
as it would appear, with the function of alimentation and are not
purely excretory organs. There is but a single pair of spermaries in
segment XI, and of ovaries in segment XIII. The sperm ducts open on
to the exterior in the position already mentioned and they are not
associated at their pore with any glands comparable to spermiducal
glands. A pair of sperm sacs depend from segment XI and traverse a
considerable number of segments, being thus long and tongue-shaped
instead of short and limited to one segment. The spermathecae are
three pairs of elongated sacs in segments VII-IX, without any
diverticula at all.

It will appear therefore that many and considerable differences
divide _Pontoscolex_ from _Notiodrilus_ and indeed from all of the
Megascolecidae whose structure has been touched upon in the foregoing
pages. The most important of these are the ornamented setae and their
arrangement and the modification of the setae upon the clitellum:
the absence of diverticula to the spermathecae: the absence of
terminal glands associated with the male ducts. Although taken in
their entirety these characters are distinctive of the American
Geoscolecidae (sub-family Geoscolecinae), there is no one of them
which is not to be found in some Megascolecid. Thus the subgenus
_Ilyogenia_ (of _Ocnerodrilus_) has sometimes no spermiducal glands:
the genus _Perionyx_ has spermathecae without diverticula in some
species, and other genera of Megascolecinae are in a like condition.
The setae of _Dichogaster_ are sometimes ornamented, while in
_Pheretima houlleti_ the clitellar setae are different from those
upon the other segments.

We can in fact only define the family Geoscolecidae by an assemblage
of characters which are mainly these: dorsal pores absent, only a
few in the neck region being occasionally present; setae generally
ornamented, those of the clitellum being larger and more marked than
the others; spermathecae without diverticula; often instead of a
pair of those pouches in the segment a large number of very small
sacs, as in _Microchaeta_, _Kynotus_. Sperm ducts without terminal
glandular or muscular sac, except in a few cases; setae always eight
in a segment except in the genus _Periscolex_ which has acquired
the 'perichaetous' condition. The range of variation shown in the
anatomy of the Geoscolecidae will be best taken in connection with
the several sub-families into which it has been subdivided. In the
first of these, the Geoscolecinae, no great differences divide the
genera from that selected as the type, viz. _Pontoscolex_. The long
sperm sacs attain to an extraordinary length in _Trichochaeta_ (or
_Hesperoscolex_) where the single pair extends through no less than
109 segments. Though as a general rule the sperm ducts open directly
on to the exterior they do so through the intermediary of a large
pouch in _Glossoscolex_ (= _Titanus_). In _Onychochaeta_ the setae on
the last segments of the body are very much enlarged and thus form a
more effective means of holding on to the soil than is possessed by
other species.

The sub-family Hormogastrinae which contains but a single genus
_Hormogaster_ is remarkable for being limited in range to the
Mediterranean coasts. The genus is mainly distinguished by
possessing three gizzards; otherwise it is not very different
from the sub-family just described. The African and Madagascar
forms are associated (together with a few Asiatic forms) into a
third sub-family Microchaetinae. These worms frequently possess a
considerable number of very small spermathecae in segments XII, XIII
or thereabouts instead of the usual paired arrangement. They have too
very often glands connected with the enlarged setae already mentioned
which are however (in the genus _Kynotus_ at any rate) usually in
front of the clitellum. The latter organ, contrary to what we find
among the Geoscolecinae, is often behind the point of orifice of the
male pores. This is so with _Kynotus_.

  [Illustration: Fig. 9. Ventral view of _Polytoreutus
  Kilindinensis_, head end (x 2). _p_ papillae, [M] male pore, [F]
  female pore.]

The last sub-family, Criodrilinae, has but three genera _Criodrilus_,
_Sparganophilus_ and _Alma_. These worms do not show any very marked
differences from other Geoscolecids. _Alma_ is noteworthy for the
facts that the male pores are borne upon long processes of the body
which bear specially modified setae and that one species at any rate
has gills.

  [Illustration: Fig. 10. Ventral view of _Polytoreutus finni_,
  head end (x 2), lettering as in fig. 9.]

Another type of structure is offered by the Eudrilid earthworms
which form rather a restricted family. These worms are as a rule
quite easy to distinguish by their external characters only. For the
apertures of the spermathecae and sperm ducts are apt to be very
large and conspicuous. They are also generally unpaired, a character
which is however not confined to the Eudrilidae; for there are
Megascolecids, such as _Fletcherodrilus_, and Geoscolecids in which
the same unpaired character occurs. The principal feature of the
family is that the ovaries are commonly enclosed in sacs--comparable
to the sperm sacs which frequently envelop the spermaries in
other earthworms--and that these sacs not only contain the mouths
of the oviducts but are directly continuous with the single or
double spermatheca. This is usually a large sac, always single or
consisting of one pair only, which opens on to the exterior close
to the oviducal pores; these spermathecae in the Eudrilidae are not
comparable to the spermathecae of other earthworms; for they are in
a way comparable to the sperm sacs, being formed as outgrowths of
the septa. There is some variation of structure within the family.
In a number which are associated into a sub-family Eudrilacea there
are two paired calciferous glands and a single unpaired one, while
the paired nephridia open by a large pore on to the exterior. In a
parallel sub-family, the Pareudrilacea, the calciferous glands are
apt to be more numerous and have a totally different structure: they
have been apparently converted into non-digestive glands bearing some
relation to the vascular system. The nephridia moreover do not open
on to the exterior by single pores, but form a network within the
thickness of the body wall and then open by numerous pores. There is
however no resemblance here to the micronephridia of _Dichogaster_
and other Megascolecids. In _Libyodrilus_ (as an example of the
Pareudrilacea) each nephridium forms a network out of the duct
leading to the exterior. In the interior of the body a series of
paired meganephridia are visible.

  [Illustration: Fig. 11. Organs of reproduction of _Eudriloides
  durbanensis_.]

The earthworms of Europe belonging to the family Lumbricidae
offer again a rather different type of structure, which is more
reminiscent of the Geoscolecidae than of the Megascolecidae or
Eudrilidae. In this family there are no glands appended to, or in
the neighbourhood of, the orifices of the sperm ducts, such as are
found in the other forms. As in the Geoscolecidae the clitellum is
furnished with setae somewhat different in form from those which
deck the body generally. These setae are never more than eight in a
segment. Dorsal pores (absent in Geoscolecidae and in Eudrilidae)
are invariably present. The spermathecae are without appendices and
nearly always simply paired, though rarely we get numerous much
smaller spermathecae in a single segment, as in _Kynotus_ among the
Microchaetine Geoscolecids. Internally the most striking feature of
this family is to be seen in the position of the gizzard at the end
of the oesophagus and at the beginning of intestine. The apertures
of the male pores are--save for two or three exceptions where they
are further forward--invariably upon the fifteenth segment, and
the clitellum, often very long, usually begins behind this point,
features which are also seen in _Kynotus_.

Finally we have the Moniligastridae which differ from all the
types hitherto considered in a few rather important particulars.
These worms are named on account of the fact that they possess
several gizzards upon the oesophagus, a character which is however
met with in the Megascolecid genus _Plionogaster_ and in certain
Eudrilids, _e.g._ _Hyperiodrilus_. The main peculiarity of the
family is that the sperm ducts are very short and open on to the
next segment to that which contains the spermaries, as in the
water-living Oligochaeta generally. The terminal sac into which the
male ducts open is moreover rather like that of such a family as the
Lumbriculidae.


THE AQUATIC FAMILIES OF OLIGOCHAETA.

It would seem to be quite possible that when the fresh waters of the
world have been as well examined for Oligochaeta as have so many
parts of the land areas, the number of purely aquatic Oligochaeta
will be found to equal those inhabiting the land. In any case we
are quite justified at the present moment in stating that there are
rather more families of these smaller Oligochaeta than there are of
the bulkier terrestrial forms. But while there are certainly seven or
eight distinct families, these do not between them contain at present
so many genera as do the fewer families of earthworms; and the number
of species of the latter that are known to science enormously exceeds
that of the 'Limicolae' as the fresh-water worms were at one time
called in common. The fact that there are purely marine forms of
these water worms was hardly appreciated at the time that the term
Limicolae was used; now however we are acquainted with a few such
forms, and even with some which live at will in either fresh, salt,
or brackish water. Of these something will be said later.

These forms have also been collectively treated of as Microdrili, a
term which expresses the undoubted fact that they are all of small
size and sometimes even minute; others however reach the dimensions
of the smaller species of earthworms. There are a certain number of
characters shared by the various families which may be considered
first of all, before dividing them into their several subdivisions.
These aquatic Oligochaetes are usually tender and transparent, the
muscular layers of the body wall being much reduced as compared with
the tougher terrestrial forms. The clitellum is also thinner and
consists of a single layer of cells only, thus contrasting with the
double layered clitellum of earthworms. As a rule the alimentary
tract is simplified, there being no gizzard or glandular appendices
of the oesophagus comparable to the calciferous glands of most
earthworms. But this rule is not without exceptions; for we find in
_Haplotaxis_ a gizzard occasionally developed, and in the remarkable
genus _Agriodrilus_ from the Baikal lake a continuous gizzard
formation along the oesophagus, while the Enchytraeidae may show
something very like calciferous glands: and even a Tubificid, called
by Pierantoni _Limnodriloides_, has a pair of diverticula of the gut.

Other internal organs show certain points of likeness in all or in
the great majority of the aquatic families. Thus the nephridia are
without a plexus of blood capillaries surrounding them, a state of
affairs which also occurs in some of the slender Ocnerodrilinae
among the earthworms. These paired organs also are very frequently
not found in the anterior segments of the body and these include
also as a general rule the segments in which the reproductive
elements are formed. Save for an exceptional case among the genera
of Enchytraeidae the dorsal pores are not found among the Limicolae,
but in some cases at least a single pore, the head pore, is found.
The sperm ducts, which among earthworms usually (and indeed always
save in the anomalous Moniligastridae) traverse a considerable
number of segments on their way from the internal opening into the
body-cavity to the external pore, do not show the same phenomenon
among the Limicolous Oligochaeta. They are sometimes indeed limited
to a single segment, that is to say the funnels and the external
pore lie in one segment. In other cases they open on to the exterior
in the segment next to that which bears the funnel, and it is only
rarely that they traverse more than one segment. There are also
points of difference of general applicability to be noted in the
sperm sacs and egg sacs. The latter are large and extensive, which
is not the case among earthworms, and the former are as a rule more
extensive in the number of segments that they occupy than among the
terrestrial forms. Another difference which they show is that their
cavity is quite simple and not divided up by trabeculae into numerous
intercommunicating chambers as in the earthworms. Finally the eggs of
the aquatic Oligochaeta are large and full of yolk and thus contrast
with the very small ova of the earthworms which are moreover much
more abundant. These features are either of general or universal
occurrence and together form an assemblage of characters which mark
out the aquatic families of Oligochaeta from their, at least mainly,
terrestrial allies.

We may also refer to certain structures which although not universal
among these aquatic families are nevertheless found only in
them--that is, are not found in any family of the terrestrial worms
of this order. The most salient of such characters are the long and
hair-shaped setae tapering to a fine point and often provided with
a series of delicate branches like a feather; such setae are often
of very great length and they occur in their various modifications
among the Aeolosomatidae, Naididae, and Tubificidae. It is clear
that these delicate setae, though they may not be due in any way to
the aquatic life, are rendered possible by it. To drag such tender
processes through stiff clay would surely break and tear them out.
It may be also mentioned that among the aquatic families as a rule
the intersegmental septa do not show that thickening in some of the
anterior segments of the body which is so general a feature of the
land-dwelling species. Finally it is only among the aquatic forms,
and among them only in the families Aeolosomatidae and Naididae, that
asexual reproduction by budding takes place. Indeed so common and
usual is it in the genera of these families that even yet there are
considerable lacunae in our knowledge of the organs of reproduction
in the said families.

Together with these general similarities are many points of
structural difference among the worms inhabiting ponds, lakes, and
rivers, which allow of their being divided into a number of quite
distinct families.

One of the most distinct of these families and lying in any case
quite at the base of the series is the family Aeolosomatidae which
includes a number of distinct species of delicate and transparent
worms, and in whose integument are embedded round cells bearing a
large brightly coloured oil drop; this is reddish or green in colour,
or--and this more rarely--colourless, but still recognisable as an
oil drop. The green sometimes even verges upon blue on the one side
and yellow on the other, while the red may approach brown or purple.
These worms are assigned for the most part to the genus _Aeolosoma_
which is found in all of the great continents and of which seven
or eight species are known. To a more doubtful genus _Pleurophleps_
are assigned a few small worms which have the general appearance of
_Aeolosoma_, but are without the coloured or colourless oil drops
in the skin. These worms have a very large prostomium which is
ciliated on the lower surface, and the body is not markedly segmented
externally by constrictions or internally by septa. The bristles are
slender and hair-like, but among them are in some species the shorter
stouter bristles bifid at the free tip, which are so general among
the aquatic families of the Oligochaeta. These worms are not uncommon
objects in pools containing weed; and they are to be found usually
crawling among the weed. They consist as a rule of but few segments
to most of which a pair of nephridia belong. The ovaries and the
spermaries are only known in a few forms and appear to be unpaired
and lie respectively in the fifth and sixth segments. There are 1-3
pairs of spermathecae, and the sperm ducts if distinct from, are at
least very like, nephridia. The ova appear to make their way to the
exterior by a large aperture in the ventral middle line of a middle
segment of the body. The vascular system contains uncoloured blood
and is greatly simplified.

The next family to the Aeolosomatidae in zoological position is
clearly the Naididae. These are also small worms, but show in some
respects a higher grade of organisation than their allies. While
asexual generation is general, the reproductive organs are more
commonly found than in _Aeolosoma_, though there are still many
hiatus in our knowledge of the same in certain genera. Where they are
known it has been found that the spermaries and ovaries are very far
forward in the body, in the fifth and sixth segments respectively.
The spermathecae are in segment six and the male ducts open into a
terminal chamber, called 'atrium,' which is on the whole not unlike
that of the Tubificidae. The blood in these worms is red as in the
higher types, and thus differs from that of the genus _Aeolosoma_.
The setae are rather varied, being in some cases long and slender,
sometimes greatly exaggerated in length as in _Ripistes_; other setae
are forked at the free end, and in _Paranais_ this is the only kind
of setae met with. A marked feature of this family is that the dorsal
bundles of setae do not always begin like the ventral setae upon the
second segment of the body. Indeed in _Schmardaella_ there are no
bundles of dorsal setae at all. The Indian genus _Branchiodrilus_
is remarkable for the fact that it has paired processes of the body
which may be termed gills and which in some segments involve the
dorsal setae. Another kind of gill is found in the genus _Dero_
(which has many species) and in the allied _Aulophorus_. These
are placed round the vent, and are ciliated. Other genera are
_Nais_, _Chaetogaster_, _Vejdovskyella_, _Amphichaeta_, _Stylaria_,
_Macrochaetina_, _Pristina_, _Naidium_.

Several genera, _Pristina_, _Nais_, _Dero_, are found in many parts
of the world; but it is not possible at present to consider very
seriously the facts of their geographical distribution.

Next to the Naids a group of aquatic worms present themselves for
consideration which are usually placed in three distinct families,
which families are however rather hard to define. These three
families are the _Tubificidae_, _Phreodrilidae_, and _Lumbriculidae_.
The Phreodrilidae were at one time placed with the Tubificidae by
Michaelsen. It is now perhaps the general opinion that they form a
family of their own, at any rate since the discovery of two other
genera _Phreodriloides_ and _Astacopsidrilus_, besides the original
genus founded by myself, and named _Phreodrilus_ from the fact that
the species was found in a deep well in New Zealand.

The distribution of this family especially of the genus _Phreodrilus_
is extremely interesting. The genus _Phreodrilus_ is the only one
genus of the aquatic Oligochaeta which has, like _Notiodrilus_, a
circumpolar range, the pole being the south pole. It occurs in New
Zealand, in Kerguelen, and, if we are to accept the opinions of Drs
Michaelsen and Benham that my genus _Hesperodrilus_ is to be merged
in _Phreodrilus_, in Patagonia also.

In this genus the male pores are upon the XIIth segment while the
spermaries lie in segment XI. Moreover the spermathecae lie behind
the male pores. In the Tubificidae on the other hand it is at least
the rule for the spermaries and male pores to be pushed a segment
further forwards. And the spermathecae are before the male pores.
_Phreodriloides_ is like _Phreodrilus_ but has no spermathecae.
It is also New Zealand in range. _Astacopsidrilus_ is Australian
and is semi-parasitic upon the Crayfish _Astacopsis_, whence its
generic name. _Phreodrilus branchiatus_ is one of the few forms of
Oligochaeta that possesses gills. Of these there are a series of
pairs on about the last eleven segments of the body. They are lateral
in position, and thus contrast with the also gilled _Branchiura
sowerbii_, where the gills, also on the posterior segments of the
body, are more numerous and lie dorsally and ventrally, a pair to
each segment.

The Tubificidae differ from the Phreodrilidae mainly in the points
already noted. There are a considerable number of genera of which
the following are the best known, viz., _Tubifex_, _Limnodrilus_,
_Limnodriloides_, _Branchiura_, _Lophochaeta_, _Ilyodrilus_,
_Psammoryctes_, _Clitellio_, _Telmatodrilus_, _Bothrioneuron_,
_Lycodrilus_.

The Tubificidae are mainly northern temperate forms, and a few of
them such as _Clitellio arenarius_ and '_Peloryctes inquilina_[1]'
are found on the sea coast. There are also a few of this family in
the southern hemisphere. These forms include _Clitellio abjornseni_
from Australia, and a few species of _Branchiura_ from New Zealand
and the islands of the Antarctic ocean. There is also to be mentioned
_Rhizodrilus_ (or _Vermiculus_) _aucklandicus_ from the island of
that name in the New Zealand area. The only tropical species appears
to be the Indian and Malayan _Bothrioneuron iris_, though this genus
also occurs in Europe and in southern South America. It is quite
likely however that _Branchiura sowerbii_, a species known at present
from tanks in hot houses, may be a tropical American species.

  [1] With many synonyms, including _Tubifex ater_ (see p. 53).

The family Lumbriculidae is yet more restricted in its range. It has
not yet been met with away from the temperate northern hemisphere,
and the great variety of species recently described from Lake Baikal
by Dr Michaelsen is a very remarkable fact. The Lumbriculidae are
entirely fresh water in habit and not even partially terrestrial.
The following are the principal known genera: _Lumbriculus_,
_Trichodrilus_, _Rhynchelmis_, _Phreatothrix_, _Claparedilla_,
_Stylodrilus_, _Mesoporodrilus_, _Sutroa_, _Eclipidrilus_,
_Aurantina_, _Athecospermia_, _Lamprodrilus_, _Teleutoscolex_.

In the worms of this family the male pores are usually upon the tenth
segment but sometimes upon the eighth or eleventh. And in addition to
a pair of funnels in the antecedent segment there is also commonly a
second pair in the same segment as that which contains the external
pore. The two sperm ducts however open into the same distended atrial
cavity before opening on to the exterior. In _Lamprodrilus_ however
each sperm duct opens by its own separate atrium on to the exterior
and in two consecutive segments. In _Teleutoscolex_ there is but one
pair of funnels opening into the same segment with the atrial pore.

Near perhaps to the Lumbriculidae comes a very small family indeed,
that of the Alluroididae. So small is it that it consists of but
a single genus _Alluroides_ of which there are two species _A.
pordagei_ and _A. tanganyikae_. Both of these species were described
by myself, one of them from but a single specimen, the other from
only two. Both species--and the name of one denotes the region--are
from eastern tropical Africa. The Tanganyika worm is purely aquatic;
the other species was found in a swamp of the mainland opposite to
Mombasa. This remarkable genus has a pair of spermaries in segment
X; but the ovaries are as in earthworms in the XIIIth segment. The
male pores are upon that segment also, _i.e._ further back than
in the aquatic worms. The spermathecae open close to the median
dorsal line of the body in one species; in _A. tanganyikae_ there is
but one spermatheca which opens actually in the dorsal median line
between segments VIII and IX. This family seems to come nearest to
the Lumbriculidae but has also obvious points of likeness to the
terrestrial Moniligastridae. It fully deserves a separate family,
which was founded for it by Dr Michaelsen.

Not obviously related to any of the other families of Oligochaeta is
the family Enchytraeidae. This consists of a very large number of
species, which are placed in about a dozen genera, and whose habitat
is nearly confined to the cold and temperate regions of the world. A
large number of species for example have been described by Dr Eisen
from Alaska, while others inhabit the verge of Patagonia. It is only
a few which are found in warmer countries. There is for instance a
solitary Enchytraeid in India and the neighbouring parts of Asia
described by myself as _Henlea lefroyi_ but doubtfully of that genus
according to Dr Michaelsen. I have also myself obtained another
Enchytraeid from British Guiana. Apart from such rare exceptions the
family is arctic or temperate in its range and is even met with upon
the ice of glaciers. These little worms--they are very rarely of more
than minute size--are both aquatic and terrestrial and inhabitants of
the sea shore. They have four bundles of short often straight and
somewhat stumpy setae; _Anachaeta_ is entirely without setae. That
they bear some relation to the lowest group of Oligochaeta, that of
the Aeolosomas and Naids, appears to be shown by the very anterior
position of the spermathecae, _i.e._ in the fourth or fifth segment.
The spermaries and ovaries on the other hand are in segments XI and
XII. They are peculiar among the aquatic families in having complex
glands appended to the oesophagus which recall the calciferous glands
of the earthworms. The funnel of the sperm duct is extraordinarily
deep and lined with glandular cells except in an apparently primitive
genus from Lake Baikal.

The remaining family of the Limicolae is that of the _Haplotaxidae_
which contains two genera, viz., Haplotaxis, better known as
_Phreoryctes_, and _Pelodrilus_. These two genera overlap somewhat
in their characters and it is in the present state of our knowledge
rather difficult, if indeed possible, to differentiate them
thoroughly. They are slender worms which frequent either damp
earth or water and are thus somewhat intermediate in habit between
the Limicolae and the Terricolae. The chief peculiarity of their
structure lies in the fact that the sperm ducts are unprovided with
any kind of terminal apparatus whatever, but open directly upon
the exterior. Moreover there are generally two pairs of testes in
segments X and XI, and in some species two pairs of ovaries in the
two following segments. The small family is very widely distributed
in more temperate regions, principally of the antarctic hemisphere.
It occurs for instance in New Zealand, South Australia, the Cape, and
in the northern hemisphere in Europe, Western Asia, and North America.




CHAPTER II

MODE OF LIFE


We have now completed a brief survey of the general characters of
the group of the Oligochaeta which will at least serve to impress
upon the reader the fact that these animals are somewhat diverse in
structure, and that even as regards outward appearance it is not
difficult to distinguish a large number of different types. These
facts become all the more remarkable when we reflect upon the very
similar conditions which surround all the species of earthworms.
It is not clear how far the influence of the soil differs in a
tropical forest in South America and in Central Africa. With
divergent external conditions anatomical differentiation becomes more
accountable. But in this case we have a lavish anatomical variation
which would appear to have no connection with any kind of need that
we can as yet appreciate. Comparing the Terricolous Oligochaeta with
other large groups of the animal kingdom, all or nearly all the
members of which lead a closely similar life, such as the frogs and
toads, or the land mollusca, or snakes, we get a much wider range of
structural change in the Oligochaeta than in any of these.

We shall now consider their mode of life and their relations to the
environment.

The mode of life of earthworms seems at first sight to need no
special chapter or section. They simply live in and beneath the soil,
leaving it at times to wander over the surface especially at night
and during wet weather. But there are a number of species which
habitually lead an aquatic life and whose characteristics therefore
demand consideration.


AQUATIC EARTHWORMS.

Although it is perhaps somewhat of a contradiction to speak of
aquatic earthworms the collocation of words will serve to emphasise
the fact that there are species of Oligochaeta belonging to the tribe
Megadrili or terrestrial group, which are as purely aquatic in their
habits as is a _Tubifex_ or _Limnodrilus_. In such cases we may
fairly assume rather a return to an aquatic life than the persistence
of such a habit. For we do not find among these genera and species
much evidence of particular resemblance with the purely aquatic
familes of Oligochaeta. It is therefore particularly interesting to
examine into the characteristics of these water-living genera; for
we may expect to be able to deduce from them some hints as to what
characters are really to be associated with the purely aquatic life.
We can in fact hope to differentiate between adaptive and fundamental
characters in these animals.

These secondarily aquatic species can be referred to two categories.
There are examples of particular species which differ from their
congeners in being aquatic; and there are whole genera, even
sub-families, which are exclusively, or very nearly so, aquatic
in habit. The former division need not detain us; for the actual
occurrence of the worms in fresh water instead of upon dry land
may be a temporary affair and not a mark of habitual sojourn. Thus
I myself found the British and European earthworm _Eiseniella_
(_Allurus_) _tetraedrus_ in the River Plym in Devonshire, while it
has been generally met with upon land. The Patagonian and Falkland
Island species _Notiodrilus aquarum dulcium_ was so called on
account of its having been collected in fresh water. But its near
ally _N. georgianus_ (which is perhaps even identical with it) was
found on the sea shore in the same region of the world. While the
differences which the small species of _Notiodrilus_ shows from other
purely terrestrial members of the same genus are trifling, further
information may prove that this case is on all fours with that of
_Eiseniella_ referred to above. There are plenty of similar instances
which I shall not enumerate.

We may now therefore pass on to the second category. These examples
are obviously much more important because they are of worms which
appear to be wholly aquatic, or very nearly so, and which belong to
definite genera easily distinguishable as such from their allies. The
examples are not however very numerous. And they belong practically
exclusively to the family Geoscolecidae, a family which, it will be
seen later, is confined to South America, South Africa, Madagascar,
certain parts of India and Burmah and of Europe. It is not a family
which has reached the greater part of the East or which has been
carried to the Antarctic parts of the globe. It is furthermore very
important to bear in mind that there are reasons for regarding this
family Geoscolecidae as one of the more modern branches of the
Oligochaeta; this latter statement tends to prove that the aquatic
life is, as already suggested, a secondary matter in these worms, and
is not due to their belonging to an ancient race which has never left
the waters of the land.

A very interesting fact offers itself first of all in considering
this family of earthworms. The Geoscolecidae are one of the only
division[2] of the Oligochaeta terricolae which are generally found
to be without those characteristic series of pores in the middle line
of the back known as the dorsal pores. They are thus eminently suited
for an aquatic life; for it is to be supposed from the fact that the
purely aquatic 'Limicolae' are also without these pores that their
existence is prejudicial to a water-living worm. Indeed the entrance
of water into the body-cavity would presumably be dangerous to the
worm. The Geoscolecidae are thus already marked out, as it were, for
an aquatic life. No modification is here necessary for them. It is
also to be noted in this connection that various species of the genus
_Notiodrilus_ to which reference has been made as a partly aquatic
genus have no dorsal pores. They too are thus fitted for at least an
amphibious life.

  [2] In the Eudrilidae also these pores are very frequently absent.

The rule however regarding the absence of dorsal pores in the
Geoscolecidae is not absolute. A few species and among them two
species at any rate of the aquatic genus _Sparganophilus_ have a few
pores between some of the anterior segments which have been spoken of
as 'neck pores.' They are not, it is to be believed, of a different
nature from the generally distributed dorsal pores of other worms but
are in fact limited to the 'neck' region.

There are no other obvious characters of the family Geoscolecidae as
a whole which might be regarded as fitting them for a purely aquatic
life.

Of this family one entire sub-family, the Criodrilinae, is very
nearly purely aquatic in habit. Two genera, viz. _Callidrilus_ and
_Glyphidrilus_, out of another sub-family, Microchaetinae, which
contains perhaps five other genera, are also aquatic in their mode
of life. In examining into the characters of the various aquatic
species with a view to searching for common characters which might
be put down to modifications induced by the aquatic life, there
are two or three which arrest attention. In the first place the
Criodrilinae never possess a well-developed gizzard, having at most
a rudimentary gizzard, or even two. However this character is not of
universal applicability, for both _Callidrilus_ and _Glyphidrilus_
have got a gizzard and a strong one. These later genera however have
no calciferous glands or oesophageal pouches of any description,
structures which are also absent among the Criodrilinae. It will
be remembered that the purely aquatic families, Tubificidae,
Lumbriculidae, etc., rarely show signs of a gizzard and rarely
possess oesophageal pouches of any kind. In view of the fact that in
the case of a life in fresh water no calcareous salts are necessary
to resist the acids of the soil, and that the mud passed through the
alimentary canal is already finely divided, it is not surprising to
find gizzard and calciferous glands absent or rudimentary.

Another not unusual feature among these aquatic Geoscolecidae is the
quadrangular form of the posterior end of the body. This is shown--as
its specific name denotes--by _Glyphidrilus quadrangulus_, by species
of _Alma_ and in all the species of the genus _Criodrilus_. It is
to be noted in this connection that a species of the partly aquatic
_Eiseniella_ has been named _tetraedrus_ on account of precisely
the same phenomenon. In these cases it is the posterior part of
the body which is thus quadrangular; the anterior segments down to
the ninth in _Criodrilus_ being rounded in the usual Oligochaetous
fashion. As the paired setae are apt to lie in the four projections
of the quadrangular body, one is tempted to see in this arrangement
of structures a faint approach to the dorsal and ventral parapodia
of the marine worms, and in any case it seems possible that by this
means the worms can cling more effectively and continuously to the
stems and leaves of aquatic plants among which they so largely live.

It is a very remarkable fact that in the genera _Criodrilus_ and
_Alma_ the vent is dorsal in position instead of being surrounded as
in earthworms generally by the last segment of the body. This fact
might be put down to the near affinity between these two genera,
were it not for the fact that _Glyphidrilus_ shows precisely the
same state of affairs. These facts gain additional significance
in my opinion from the fact that among the leeches which are now
universally admitted to be allies of the earthworms the same position
of the vent is met with. This abnormal position of the end of the
alimentary canal may thus be fairly quoted in connection with
structures modified by the aquatic life.

Finally, and this seems to be very important, the only genus among
the Megadrili which possesses gills is the Nile worm _Alma nilotica_.


MARINE SPECIES.

There are a few, but relatively speaking very few, worms of the order
Oligochaeta which lead a marine life. And of these the majority
are shore forms not extending into the waters of the sea. The most
salient example, at any rate the best known perhaps, is the genus
_Pontodrilus_, the name of which fixes its habitat, and was naturally
given to it on that account. It was originally found on the sea
shore of the South of France, and I have myself examined examples
from Nice. The worm lives among bunches of sea-weed cast up by the
sea, and which are thoroughly salt. Besides the two forms that have
been met with in this Mediterranean region but which are united by
Dr Michaelsen into but one species, other _Pontodrilus_ have been
described from so many and such diverse parts of the world as the
following. The West Indies (Bermudas, Jamaica etc.), the coasts
of South America, of both West and Eastern Africa, the Red Sea,
Christmas Island near Java, Sharks Bay in West Australia, the Hawaii
Archipelago, Celebes, South West Australia etc. In fact there is no
great tract of the ocean excepting the antarctic region where this
genus is not to be found. It is possible however that this latter
statement is not correct and that New Zealand ought to be added.
But the species described from those islands, viz. _Pontodrilus
lacustris_, is not a marine form at all as its specific name denotes;
nor is it quite certainly to be included in the genus. On the other
hand a form from the Chatham Islands in the same quarter of the
globe, described originally as _Pontodrilus chathamensis_, is to
be referred to the antarctic region. Altogether some dozen species
of _Pontodrilus_ have been described by different naturalists; but
quite recently Dr Michaelsen has reduced these to three only, which
are _P. bermudensis_ (F. E. B.), _P. litoralis_ (Grube) and _P.
matsushimensis_ (Izuka), with the doubtful addition of _P. lacustris_
already referred to. Whatever may be the ultimate verdict upon this
question of species it is clear that the genus is widely spread upon
the sea shores of the world and that forms from different regions
show some fixed variations, which others may eventually agree with
their original describers in regarding as definite species.

It cannot be said that any salient characters in the organisation
of these worms mark them out from either terrestrial or fresh-water
Oligochaeta. There are no such important variations of structure as
can be seized upon to characterise them as inured to salt water.
The genus agrees with many aquatic forms in the fact that the
nephridia are not present in the earlier segments of the body, not
indeed putting in an appearance until about the thirteenth segment
or even later. They are thin delicate worms; but there is nothing
distinguishing about this, while the feeble or absent gizzard is
a character which is really difficult to correlate with habitat.
Still we have here a whole genus which is marine in its habit. Among
the Megadrili or earthworms there are not many other examples of
these 'euryhaline' forms as they have been named. On the shores of
Patagonia however and Kerguelen shore-living species of the mainly
antarctic genus of earthworms _Notiodrilus_ have been met with.
And there are a few allied cases among the antarctic genera of
Acanthodrilinae.

In addition to these terrestrial forms there are a few limicoline
genera which are partly marine in their habit. Thus several species
of the prevalently arctic and antarctic family Enchytraeidae are
shore living. There are also marine Tubificids such as _Clitellio
arenarius_ and _Tubifex ater_ (not uncommon on British shores),
marine Lumbriculids and a marine Naid from the Italian coast. These
forms show no great differences from their fresh-water allies.


EARTHWORMS ORIGINALLY PURELY AQUATIC ANIMALS.

The very name Earthworm, so distinctive as it is of the habitat of
these animals, seems to have been expressly invented in order to
crystallise into one word the remarkable distributions of these
creatures. They are with very few exceptions the most purely
terrestrial animals that we know. There are a few Mammals like the
mole and several underground Amphibians and Snakes in the tropics
which share this habitat with the worms, probably because they
chiefly prey upon them. But there is no group of animals that is
characterised by a subterranean existence in the way that earthworms
are. For we cannot put burrowing animals, such as the prairie dog
and many rodents, into the same category. These make and seek their
burrows for protective purposes and in order to bring forth their
young in security. They do not feed beneath the surface of the ground
or pass their entire lives in that situation. We have already in a
previous chapter dealt with such exceptional forms of earthworms as
do not lead an entirely subterranean existence; but as was pointed
out in chapter I these exceptions are but few and the immense bulk of
earthworms fully justify their name.

Nevertheless there are many arguments which tend to show that these
purely land-dwellers have grown out of exclusively water-dwellers and
even that the change from the one mode of life to the other has been
accomplished comparatively recently. For there are here and there
vestiges of structures which seem only fitted for an aquatic life;
and in other cases structural changes have commenced which would
appear to be in definite relation to the underground mode of life
prevalent to-day. Let us consider for a moment the differences which
obtain between the conditions of life in water or in soft mud at the
bottom of pool or river, and those which are undergone by a dweller
in stiff soil or vegetable débris. In the first case the medium is
fluid or at most very soft, while the soil is at least stiffer and
harder to traverse.

Secondly the transition between the very bottom of a pool and the top
layers of the water is more or less gradual, while the stiff soil
ends abruptly in the tenuity of the atmosphere.

A third point of difference is doubtless the smaller supply of
readily available oxygen in the still pools and even rapid rivers,
which in certain stagnant pools and in the bottom waters of deep
lakes must produce a very vast difference in physiological conditions.

We have already dealt with the characteristics of the aquatic genera
of earthworms, not only in detailing the general characters of the
families which are found in this situation but also in studying
the features which earthworms show in those cases where they have
reverted to an aquatic mode of life. It remains in the present
section to attempt to descry in the purely terrestrial forms the
remnants of adaptations to an aquatic life which are no longer of
service to them.

It is a noteworthy fact, that the continuous circle of setae which
is met with in certain earthworms is by no means a character of such
classificatory importance as it was at one time, perhaps, thought
to be. It is true that we meet with this character in the genera
_Megascolex_ and _Pheretima_ which are not very far from each other
in the system and are at any rate members of the same sub-family,
the Megascolecinae. But we also find the continuous circle of setae
well developed in _Plagiochaeta_ which is not so near to _Pheretima_,
and an approach towards it in _Dinodrilus_ and _Dinodriloides_ which
are equally remote perhaps from both _Pheretima_ and _Megascolex_
on the one hand and _Plagiochaeta_ on the other. Still it may be
urged that all of these genera are at least members of the family
Megascolecidae and that the question of a character which thus
merely shows affinity is not yet eliminated. It is therefore of
particularly great importance that Dr Cognetti de Martiis should have
met with the South American genus _Periscolex_ which, undoubtedly a
member of a totally distinct family, the Geoscolecidae, yet shows
the same complete circle of setae. The reason for dwelling upon this
particular anatomical character in the present connection is because
it would seem to be a character specially suited to an underground
life where there is an equal pressure all round the body and where
progression would seem therefore to be best attained by a continual
leverage round the circular body. And this view is strengthened by
the sporadic occurrence of this modification in different families.
We thus come to the conclusion that the opposite state of affairs is
a remnant of an aquatic life, a conclusion which it is the object of
the present section to discuss. More than this, it would seem that
an equal disposition of the two bundles of setae on each side of
the body was a less modified state of affairs than the restriction
of the two bundles or pairs of setae to the ventral surface, such
as occurs for example in the genus _Dichogaster_ and which is very
obvious in some of the larger-sized members of this extensive genus.
For the restriction of the setae to the ventral surface obviously
favours progression upon a surface and not through a medium. And
it is only among the terrestrial Oligochaeta that this mode of
progression occurs. It might also be urged, and with some reason,
that the retention of rather longer setae upon the clitellum in the
Lumbricidae and Geoscolecidae, and the possession of equally long
or in many cases much longer setae corresponding to one of the two
pairs of setae of the generative segment in certain Megascolecidae,
is a feature in which an aquatic condition--so to speak--is retained.
The setae would represent a vestige of the general presence of long
setae over the body generally such as is convenient or at least
not inconvenient to an Annelid living in water or soft mud. But
probably it will be thought the modified genital setae are a recent
development and not a retention.

There is no more thoroughly terrestrial family of earthworms than
that of the Moniligastridae and yet this family in its general
anatomical characters shows many points of likeness to aquatic forms
as has been now pointed out by many observers. It is true that these
characters are not those which might be associated at first sight
with an aquatic life. But none the less they are characteristic of
most of the families which live in the waters of the earth. Thus
_Moniligaster_ and its allies (_Eupolygaster_, _Drawida_, etc.) have
quite short sperm ducts which open on to the exterior at furthest in
the segment next to that in which their internal funnel lies. Again
the simple structure of the terminal gland into which they open and
which in its turn opens on to the exterior is very like that of such
a family as that of the Lumbriculidae. Another fact is the simple
undivided cavity of the sperm sacs which is unlike that of typical
earthworms but again like that of all of the Limicolous families.
We may fairly see in these worms evidence of origin from aquatic
ancestors. Evidence of the same nature, _i.e._ not as showing the
retention more or less of anatomical characters commonly associated
with a life in water, but as affording indirect evidence of an origin
from actually aquatic forms, is to be seen in certain members of
the families Geoscolecidae and Eudrilidae. In both of these it not
infrequently happens that the sperm sacs are but a single pair and
that that pair consists of sacs of extraordinary length. Thus in
_Trichochaeta_ (or _Hesperoscolex_) _barbadensis_ Miss Fedarb and I
have shown that the long thin sperm sacs extend through no less than
109 segments, which is vastly in excess of the length of those of
the majority of earthworms in which they are most commonly limited
to a single segment. In the same way the Eudrilid worm _Polytoreutus
magilensis_ has a pair of long and thin sperm sacs which extend
through some fifty segments. This elongation of the sperm sacs in the
ripe worms is a very common feature of the Limicolous genera.




CHAPTER III

THE EXTERNAL FEATURES OF EARTHWORMS AND THEIR RELATION TO HABIT AND
ENVIRONMENT


To the very inexperienced eye all earthworms might appear to be
quite similar in detail as they undoubtedly are in general form.
But it needs not a great deal of examination to detect even salient
characteristics whereby one kind may be distinguished from another;
to the expert it is possible in very many cases to go no further
than the outside before assigning its correct place in the system
to a given example. The general external features of this group
of worms have been already dealt with in another chapter. To some
of these we again direct attention in a more elaborate fashion in
order to emphasise the possible meanings of the variations met with
apart from their use in systematic arrangement. It is difficult to
say in comparing one worm with another what is the most salient
external difference. There are however a few which may be regarded
as equally conspicuous on a nearer examination of the specimens.
The varying position and greater or less extent of the clitellum,
the longer or shorter retractile or nonretractile prostomium, the
position of the usually conspicuous male pores, and the existence
of in the first place and--when present--the numbers and situation
of the so-called genital papillae are among the most obvious. The
setae and their position we treat of under the heading of the
modification of the worms to lead a terrestrial life; and though
these chitinous organs differ greatly they do not concern us in
the present section. The girdle or clitellum ('eminentia quasi
ulcerata') has been long observed as a character of these animals
and it is one which distinguishes them from all other worms except
the leeches and a very few marine Polychaeta. This modified region
of the body is often of a different colour to the rest and has a
glandular look which readily enables one to recognise its position
and limits, though its obviousness is less in some cases. It either
forms a complete ring round the body or is developed upon the dorsal
surface and only to a slight extent upon the ventral surface. Its
use, as is well known, is to secrete the cocoon in which the eggs are
deposited; and the epidermis which forms it is thickened and more
glandular than that in other regions of the body. Among earthworms
it is doubtful whether the clitellum ever occupies less than three
segments; it consists of three only in the great majority of species
of the marked genus _Pheretima_. From this lowest level it extends
in other forms, and in the partially aquatic African genus _Alma_
it may occupy as many as forty segments. The position also varies
from genus to genus and from species to species. It is sometimes
further forward and sometimes further back. In the remarkable family
Moniligastridae this organ is developed earlier in the body than in
any other group of true earthworms, consisting of four segments or
so commencing with the tenth. As a rule the clitellum begins further
back than this--the thirteenth or fourteenth being a common place for
the first commencement of the organ among the Megascolecidae, while
among the Geoscolecidae and Lumbricidae it is generally much further
back, commencing in _Alma_ at the forty-fifth. These details might
be increased to many pages; but enough has been said to emphasise
the variability of the organ. What reason can be assigned to this
variability, which might be supposed unnecessary in view of its
functions? There are perhaps two suggestions that may be made, though
many facts are lacking which might offer confirmation or refutation
of either of these. It is to be noticed that on the whole the older
types such as the Moniligastridae and the Megascolecidae (including
for this purpose the Eudrilidae) have clitella which are short. There
are a few but not many exceptions. These older types do not seem
capable of extending their range with any rapidity. It is true that
here again there are exceptions, notably many species of _Pheretima_
which are considered under the section which deals with the
migration of these animals. On the other hand the Lumbricidae have on
the whole a more extensive clitellum and so have many Geoscolecidae.
It is obvious that of all earthworms the Lumbricidae is the family
which has the greatest capacity of migration and adaptation to new
circumstances. The reason for this may be that in the latter case
the more extensive clitellum produces a larger cocoon which in
its turn can hold and cherish while they reach maturity a larger
number of embryos. Much remains to be learnt under this heading. But
the comparatively small clitellum of the large Ceylon _Megascolex
coeruleus_ only contains two embryos, while the also comparatively
small cocoon of the large and restricted _Octochaetus multiporus_
(limited to the South Island of New Zealand) only contains a single
embryo. This latter fact may be regarded as fairly well established
since I myself examined quite fifty cocoons.

On the other hand larger numbers seem to arrive at maturity in the
cocoons of _Allolobophora_. The more extensive clitellum must produce
a relatively larger cocoon, and it is interesting to note that the
cocoon of the widely distributed genus _Criodrilus_ (Europe and South
America) is very long although not of great diameter. However the
facts are not sufficiently great to dogmatise much upon this subject.
Another conceivable reason for differences in the clitellum is--as
I also think is the case with the genital papillae--to prevent
hybridisation. That the sense of touch is delicate in these animals
seems clear from the abundant development of epidermal sense-organs.
It may be that the feel of the clitellum during union enables two
individuals of a given species to come together and prevents those
of different species from mating. In any case there is no positive
evidence that hybridisation does occur in this group of animals.
The astounding variability and yet constancy in a given species
of the genital papillae is in favour of regarding these organs as
tactile recognition marks; and it will be noted that they are not by
any means characteristic of some of the older types of earthworms.
Furthermore they are particularly conspicuous in such genera as
_Pheretima_, _Megascolex_ etc., which possess a large number of
species. In these of course mutual recognition would otherwise be
more difficult.

  [Illustration: Fig. 12. Ventral view of _Pheretima solomonis_ to
  show papillae which are to be compared with those of fig. 13. (x
  2.)]

  [Illustration: Fig. 13. Ventral view of _Pheretima sedgwickii_.
  (x 2.)]




CHAPTER IV

SENSE ORGANS AND SENSES OF EARTHWORMS


As this is not an anatomical treatise we shall not attempt any
detailed anatomical and histological account of the sense organs in
this group of worms. But a few facts must be given in illustration
and explanation of the senses of touch and sight that the Oligochaeta
undoubtedly possess. These Annelids, unlike their allies the marine
Polychaeta, and even their allies on two other sides, the leeches and
flatworms, have no complexly organised eyes or other sense organs.
They have in fact no organs to which a definite sense can be attached
on histological grounds. There is nothing comparable to an eye or to
the auditory sacs of other low worms. There are only particular cells
of the epidermis, often associated into small groups, and those again
into larger associations of rows of such groups of cells. It is to be
presumed that these modified groups of cells have a sense function;
but no more can be said than that they are doubtless tactile and
also to some extent receptive of the influence of light. True visual
cells have been asserted to exist in earthworms, consisting of cells
of which a part is clear and transparent and has been supposed to
serve as a lens for the rest of the cell which represents a retina.
But belief in the function of these cells is by no means unanimous.
On the other hand many investigations have proved the existence
of groups of epidermic cells of an oval form which are at present
arranged in definite rows upon the segments of both terrestrial and
aquatic forms, which are moreover connected with nerve terminations,
or are at least--according to more modern views--in close contact
with the terminations of nerve fibres. These are furnished often at
their free tips with minute sensitive processes. There is nothing in
the structure of these to associate them definitely with any sense in
particular. They suggest of course tactile organs more than organs
of any other sense. In addition to these are certain problematical
organs which are found in the Eudrilidae and are present in
the members of one section of that group, the section which is
represented by the universally found _Eudrilus_.

These bodies have been compared to a Pacinian body (a sense-organ
found in Vertebrates) and they bear no little resemblance to it.
For each consists of a darkly staining core surrounded by a layer
of cells arranged like the coats of an onion. In any case it would
appear that these bodies must be looked upon as of a sensory nature,
though they do not reach the surface of the body but underlie the
epidermis. Their function must remain purely a matter of guesswork
at present, for nothing to the point is known of the habits of the
Eudrilidae. It has been suggested by Dr Gustav Eisen that these cells
are auditory and serve to warn the worm of the footsteps of birds and
other enemies. That too is his view of certain peculiar but different
cells found in the epidermis of _Pontoscolex_. In the latter
something like an otosome has been found which is certainly lacking
in the Eudrilidae, whatever may be the function of the cellular
epidermic bodies here briefly referred to.

While there is thus nothing very positive to be gleaned from an
examination of the structure of the Oligochaeta as to the senses
which they may possess, experiment has done something towards an
elucidation of their behaviour under stimuli and their reaction
to light and to other forces which come into play during their
lives. There is some evidence that earthworms can see, using that
expression of course in a very broad way. At any rate they react to
changes in the intensity of light. The gross experiment of flashing
a lantern upon earthworms which are reaching out from their burrows
with the tail end inserted in those burrows shows that they have an
appreciation of light. More refined experiments have been conducted
upon the sense of light. Dr Graber used a box with two compartments,
the one of which was dark and the other illuminated with diffused
daylight, _i.e._ not direct sunlight. The worms were allowed equally
free access to both and were examined at the close of every hour, and
their positions noted. The investigator found that on the average the
dark half of the box contained 5·2 times as many worms as the light
chamber, thus indicating a very marked preference for absence of
light.

Not only is this the case, but the same observer proved that
earthworms can distinguish between degrees of intensity of light.
This obviously indicates a more complete 'visual' sense. He
illuminated the light-box of the former experiment with light
admitted through a ground glass screen, thus diminishing its
intensity. The other chamber was left as before but the screen was
removed, thus admitting full daylight. In this experiment the number
of worms in each compartment proved to be different. The results were
not so striking as before, since only rather more than one-half were
found in the more dimly illuminated chamber. It is a well-known fact
that if earthworms are abroad at all from their burrows, it is during
the night that this movement takes place, the numbers decreasing
towards morning though worms are often seen crawling about well after
sunrise. Some experiments have been made in attempt of explanation of
this apparent anomaly. It would appear from these experiments that
while worms are negatively phototropic to strong and moderate light
as has already been pointed out, they are positively phototropic to
very dim light; hence the advent of evening calls them forth from
their burrows. It will be noted that this perception is of very great
advantage to the worm since its more active enemies above ground are
diurnal. It was held originally that the head end of the worm only
was thus sensitive to light; but more recent experiments have shown
that this is not the case, and that all of the body is sensitive.
This disposes of course of the existence of special light-receiving
organs in the anterior part of the worm's body. Not only this, but an
interesting extension of the view has been promulgated. It has been
shown by Prof. G. Parker and a colleague that in the common Brandling
worm, _Allolobophora foetida_, the response to light stimuli was
related to the amount of the body exposed to its influence. This
is very important as showing that the light perception in these
creatures is probably not due to special organs having a limited
position on the body, but is due to collective sense impressions of
many cells scattered over the whole body, the impression being the
greater when the whole body is exposed and less when only parts of
it are exposed. Furthermore, and this has quite another importance,
these observers noted that the reaction effects differed when only a
part of the body was exposed; that they were greater in the front of
the body, less in the middle, and less still at the tail end. Indeed
they found that the reactions in the case of the front end of the
body alone being exposed were rather more than one-third as compared
with those which were shown when the whole body was subjected to the
light stimulus. The fact that the least sensitive region of the body
is the posterior end has, it is true, only been definitely proved in
the case of the worm whose specific name has been mentioned. But
it is possible that others are similarly affected. And it is highly
important to note the prevalent habit among the Tubificidae of lying
with the head end imbedded in the mud of the pool which they inhabit,
while the tail end emerges and waves freely in the flood. The
additional fact that this tail end occasionally bears gills (as in
_Branchiura sowerbii_ and _Phreodrilus branchiatus_) has a collateral
importance not to be mistaken.




CHAPTER V

RELATIVE FREQUENCY OF EARTHWORMS IN DIFFERENT REGIONS OF THE WORLD


It will be of use for various purposes to be considered later to
arrive at a comprehensive view of the relative numbers of species
and genera of earthworms in the four quarters of the globe. And in
making this general census we shall not take into consideration the
purely aquatic forms, but shall limit ourselves to the earthworms,
_sensu stricto_, or Megadrili, of which, however, it is true that
some members are actually lake and river dwellers. This latter fact
will not, however, interfere with the usefulness of the comparative
survey.

Two preliminary remarks are necessary. The opinions of naturalists
vary as to the limits of genera; and a species may be a species to
one and a mere variety to another. Thus it will be impossible to give
a summary of the facts to be enumerated presently, which will be
either absolutely accurate or which will satisfy everyone in every
detail. But it is asserted that the following survey is substantially
correct.

In the second place it is often possible to eliminate from the
fauna of a given region those species and even genera which have
been accidentally imported, a matter which will receive careful
consideration on a later page. Such forms are therefore, in those
cases at any rate where the evidence seems to be overwhelming,
withdrawn from the list. In other cases, particularly in the Eastern
region of the world, it has been found less easy to rectify the
catalogues by removing what Dr Michaelsen has termed 'peregrine'
forms.

We shall commence with a census of South America; the entire
Continent will be divided for the present purpose into three
divisions, viz. South America, Central and North America, and in the
third place the West Indian Islands.

In South America we find that the bulk of the indigenous earthworms
belong to the family Geoscolecidae and to a definite sub-family,
viz. Geoscolecinae. These genera are _Onychochaeta_ with one
species, _Hesperoscolex_ of which one species is known from the
area, _Periscolex_ with one species, _Anteoides_ with two species,
_Pontoscolex_ one species. _Opisthodrilus_ with two species,
_Rhinodrilus_ (including either as synonyms or as sub-genera,
_Thamnodrilus_, _Anteus_, _Tykonus_, _Urobenus_ and _Aptodrilus_)
with no less than 49 species: _Andiodrilus_ with five species,
_Holoscolex_ with one species, _Glossoscolex_ ten species,
_Fimoscolex_, _Andiorrhinus_ and _Enantiodrilus_ with one species
apiece.

Thus of this sub-family of Geoscolecidae we have in South America
a large number of genera and a much larger number of species. Of
a second sub-family of Geoscolecidae there are three species of
_Criodrilus_ found in the South American Continent.

We now turn to the Megascolecidae of which a large number of species
occur within the area now under consideration. The bulk of these
belong to the sub-family Acanthodrilinae and they are as follows:

Of the genus _Notiodrilus_ there are ten species, of _Microscolex_
two species, of _Chilota_ 19, of _Yagansia_ 13.

A second sub-family Trigastrinae also occurs in this Continent and
the following genera are permanent inhabitants, viz.:--

_Dichogaster_ (= _Benhamia_) with only three species, of which
two at least are found elsewhere, and of which therefore the
autochthonism is doubtful.

Finally, there is the sub-family Ocnerodrilinae comprising the
following genera: _Kerria_ with ten species, _Ocnerodrilus_ (with
sub-genera _Liodrilus_, _Ilyogenia_ and _Haplodrilus_) four species,
which again are rather doubtful indigenes of the South American
Continent.

Leaving aside certain species (of the genera _Lumbricus_,
_Pheretima_, etc.) which are clearly not indigenous, the South
American Continent harbours 150 kinds of earthworms which are
distributed in some 19 genera. But of these a few species (_e.g._
_Onychochaeta windlei_, _Kerria macdonaldi_) stray into neighbouring
regions, _i.e._ the West Indies and California. It is doubtful
therefore whether they are to be referred to as limited to one of
these regions and accidentally imported into the others, or whether
they are genuine inhabitants of both.

The South American Continent shares with the West Indies the
following genera, but the species (except in the case of
_Onychochaeta windlei_, _Glossoscolex peregrinus_) are distinct;
these genera are _Hesperoscolex_, _Pontoscolex_, _Dichogaster_, and
_Ocnerodrilus_. _Diachaeta_ is limited to the West Indies.

The following South American genera are also found in Central and
warmer North America (Mexico, California), viz. _Hesperoscolex_,
_Periscolex_, _Rhinodrilus_, _Pontoscolex_, _Andiodrilus_,
_Glossoscolex_, _Notiodrilus_, _Microscolex_, _Dichogaster_, _Kerria_
and _Ocnerodrilus_. But with the exception of _Hesperoscolex_, which
seems to belong rather to Central America and the West Indies,
_Microscolex_ and _Pontoscolex_ which are world-wide and whose
original home is therefore difficult to fix, and _Dichogaster_
and _Ocnerodrilus_ which would seem rather to be rare immigrants
(perhaps not truly indigenous) into South America, these genera are
practically distinctively South American.

Thus we may fairly say that the genera _Anteoides_, _Opisthodrilus_,
_Andiodrilus_, _Holoscolex_, _Glossoscolex_, _Rhinodrilus_,
_Andiorrhinus_, _Fimoscolex_, _Enantiodrilus_, _Notiodrilus_,
_Chilota_, _Yagansia_ and _Kerria_ are at least very distinctive of
S. America and that they are represented by the large majority of
species found in that continent, the total being 120 or slightly more.

Leaving the American Continent and adjacent islands and archipelagos,
the next great land-mass to receive attention will be the Continent
of Africa. In giving a census of the species and genera of earthworms
which inhabit this quarter of the globe it must be premised that
the facts relate only to Africa south of the Sahara. But little is
known of the genera which occur in Algeria and Morocco, but from what
little is known it is clear that they should come into consideration
in connection with the fauna of Europe and not with that of tropical
and south temperate Africa.

We have in the first place to consider an entirely peculiar family,
of fair extent in genera and species, which is limited to this region
of the world; that is to say with one apparent exception which is
clearly only apparent. The genus _Eudrilus_ is one of the few kinds
of worms that turns up in collections from every tropical part of
the world and even at times from more temperate countries. It is one
of those 'peregrine' forms, as Dr Michaelsen has termed them, which
possess unusual facilities for extending their range. Presumably its
real home is Africa. This family is known as the Eudrilidae though
by some it is only regarded as a sub-family of the Megascolecidae.
In this family we have the following genera: _Eudriloides_
with 11 species, _Platydrilus_ with 11 species, _Megachaetina_
with two species, _Reithrodrilus_ with one, _Stuhlmannia_ with
five species, _Notykus_ and _Bogertia_ with one species each,
_Metadrilus_ also with but one species, _Pareudrilus_ with perhaps
five, _Libyodrilus_ with one and _Nemertodrilus_ with two species,
_Metschaina_ with two species, _Eudrilus_ with two (or possibly
more?) species, _Parascolex_ with four species, _Preussiella_ with
two, _Buttneriodrilus_ with two, _Eminoscolex_ with 16 species,
_Hyperiodrilus_, _Heliodrilus_, _Alvania_, _Rosadrilus_, _Kaffania_,
_Euscolex_, _Metascolex_, _Beddardiella_, _Gardullaria_ with only one
species to each genus, _Bettonia_ with three species, _Teleudrilus_
with 15 species, _Polytoreutus_ with 22 species, _Iridodrilus_
with two, _Malodrilus_ also with two, _Neumanniella_ with eight,
_Eupolytoreutus_ with two and _Teleutoreutus_ with only one species.

This completes the list of the Eudrilidae. We will take the huge
family Megascolecidae next; and we find in tropical Africa that the
genus _Dichogaster_ alone contains at least 93 species confined to
Africa, as well as a few which it shares in common with America,
and the common and widely spread _D. bolavi_ which has even made
its way to Europe. Of the sub-family Ocnerodrilinae we have the
genus _Gordiodrilus_ with seven species, a genus which also occurs
in Madagascar and the West Indies: _Nannodrilus_ with three
species, _Diaphorodrilus_ with one species and a few examples of
_Ocnerodrilus_ and its sub-genera, some of which are also forms that
occur elsewhere in the world, for example _Nematogenia panamensis_.
Of _Pygmæodrilus_ there are eight or nine species.

Of the sub-family Acanthodrilinae the Cape region of South Africa
harbours some seven species of the genus _Notiodrilus_, of which one
however is a West African form. The allied _Chilota_ has 13 species,
and there is a peculiar genus _Holoscolex_ near to _Yagansia_ with
one species. We next have to deal with the Geoscolecidae, of which
a sub-family, Microchaetinae, is mainly found in Africa, the rest
being found in the neighbouring Madagascar and some few in the East.
_Microchaetus_ contains about 14 species, _Tritogenia_ perhaps
three, _Callidrilus_ two, and the genus _Glyphidrilus_, mainly found
in Asia, has one species in the region now under consideration. In
addition to these Geoscolecids there is the peculiar and largely
aquatic _Alma_ with six or seven species in East, West, and Central,
Africa, and in the Nile region.

Summing up the genera which are found in tropical and South Africa we
find that there are 44 which are abundant in, or entirely confined
to, the region. In addition to these four or so occur in Africa but
are either more abundant elsewhere or (as in the case of _Chilota_
and _Notiodrilus_) are equally distinctive of other parts of the
world. The number of species may be estimated at 270, possibly rather
more. Clearly therefore this part of the world is much richer than
South America, both in numbers of genera actually found, and peculiar
to the country, and numbers of species.

Passing from Africa the next definite quarter of the globe which
will detain us here is the island of Madagascar, so remarkable for
the Mammalian fauna which characterises it, for its lemurs, peculiar
Insectivora and Carnivora, and above all by reason of the absence of
the prevalent African types such as antelopes, zebras, rhinoceros
etc. It is probable that a good deal still remains for discovery
among the earthworms of this island; but a considerable number are
already known and they are as follows:

The Eudrilidae are completely absent, a state of affairs which is
paralleled by the absence of antelopes among mammals.

The sub-family Acanthodrilinae of the family Megascolecidae are
represented by four species of the genus _Notiodrilus_, with which
perhaps _Maheina braueri_ from the Seychelles should be included as
it presents small differences from _Notiodrilus_.

Among the Megascolecinae a good many species of _Pheretima_ have been
collected both on Madagascar and on certain adjacent islands; but
these, with one possible exception, are forms which occur elsewhere
and are often indeed very widely distributed 'peregrines,' so that
it is hardly permissible to place them among the real inhabitants of
Madagascar. The same arguments hold in the case of _Lampito mauritii_
and the ubiquitous _Eudrilus_ and _Pontoscolex_. But there is the
peculiar _Howascolex_.

Among the Ocnerodrilinae a distinct species of _Gordiodrilus_ occurs,
and an obviously introduced _Ocnerodrilus_.

It is among the Geoscolecidae that the most characteristic forms are
met with. These belong exclusively to a genus of the Microchaetinae,
_Kynotus_, which is found nowhere else but in Madagascar, where it
is represented by at least twelve different species.

We have therefore in Madagascar and the surrounding islands only two
peculiar genera, only four genera which can be regarded as endemic,
and only about seventeen peculiar species.

Passing eastward and crossing the Indian Ocean we come to the
Continent of Asia, and we shall first direct attention to the
peninsula of India and adjacent parts of Burmah and the island of
Ceylon, of which there has been accumulated a great deal of knowledge
concerning the Oligochaetous fauna.

This quarter of the globe differs quite as much from any that have
been hitherto considered as they do among themselves. We have left
behind us the Geoscolecidae with the exception of the ubiquitous and
clearly peregrine _Pontoscolex_, and the genus _Glyphidrilus_, which,
being aquatic at times, is perhaps hardly to be considered in the
present survey.

We have also in this Indian region the equally ubiquitous _Eudrilus
eugeniae_ which need not of course detain us further. The Lumbricidae
are for the most part of European forms with the exception of
_Helodrilus indicus_ conceivably an actual inhabitant of India.
It is among the Megascolecidae that the vast majority of the
forms endemic in India are to be found. We shall take this family
according to its sub-families. In the first place we note that the
sub-family Acanthodrilinae is totally unrepresented. The large
sub-family Megascolecinae has very numerous representatives. Of
the genus _Megascolex_ itself there are some 30 species, of which,
however, two or three are looked upon as varieties. Of the allied
genus _Notoscolex_ there are ten species and of _Perionyx_ about
13. The genera _Megascolides_, _Diporochaeta_, _Spenceriella_ and
_Woodwardia_ have only six species between them of which three belong
to _Megascolides_. _Plutellus_ has five species in this region.
_Lampito_, which is a widely spread form with but one species _L.
mauritii_, may or may not find here its original home. It also occurs
in Africa and Madagascar. _Pheretima_ is represented by no less
than 12 species, of which at any rate the very great majority are
peregrine forms, and not to be safely regarded as forming an integral
part of the fauna of the Indian peninsula and adjoining countries.

Of the sub-family Octochaetinae the type genus _Octochaetus_ has
ten species in this region and _Eutyphoeus_, which is restricted to
the region, has 15 species or perhaps rather more. _Hoplochaetella_
has but one. Of the Trigastrinae there are four or five examples
of the genus _Dichogaster_ which occur within the region now
under consideration; but it is doubtful whether they are truly
indigenous. On the other hand _Eudichogaster_, closely allied both to
_Dichogaster_ and _Trigaster_, seems to be confined to this part of
the world where it is represented by five species. Finally we come to
the last sub-family--that of the Ocnerodrilinae, which is represented
by a species apiece of the genera _Ocnerodrilus_, _Nematogenia_ and
_Gordiodrilus_. The latter species _G. travancorensis_ is alone
to be regarded as endemic and it is very near to the African _G.
zanzibaricus_, described some years since by the present author.

The remaining family of terricolous Oligochaeta found in India is
the family Moniligastridae which is practically limited to this part
of the world and consists of at least twenty species distributed
among the genera _Moniligaster_, _Eupolygaster_, _Desmogaster_ and
_Drawida_, the majority belonging to the last-named genus.

This quarter of the globe is therefore inhabited by 18 genera which
are certainly truly endemic, and which comprise between them about
120 species. But only four or five genera are peculiar.

The remainder of the Asiatic Continent is not very well explored
with regard to its earthworm inhabitants. It seems clear however
that the southern and coastal region of China and Japan with the
Malay peninsula are really continuous with the mass of islands which
lie between India and Australia and form together a tract of land
which is characterised by an Oligochaetous fauna differing from both
that of India on the one hand and Australia on the other. We shall
therefore consider this huge portion of the globe as one region
comparable to the other divisions that have been hitherto considered.
With reference to the Lumbricidae and Geoscolecidae the same remarks
may be made as in the case of India. The indigenous forms of the
latter family are to be looked upon as outside of the present survey
since they are largely or entirely aquatic forms. _Pontoscolex
corethrurus_, and _Eudrilus eugeniae_, need not detain us for reasons
already amply stated. We now come to the great family Megascolecidae.
Of this family the genus _Pheretima_ stands at the head; and of the
200 or so species that have been or can be assigned to this genus
all, with merely two or three exceptions, are natives of the Eastern
Archipelago and adjoining mainlands of Asia. Of other Megascolecinae
the region has yielded the following genera. In Java one species
of _Woodwardia_ (_W. javanica_) has lately been described. It is
regarded by Michaelsen however as doubtfully indigenous. And the same
remark may be made of _Perionyx_. _Plionogaster_, however, with four
or five species, is limited, as far as our present knowledge goes, to
the Philippines and to neighbouring islands. The Acanthodrilinae and
Octochaetinae are totally absent from this part of the world, there
being no record even of peregrine species of these sub-families.
The sub-family Trigastrinae is not however unrepresented; for of
_Dichogaster_ several species occur, such as _D. malayana_ and _D.
saliens_. But these are by no means certainly to be looked upon as
real natives of the situations within this area where they occur.
This completes the scanty list of genera found in the region under
consideration; for one Moniligastrid (_M. barwelli_) is hardly to
be looked upon as indigenous. We have therefore to record here but
five genera, of which only two are certainly indigenous and probably
also confined to the region; they contain between them certainly two
hundred species.

We next come to the Continent of Australia. The earthworm fauna is
again quite without Geoscolecidae and of course Lumbricidae. As to
the former there are not even doubtful cases like _Glyphidrilus_ of
the East; for we find only recorded _Pontoscolex corethrurus_, a
species concerning whose extraordinary powers of migration there is
no possible doubt. The same may be said of _Eudrilus_ also recorded
from Australia. Here the Megascolecidae are as conspicuous as in the
old world generally. We find, however, a great many members of the
sub-family Acanthodrilinae. Of the antarctic genus _Notiodrilus_
there are some six species. _Microscolex_ occurs; but the real
habitat of this genus is very doubtful. _Diplotrema_, with one
species, _D. fragilis_, is not only indigenous to, but confined to,
Australia.

The most prevalent sub-family is that of the Megascolecinae. Of
the genus _Pheretima_ there are two species which may or may not be
truly indigenous. Of _Plutellus_ there are forty species; there are
over thirty of _Diporochaeta_, while _Notoscolex_ (with which Dr
Michaelsen associates _Digaster_, _Didymogaster_, _Perissogaster_)
includes more than forty species, _Fletcherodrilus_ has but one
species, _Megascolex_ has seventy species, _Woodwardia_ sixteen,
_Spenceriella_ five, and _Megascolides_ seventeen. There are four
species of _Perionyx_ which were at one time regarded by Michaelsen
as necessitating a new but allied genus _Perionychella_; the two are
now merged. Thus there are not far short of 150 species and eleven
genera represented, of which only two are limited to Australia.

Having completed the survey of the central and southern land masses
of the globe we next direct attention to the northern land masses,
viz. North and Central America on the one hand, and Europe and
northern Asia on the other. With regard to America we find besides
many species of Lumbricidae the genera _Notiodrilus_, _Microscolex_
among the Acanthodrilinae, _Megascolides_ and _Plutellus_ of the
Megascolecinae, no member at all of the Octochaetinae, a considerable
number of species of _Ocnerodrilus_ and sub-genera belonging to the
Ocnerodrilinae, a good many species of _Dichogaster_ and at least
one of _Trigaster_ among the Trigastrinae, while one sub-family,
that of the Diplocardiinae, is only found here and contains two
genera _Diplocardia_ and _Zapotecia_ with quite ten species between
them of which only one belongs to the last-named genus. There are
no Geoscolecids (except the chiefly aquatic _Sparganophilus_); this
family stops short in the West Indies where their presence has been
briefly referred to in considering the worms of South America. The
vast majority of the genera enumerated here are only found in the
warmer parts of the North American continent. We have therefore
in this division of the world some nine genera of which at least
one, viz. _Diplocardia_ (and its close ally, hardly perhaps to be
separated, _Zapotecia_), is confined to it, while _Trigaster_ only
extends as far south as to the West Indies.

Europe and northern Asia, of which Europe alone and certain
limited tracts of Asia are at all known, contain all the genera
of the family Lumbricidae which are (according to Michaelsen)
_Eiseniella_, _Eisenia_, _Helodrilus_, _Octolasium_, _Lumbricus_
with various sub-genera of _Helodrilus_, such as _Dendrobaena_ and
_Allolobophora_. Besides these forms, which amount to at least 130
species, we find the genus _Hormogaster_, with two species, the only
genus of the Geoscolecid sub-family Hormogastrinae; there is also
a species apiece of the genera _Criodrilus_ and _Sparganophilus_
(sub-family Criodrilinae). But as these are at least largely aquatic
they come under another set of rules and are not important in the
present survey of the earthworms of the world. It will be thus seen
that with these two last exceptions the area in question contains but
six genera of which _all_ are peculiar.

We have now completed the survey of the principal land-masses of
the globe. We shall conclude with a reference to one of the largest
islands of the world, viz. New Zealand, with which will be included
a few outlying islands such as the Aucklands, Snares Island, etc.
The reason for not ignoring these islands as we have ignored certain
other islands of large size, such as Japan, will be apparent from
the peculiarity of the earthworm fauna which they show. From New
Zealand the Moniligastridae, Geoscolecidae, Lumbricidae, and, of
course, the Eudrilidae, are absent, save the ubiquitous _Eudrilus_.
The only family of earthworms which is here represented is that of
the Megascolecidae. Of this family the Acanthodrilinae are very
well represented. We have at any rate seven species of the genus
_Notiodrilus_ of which the bulk are from the small adjacent islands
and not from the mainland. The genus _Rhododrilus_ with nine species
is actually limited to the New Zealand group and so is an allied
genus consisting of two species only, viz. _Leptodrilus_. This
latter genus is confined, so far as present information goes,
to the Auckland and Campbell islands. _Dinodriloides_ with two
species is also limited to New Zealand and to the North Island.
_Maoridrilus_ with some ten species is another native and restricted
genus. So too is the allied _Plagiochaeta_ with numerous setae on
each segment but with the alternating and single nephridial pores of
_Maoridrilus_. There are several species of _Plagiochaeta_ of which
one has been lately regarded by Dr Michaelsen as really falling
within the otherwise Indian genus _Hoplochaetella_, while for another
he has formed the genus _Pereiodrilus_. _Neodrilus_ with but one
species _N. monocystis_ is another peculiar New Zealand genus. The
family Octochaetinae contains only four genera, of which one, viz.
_Dinodrilus_ (with four species), is limited to New Zealand, while
_Octochaetus_ has about five representatives. _Hoplochaetella_
(if Dr Michaelsen's surmise referred to above be correct) has
one species in New Zealand. The Megascolecinae are less numerous
than the Acanthodrilinae, but there are ten species of the genus
_Megascolides_ (which includes Benham's genus _Tokea_), perhaps seven
species of _Diporochaeta_, and two other species which Michaelsen has
removed from the genus _Diporochaeta_ and placed in _Spenceriella_.
One _Plutellus_ (which however may have been introduced) completes
the New Zealand Megascolecines. We have therefore in this part of the
world fifteen genera including between them some 58 species; eight
of the genera are peculiar to the islands.

From this brief statement of facts some inferences of interest can
be drawn. It is in the first place plain that every part of the
world except the extreme north and south has a considerable fauna of
earthworms. The one exception would appear to be the northern part
of the North American continent. Here we meet with members of the
family Lumbricidae which are however species that are met with in
the Euro-Asiatic province and are thus to be regarded as possibly
later immigrants introduced probably by man. Thus temperature
short of a constantly frozen condition of the ground is not a bar
to the existence of earthworms. Even a freezing of the ground for
lengthy periods is not a complete obstacle to the existence of those
Annelids; for I have myself received examples of Lumbricidae from
the arctic island of Kolguev. Moreover the temperate regions would
seem to be as fully populated in the way of individuals, and even of
species, as are the tropical regions. Indeed as to individuals it
seems that the temperate regions are more fully supplied than much
of the tropics. This however is not quite the object of the present
section to discuss. We are here concerned with the relative frequency
of genera and species. There are according to a recent estimate of
the Rev. H. Friend some forty species recognisable in Great Britain.
And as already has been stated the earthworms of Europe amount to
perhaps 130,--at any rate well over one hundred. In tropical America
there are hardly more. But in the latter case the number of genera is
very greatly in excess of that of Europe. We cannot however say that
an abundance of generic types is quite characteristic of the tropics.
For the Eastern Archipelago, though rich in species, is but poor
in genera, not possessing more than half a dozen or so. And on the
other hand the temperate climate of New Zealand has produced a very
considerable series of genera, much more than those of the islands
of the East and nearly as many as those of, for instance, Central
America and the West Indies.

This conclusion is in its turn contradicted by the conditions
observable in Chili and the temperate regions of South America, where
the number of species is large but the number of genera small. In
short no general laws, in the present state of our knowledge, can
be laid down as to the connection between species and genera on the
one hand and climatic conditions on the other. In this department of
our subject we cannot do more than has already been done, _i.e._ to
state the actual facts. One is tempted in comparing the rich fauna
of tropical Africa with the very limited fauna of Madagascar to
associate a richness of types with extent of land surface. In the two
cases cited this conclusion is obvious. It may also be extended--if
we confine ourselves to species and not to genera. For the two great
islands of New Zealand have not between them more than fifty species
of earthworms, while Australia has four or five times that number. It
will be noticed however that we cannot associate poverty of generic
differentiation with limited land masses; for New Zealand has a large
number of generic types, very many more than the huge Euro-Asiatic
tract of continent.


THE RANGE OF GENERA.

We have seen, and shall again refer to the fact, that individual
species of earthworms have not as a rule a range over a great
extent of country, save only in those cases such as _Pheretima
heterochaeta_ which belong to that physiological section of these
worms called 'peregrine' forms; these appear to possess some means
of extending their range by the assistance of man which is denied to
other forms. Apart from these instances, which do not come under the
present category, it is only _Lumbricus_ and its immediate allies,
_Helodrilus_, etc., of which certain species are found to exist over
wide tracts of land. There are however many genera which have a wide
range and which may be contrasted with others in which the range is
very limited. The two extremes are moreover connected by forms with
an intermediate range. There is no doubt whatever that the genus
with the widest range is _Notiodrilus_ of which species are found
throughout the antarctic region, viz., in Patagonia, the islands of
the Antarctic Ocean, the Cape of Good Hope, New Zealand, and also
further to the north, sometimes even to and beyond the tropics in
America, Australia, and Africa. There is no other genus of which the
genuine extension (_i.e._ not in any way due to man) is so great as
this genus _Notiodrilus_. And this fact gains much significance from
the now generally accepted view that in its anatomical structure
_Notiodrilus_ comes near to the original type of earthworm.

Perhaps the next most widely distributed genus is _Helodrilus_ of
the family Lumbricidae which occupies Europe and Asia to the extreme
east, and is thought also to be indigenous to certain parts of North
America. But this range, though equally wide perhaps in mileage, is
less impressive than that of _Notiodrilus_, since the land areas
inhabited by the genus are continuous--almost so if we accept North
America as its real habitat. Here we have a case precisely the
opposite of that of _Notiodrilus_; for while there are reasons for
regarding _Notiodrilus_ as an ancient form of Lumbricid, there are
equally good reasons for regarding the Lumbricidae as the most modern
family of earthworms.

To find other instances of widely spread genera we must recur to the
great family Megascolecidae. There are practically no Geoscolecidae
which have a really extensive range. The only instances are
_Criodrilus_ and its ally _Sparganophilus_ which occur in America,
whether North or South, and in Europe; but as these forms are at
least largely aquatic the facts are not quite comparable with those
now under consideration.

The genus _Dichogaster_ (which includes as synonyms _Benhamia_,
_Millsonia_, _Microdrilus_) is unquestionably indigenous to tropical
Africa and certain parts of America including the West Indies. It has
been also met with in the East; but as the species there occurring,
such as for example the species originally described by myself as
_Microdrilus saliens_, are of small size, an accidental introduction
is quite possible, and it is by no means certain that it has not
occurred. In any case the genus is known to possess species which
are undoubtedly to be reckoned among peregrine forms--such as _D.
bolavi_, which has turned up in Europe. _Gordiodrilus_ and also
_Ocnerodrilus_ with its sub-genera have very much the same range as
has _Dichogaster_. It is to be noted however that these forms are
circumtropical, and that their distribution is thus less continuous
than that of _Notiodrilus_; they do not however show the markedly
discontinuous range of certain other genera of Megascolecidae. For
instance _Octochaetus_ is well known from New Zealand, and, not
occurring in the intermediate tracts, is again met with in India.
_Hoplochaetella_ is believed by Michaelsen to present us with another
precisely similar instance. Then also the genera _Woodwardia_ and
_Notoscolex_ are to be found in Australia and again (absent from the
immense tract lying in between) in Ceylon. _Megascolex_ has much the
same range, showing also this marked and remarkable discontinuity.
Stranger still, perhaps, is the range of _Plutellus_ and
_Megascolides_, of which the former, chiefly found in Australia and
Tasmania, not only extends its habitat to Ceylon but also to North
America; it is there represented by Eisen's species _Argilophilus
marmoratus_, referred by him, and not unnaturally, to a distinct
genus, but placed by Michaelsen in _Plutellus_. _Megascolides_ is
Australian and from the North Island of New Zealand, where its
species were regarded by Benham as of a distinct genus, _Tokea_.
There is also one form, _Megascolides americanus_, in the western
region of North America.

The two genera _Yagansia_ and _Chilota_, closely related to
_Notiodrilus_, have a range which is short of that of _Notiodrilus_,
and we shall see later that there are reasons for regarding these
genera as derived from _Notiodrilus_. They are met with only in the
south of South America, and in the Cape of Good Hope region.

The range of _Microscolex_ seems to be much the same as that of
_Notiodrilus_; but it is a little uncertain how far the genus is
really autochthonous in the countries where it occurs; and in any
case it differs from _Notiodrilus_ in occurring in Europe, where the
species has been named for a long time _M. phosphoreus_. We do not
positively know whether this is 'peregrine' in Europe or not.

The range of the antarctic Acanthodrilinae is in a sense continuous;
for they argue the former northward extension of the antarctic
continent and in any case they occupy neighbouring land masses. In
_Octochaetus_ and _Plutellus_ the case is different and one of real
discontinuity. There are however cases of wide range which is also
actually continuous and such is afforded by the genus _Pheretima_.
This genus appears to be possibly indigenous to Australia; in any
case it reaches from the Solomon Islands on the east to India towards
the west, being found in all intermediate continents, while it
reaches Japan on the north side of this large area.

There are other genera which extend their range over a considerable
area, but which are not so widely distributed as these which we have
just been considering. Thus _Diporochaeta_ is chiefly Australian but
also reaches even the South Island of New Zealand and the southward
lying antarctic islands. _Desmogaster_ and _Eupolygaster_ among the
Moniligastridae range from Burmah in the east to Sumatra and Borneo
further east, though they are not recorded from intermediate islands.
_Perionyx_ is found in Burmah, India, Zanzibar, Sumatra, and Java.
There are other examples of genera which have much the same range
as those enumerated. Finally there are those which are confined to
one land mass and very often to a restricted region of that. Thus
_Kynotus_ is confined to Madagascar, all the genera of Eudrilidae to
tropical Africa, some of them, _e.g._ _Beddardiella_ and _Euscolex_,
to very limited tracts, others to wider or less wide areas in that
continent. _Maoridrilus_ is only found in New Zealand, to the South
Island of which also is confined the genus _Neodrilus_. To the Cape
region of Africa is limited _Microchaetus_; and to a belt running
across the northern part of the tropical region and extending down
the Nile, the remarkable, partly aquatic, _Alma_.

As a kind of appendix to these facts and conclusions we shall next
deal with certain widely spread forms that have been already referred
to, with the range of different genera over great land masses of the
world, and with the earthworms of oceanic islands.




CHAPTER VI

PEREGRINE FORMS


Dr Michaelsen has used this term to describe those species which
possess some powers of migration over the sea, denied to the majority
of worms, and probably due to the direct interference of man. Thus
we find in collections of earthworms from various parts of the
world not only examples of forms which do not come from other parts
of the world, but also a few which occur in many or even most of
such collections. It is for example to be actually expected that
a collection of earthworms made in South America, the Philippine
Islands, or Australia will contain examples of the apparently
ubiquitous _Pontoscolex corethrurus_. This is what has actually
happened in cases of which I have personal knowledge, as well as in
many others recorded in the literature of the subject. I have myself
received this worm from the three parts of the world mentioned, and
also from Hawaii. Others have increased its known range to other
parts of the South American continent, to Central America, the West
Indies, the islands of Sumatra, Java, Borneo, Celebes, Mauritius,
and Madagascar, etc. It is in fact found everywhere in the tropics.
With this range may be contrasted that of another genus of the same
family (Geoscolecidae), viz. _Kynotus_, which, though consisting of
many species, is not found outside of the Madagascar district. It
should be added that _Pontoscolex_ does not appear to contain more
than two species, the one not mentioned in the above survey of its
distribution being _P. insignis_ of Kinberg, which is apparently the
same as _P. liljeborgi_ of Eisen, and is limited to certain parts of
America.

Before attempting to grapple with the remarkable facts implied by the
distribution of this genus, it will be well to survey the whole group
of Oligochaeta and to reduce to as short a space as possible the
total series of facts which are of the same nature.

A case, even more striking than that of _Pontoscolex_, is afforded
by the Eudrilid genus _Eudrilus_. As with _Pontoscolex_ there are
two species of this genus, one, _E. pallidus_, being confined to
West Africa, the remaining one, _E. eugeniae_, being world-wide in
range. This latter species has received the following names, viz.
_E. decipiens_, _E. lacazii_, _E. peregrinus_, _E. sylvicola_, _E.
boyeri_, _E. jullieni_, _E. erudiens_, and _E. roseus_; they appear
to be all synonyms of the name originally given by Kinberg who
however did not recognise the distinctness of the form as a genus. It
is now known as _Eudrilus eugeniae_. The variety of names given to
supposed different forms (for two of which I am myself responsible)
is due to the fact that in earlier days when nothing was known
about the geographical distribution of this group of animals it was
thought by no means unreasonable that a given genus represented by
several species should range over the globe. This fact coupled with
imperfect description of structural details led to the multiplication
of supposed species, a position which is no longer tenable. This
worm is quite as abundant in gatherings from all parts of the world
as is _Pontoscolex corethrurus_; and in addition to the countries
inhabited by the latter, _Eudrilus eugeniae_ has been met with in New
Caledonia: tropical Africa is probably its original home.

The two families that have been hitherto considered offer no further
instances, among their many species, of worms with so wide a range
as those just dealt with. There are indeed one or two forms, _e.g._
_Criodrilus_ and _Glyphidrilus_, which have a considerable range
though not nearly equalling that of _Eudrilus_ and _Pontoscolex_.
These are, however, aquatic forms and the range of aquatic forms is
determined as far as we can see by a different series of causes to
that of terrestrial forms, which are referred to later.

Among the Moniligastridae we have apparently an instance of a
peregrine form. The genus itself has its headquarters in Ceylon and
extends a little way in other eastern regions; there is, however,
one species, _Moniligaster bahamenis_, described some years since
from the Bahamas which must surely be an example of a peregrine form,
particularly since it is probably identical with _M. japonicus_ whose
name is indicative of its habitat.

Among the huge family of the Megascolecidae there are a considerable
number of species which apparently possess the same facilities for
making their way in different directions and across seas from the
locality that is thought to be their real home.

Of the very many genera, however, of which this family is composed,
a comparatively small number are thus peregrine in habit at times.
All the species known which are distributed broadcast, more or less,
over the tropics belong to the genera _Pheretima_, _Microscolex_,
_Dichogaster_, _Megascolex_, _Perionyx_, _Ocnerodrilus_, _Kerria_.
These several genera are placed in order of frequency of exotic
occurrence. Indeed of the two latter genera their frequent life
in fresh water may really remove them from the present category
altogether. In addition to these are some perhaps more questionable
instances, such as the genus _Gordiodrilus_ which, prevalently West
African, has also been found in the West Indies, in East Africa, and
in India and Madagascar. These instances I propose to leave out of
consideration in the present sketch. The most obviously peregrine
genus of all those enumerated is _Pheretima_, which according to
my experience turns up in almost all gatherings of earthworms from
any part of the tropical or even sometimes temperate regions of the
world. It seems to be fairly well settled that this extensive genus
has its real home in the islands of the Eastern Archipelago, perhaps
extending a little in various directions from that centre. But
examples of the genus have been found in almost all other regions.
And what is especially to the point in considering the facts, as will
be pointed out with more emphasis later, the assumedly peregrine
species do not differ from those found in the real district in which
the genus is indigenous.

Dr Cognetti de Martiis enumerates in the Neotropical region, that
is in South and Central America and the West Indies, the following
species: _Pheretima biserialis_, _P. californica_, _P. capensis_,
_P. elongata_, _P. hawayana_, _P. hesperidum_, _P. heterochaeta_,
_P. houlleti_, _P. posthuma_, _P. rodericensis_, _P. schmardae_ and
_P. violacea._ Of these twelve species it is quite certain that the
last six occur in the East, where they are doubtless indigenous. So
too do the species _P. biserialis_, _P. capensis_, and _P. hawayana._
The synonymy of the different species of this large genus is not yet
in a completely settled condition. But in the meantime it is in my
opinion quite possible that both _P. hesperidum_ and _P. californica_
are identical with species also occurring in the East. There remains
the somewhat doubtful _P. elongata_ from Peru which has not been very
fully described. There is thus no convincing evidence of species
really indigenous to and confined to any part of America. Some of
these species also occur in many other parts of the world. For
instance, _P. heterochaeta_ is very widely spread indeed, occurring
as it does in Australia, New Caledonia, Madagascar, and even England
(in hothouses). This species indeed is the most prevalent of all
Pheretimas and seems to be abundant in gatherings of earthworms from
various localities as are _Eudrilus_ and _Pontoscolex_.

From the island of Madagascar and neighbouring islands the following
species of _Pheretima_ have been obtained and identified by Dr
Michaelsen: viz. _Pheretima pentacystis_, _P. peregrinus_, _P.
heterochaeta_, _P. biserialis_, _P. rodericensis_, _P. houlleti_,
_P. robusta_, _P. mauritiana_, _P. taprobanae_, and _P. voeltzkovi_.
It will be noticed that the majority of these are also included in
the list from South America, and that many of them are also found
in other parts of the world, and nearly all of them in the East.
There remain a few which are doubtful. It is quite possible that _P.
mauritiana_ is the same as _P. hawayana_ and _P. bermudensis_, in
which case it has a world-wide range. _P. taprobanae_ is well known
as a Ceylon species. _P. robusta_ also occurs in the East Indian
islands. There remain _P. pentacystis_, _P. peregrinus_, and _P.
voeltzkovi_. _P. peregrinus_ is known from Australia and also from
Sumatra, so that that species need not concern us in enumerating
those which are possibly endemic. In fact it is only _P. pentacystis_
and _P. voeltzkovi_ which may be really Mascarene.

Another peregrine genus belonging to the sub-family Acanthodrilinae
is _Microscolex_. But the limits of this genus may be regarded as
at present rather uncertain. And this difficulty somewhat affects
the bearing of the facts to be related, though it hardly affects the
value of the facts themselves. Dr Michaelsen referred to the genus
in his great work seven well-defined species, and four others not so
plainly distinct. Of these eleven, two are confined to New Zealand,
four to North and Central America, one to Hawaii, one to Madeira,
one to Algeria, while the remaining two are found pretty well over
the whole surface of the world. More recently the same authority
has somewhat extended his view of the generic characters, so as to
include a number of forms found in Patagonia, Cape of Good Hope, and
the antarctic region generally, while he has lumped together into two
species only, viz. _M. phosphoreus_ and _M. dubius_, the eleven forms
just mentioned, which species therefore are absolutely world-wide in
range, and thus form an excellent example of a peregrine form. These
species moreover differ from _Pontoscolex_ and some others in that
they have been able to establish themselves in Europe. Dr Michaelsen
also relates that in the cultivated lands of South West Australia,
_Microscolex dubius_ and _Helodrilus caliginosus_ are actually the
commonest species; and he calculates that they form together quite
90% of the earthworms gathered in any locality belonging to this
region.

Some of the other Megascolecid peregrine forms will be referred
to later. There is no doubt that the family Lumbricidae offers by
far the greatest number of peregrine forms and that these are most
abundant in collections from extra-European countries, where the
collector has searched in cultivated lands. There are at least eight
or nine species which are common in many parts of the world though
their original home is undoubtedly Europe.

This is a brief review of the facts, more detailed in some cases than
in others. It remains to review and compare the results arrived at.

The first general statement that may be made is that this faculty
of extending their range beyond the limits assigned by nature is
not confined to any one family. For all the chief sub-divisions of
the terrestrial Oligochaeta seem to possess it, though in unequal
degrees. But the inequality may be more apparent than real. For if
it be recollected that the species of the family Megascolecidae are
very much more numerous than those of the Eudrilidae or even the
Geoscolecidae, the fact that there are more peregrine Megascolecidae
will lose some of its importance. With the Lumbricidae the case seems
to me to be different. Here the preponderance, not only in species
(relatively speaking) but in individuals, is much above that of other
families. This preponderance I should be disposed to assign to the
newness of the family coupled with the vigour seen in new races. That
this is a possible explanation is borne out by the fact that the
'Perichaetidae' (_i.e._ the genus _Pheretima_) is the most salient
race of peregrine Megascolecidae, and it is now generally held that
this group is the most modern of that enormous family.

Another general statement may be made with even more confidence,
viz. that it appears to be an undoubted fact that some species are
more capable of extending themselves than others. Thus _Eudrilus
eugeniae_ occurs everywhere on the great land masses of the globe,
except in Europe; it is in fact circummundane in the tropical zone,
as is also _Pontoscolex_. _Dichogaster bolavi_ is again a trifle more
restricted in its range, having been recorded from tropical Africa,
South America, West Indies, Madagascar, and India. Its occurrence
near Hamburg in Europe is also to be noted. A little more restricted
still is _Nematogenia panamaensis_ whose range is in Central America,
tropical West Africa, and Ceylon. Lastly there are cases such as
_Pheretima taprobanae_ which, a native of Ceylon, is also found in
Madagascar.

It may be asserted in the third place that there are no peculiarities
of structure shared by all of these peregrine forms which might
account for their physiological similarity, except indeed the
somewhat negative feature which they have in common, that is of
being of small or moderate size. _Eudrilus_ and _Pontoscolex_ are
not isolated types in their respective families; nor do they seem
to approach each other in any respect. Nor can it fairly be said
that these peregrine species are marked by any great variability of
structure as compared with other forms, which might allow for their
suiting themselves to various climates and conditions. It is true
that _Eudrilus eugeniae_ has received many names which might at first
argue some variability. But these names have been perhaps given by
persons rather under the influence of the idea that remote habitat
implied specific difference, and who were thus inclined to see minute
differences, and who perhaps were furthermore led astray in the
matter by imperfectly accurate descriptions on the part of others.
Certainly some of the peregrine species of _Pheretima_ are subject to
some variation, particularly in the number and arrangement of their
genital pupillae. But this feature is by no means confined to those
species and cannot be utilised as in any way an adaptation to wide
distribution.

But while we can lay down no general explanation of the phenomenon,
it is possible to furnish some explanation of particular cases. Thus
the genus _Microscolex_ is the only exotic genus which appears to
have established itself in Europe, from which country indeed it was
early known as an apparently indigenous inhabitant. We must put this
and some similar cases down to ability to do without great heat.
It is probable in fact that the original home of _Microscolex_ is
the antarctic half of the globe; and this of itself would allow of
its establishing a new home in the northern hemisphere, did other
circumstances allow of it.

It might be urged that this genus has been able to establish itself
in Europe because it has in fact had the chance denied to other
species. There are a good many, however, which would in that case be
in the same category. Some years ago I received from time to time a
very large number of earthworms from the Royal Gardens at Kew which
had been accidentally imported thither from many quarters of the
globe, among which I described some eighteen or twenty new species
including, for instance, the African genus _Gordiodrilus_. There are
plenty of facts of a similar nature and Dr Michaelsen has pointed out
that botanical gardens act as centres of dispersion for accidentally
introduced Oligochaeta. We must therefore come to the conclusion
that temperature is at least one of the causes of a difference in
the capability of extending their range shown by the Oligochaeta, a
cause which doubtless operates as a check upon extension of range in
non-peregrine forms also, and prevents for instance the dispersion of
the tropical African Eudrilidae into the region of the Cape.

We may, as it appears to me, confidently look upon indifference
to varying temperature as a condition of ability to colonise new
countries. But it is obvious that this is not of itself a sufficient
cause to explain the facts. Otherwise this country and N. Europe
would contain many antarctic earthworms; the only one that has been
recorded to my knowledge is _Microscolex_.

Though an inability to endure a temperate climate may have rendered
the movements of peregrine species more limited, the same or rather
the exactly opposite cause does not seem to have played any important
part in this direction. For it is above all the Lumbricidae, normally
dwellers in temperate climates, that are so remarkable for their
wide range over the world. Nor can it be convincingly asserted that
the extra-Palaearctic Lumbricids are real indigenes of those--often
tropical--countries. For if so we should expect them to be at
least of different species. Lumbricids however from South America,
Australia, etc., are specifically identical with European forms.

There is no doubt that wherever land has been at all long under
cultivation in any part of the world that land will be found to
produce species of the European genera _Lumbricus_, _Helodrilus_,
_Eisenia_, etc. More than this the recently imported European forms
will be found to have largely or almost entirely driven out the
native species, which have retired more into the interior of the
country. There is thus here no barrier placed by temperature. It
should be remarked, however, that while these earthworms are most
abundant in the less tropical regions, they occur in such tropical
districts as Peru, though in less striking numbers. Whether those
of North America are really indigenes or not remains perhaps a
matter for discussion; but it is at least noteworthy that the vast
preponderance of species occurring there are also European and even
British. In this particular case, which is on the whole the most
emphatic of all the cases of peregrine earthworms, some explanations
are possible, or at least have been offered. In the first place
it would appear that earthworms are more abundant as individuals
in northern countries where the soil is rarely dry for prolonged
periods. And as has been already pointed out there is a close
relation between earthworms and agriculture. Dunghills are fertile
gathering grounds for some species, and ploughed fields and gardens
always swarm with several species. In the tropics these animals are
not so evident; and the strong rays of the sun appear to drive them
further underground and into marshes; this obviously lessens the
chance of their accidental transference by man. Furthermore Dr Eisen
has pointed out that the European species are apt to have clitella
and to be fertile all the year round, which is not always the case
with other genera. That naturalist has added to this observation the
fact that in rich cultivated soils in California it is impossible to
find anything but imported European species, since cultivation itself
appears actually to drive away the native forms.




CHAPTER VII

THE EARTHWORMS OF OCEANIC ISLANDS


Oceanic islands are islands that have always been islands, a
definition that seems tautological until we compare it with some
other land masses that may be termed 'islands.' Geology teaches us
in fact that from the point of view of their origin islands may be
divided into two quite sharply contrasted classes. There are those
detached land masses usually lying near to or comparatively near to
some continent, which have been in the course of time detached by the
action of the waves from that continent, such as for instance the
British Isles, which undoubtedly represent a portion of the European
continent which was once quite continuous with Europe. On the other
hand we have the Hawaiian archipelago, St Helena, Fernando Noronha,
and other similar islands, which are more remote in their position
from continents and concerning which it seems clear that they have
originated _de novo_ by the action of submarine volcanos or of the
growth of coral, combined with subsidence, following elevation, or
from several of the causes combined. In any case the islands which
are termed oceanic islands have never formed part of a continent.
They are not relics of previously existing continents. It becomes
a matter of great interest to compare the earthworms which are to
be found upon oceanic islands with those which inhabit continental
islands. Fortunately there are a good many facts at our disposal for
this purpose; and we shall compare the earthworms of the Hawaiian
archipelago with those which are found upon certain small islands
lying to the south of New Zealand, viz. Campbell and Auckland islands
and the more southern Macquarie islands.

The earthworms of the Hawaiian archipelago have been studied by a
good many persons, and altogether a number of species have been
described from that group of islands of which the following is a
list: _Pheretima hawayana_, _P. heterochaeta_, _P. peregrina_,
_P. schmardae_, _P. hesperidum_, _P. morrisi_, _P. perkinsi_,
_P. biserialis_ (= _P. elongata_), _Allolobophora putris_ (=
Kinberg's _Hypogaeon havaicum_), _A. foetida_, _A. caliginosa_, _A.
nordenskiöldi_, _A. limicola_, _A. rosea_, and finally the well-known
_Pontoscolex corethrurus_. Of these species there is only one which
is even possibly a form limited to the Sandwich Islands, and that is
_Pheretima perkinsi_, a species which I myself at first described as
a new form, but which was afterwards regarded as identical with _P.
heterochaeta_ by Michaelsen, and later still resuscitated by Ude. All
the others are found in many parts of the world and not only in the
nearest mainland to the archipelago which we are now considering.
I have had already occasion to speak of some of them as peregrine
forms, especially of _Pontoscolex corethrurus_ which occurs all over
the world.

The conditions which have been recently revealed by an exploration
of the antarctic islands mentioned above are totally different. Dr
Benham has enumerated the following species from those islands,
viz. _Notiodrilus haplocystis_, _N. fallax_, _N. aucklandicus_,
_N. campbellianus_, _N. macquariensis_, _Plagiochaeta plunketi_,
_Rhododrilus cockayni_, _Leptodrilus leptomerus_, _L. magneticus_,
_Plutellus aucklandicus_, _Diporochaeta heterochaeta_, _D.
brachysoma_, _D. helophila_, _D. perionychopsis_ among the
Megascolecidae, besides _Phreodrilus campbellianus_, _Pelodrilus
tuberculatus_, _P. aucklandicus_ and the Lumbricid _Helodrilus
constrictus_. There were also four species of purely aquatic
Oligochaeta which we shall leave aside from the present enumeration,
as their range in space is a matter requiring a different explanation
from that of the terrestrial forms. Here we have a series of worms,
all of which, save the widely spread Lumbricid, are apparently
absolutely indigenous to the islands mentioned since they are all
different as _species_ from those found elsewhere. Indeed there is
a whole genus _Leptodrilus_, consisting, it is true, of but two
species, which is a native of the Campbell and Auckland islands and
of those only. The other genera are found in the antarctic region,
while _Pelodrilus_ is still more widely spread.

These facts as will be observed contrast about as strongly as they
can with those supplied by the fauna of Honolulu and its adjacent
islands. Not only are the worms of the antarctic islands different
species from those found elsewhere, but the majority of them do not
consist of widely ranging peregrine forms. It appears therefore most
probable that these islands are not oceanic islands but a portion of
the former existing northern portion of the antarctic continent. Were
the species _identical_ with those of New Zealand this conclusion
would have of course to be reconsidered. The barriers to migration
(see chap. VIII) explain the contrast recorded in the foregoing
pages.




CHAPTER VIII

MOVEMENT AND MIGRATION AMONG EARTHWORMS


That earthworms can move upon the surface of the ground at a rapid
pace is probably well enough known to everyone, and that they can
also burrow with considerable celerity. Multiplying the inches of
progress in minutes of time by centuries with the resulting miles, it
is quite clear that there is no reason to suppose that an individual
earthworm might not enormously extend its range under favourable
circumstances. Their powers of locomotion are such that they could
in the course of comparatively few centuries people a continent. As
a matter of fact these animals are frequently very widely spread
upon a given land surface; but on the other hand they are sometimes
equally limited. It behoves us therefore to enquire the reasons
for the possibility of extended migration and the causes which
have led to its restriction. We are now, it must be borne in mind,
considering these animals as purely terrestrial animals moving over
the surface of the land by their own unaided efforts. We leave out
of consideration any possible assistance in crossing water, whether
fresh or salt. We have to consider in fact in the present section
the earthworm inhabitants of larger and smaller tracts of continuous
land such as the African continent, which will serve as an excellent
example wherewith to test the facts and inferences.

And as a 'control' we can compare this continent with the very
different continent of Europe.

As an excellent instance, because of the certitude of specific and
in most cases of generic distinctions, we may take the Eudrilidae as
illustrative of the facts that are to be considered in the present
section. That family consists, as will be remembered, of 33 genera
at most, which have the following more exact range on the African
continent. The genus _Eudriloides_ occurs in British and German East
Africa and has been met with as far south as Mosambique and even
Durban, in which latter locality it has been thought that it is
really an accidentally introduced stranger. _Platydrilus_ is limited
to eastern equatorial Africa, thus not having quite the range of
_Eudriloides_.

The small genera (that is small in numbers of species)
_Reithrodrilus_, _Bogertia_, _Megachaetina_, _Metadrilus_, _Notykus_
have the same limitation of range as the last genus. _Metschaina_
has a wider range from tropical North East to lake Tanganyika.
_Stuhlmannia_ has a wider range still being found as it is in the
Tanganyika district, in tropical North East Africa, and in British
and German East Africa near the coast. _Pareudrilus_ reaches still
further north while _Nemertodrilus_ is limited to the Mosambique
region and to the Orange River district further south. The only
remaining genus of this sub-family of the Eudrilidae is _Libyodrilus_
which is purely West African and equatorial.

Of the remaining genera which are usually grouped together
into a second sub-family, five, viz. _Malodrilus_, _Kaffania_,
_Gardullaria_, _Teleudrilus_ and _Teleutoreutus_, are confined to
tropical North East Africa. _Eminoscolex_ occurs in the same district
but also to the south in the great lake region. The most remarkable
fact about this genus is that one species _E. steindachneri_ comes
from the Cameroons, and another _E. congicus_ from the Congo,
and thus the range of the genus is right across the continent.
_Neumanniella_ has much the same range. _Polytoreutus_ is a purely
equatorial East and Central genus, reaching from the coast to the
lakes. _Bettonia_ known by three species is from British East Africa.

The remaining genera, viz. _Hyperiodrilus_, _Heliodrilus_, _Alvania_,
_Iridodrilus_, _Rosadrilus_, _Euscolex_, _Parascolex_, _Preussiella_,
_Buttneriodrilus_, _Beddardiella_, _Metascolex_, are all West African
and the vast majority equatorial. We thus see that with one exception
the genera of East Africa are totally different from those of West
Africa and that the family as a whole is restricted in its range to
a comparatively small part of the vast African continent. It also
obviously follows, and it is advisable to state this fact however
obvious, that no species are common to the two sides of the continent
except indeed the ubiquitous _Eudrilus_, whose range over the world
has been more than once referred to in this book.

On the other hand the genus _Dichogaster_ offers quite different
facts, which are in contradiction to those just enumerated. This
genus as already said is very characteristic of tropical Africa,
and a large preponderance of the known species are confined to that
continent. Although there is some variation in structural characters
among the many species which compose this genus, there is but little
doubt that they are all rightly referred to one genus with perhaps
some doubtful, though not very striking, exceptions. In any case the
utmost divergence of structure between worms usually placed together
in this genus is nowhere near to that which separates the genera of
Eudrilidae from each other. Of the African members of the genus the
species are pretty evenly divided between the eastern and western
halves of the continent; they are, like the Eudrilidae, tropical in
range, not occurring to the southward, where their place is taken by
the Acanthodrilinae and Geoscolecidae. There are it is true a few
species, such as _D. gracilis_ and _D. bolavi_, which are common to
the two sides of Africa; but in these cases we clearly have to do
with those rather mysterious species which can apparently unduly
extend their range and which are known as peregrine forms; for they
also occur in other parts of the world besides Africa. We have
therefore in _Dichogaster_ the case of a genus which ranges all over
the tropical parts of Africa, but whose species are not common to the
Atlantic and Indian shores of that continent.

We will now contrast these conditions, which exemplify certain
facts shown by the characteristic Oligochaeta of tropical Africa,
with those which obtain in Europe. In this region of the world the
prevalent and practically the only genera which need be taken into
consideration in surveying the Oligochaetous fauna from the present
point of view, are _Lumbricus_ and the genus _Allolobophora_ of
Eisen which has been variously rearranged into genera and sub-genera
known by the names of _Helodrilus_, _Bimastos_, _Octolasium_, etc.
The structural differences which divide these genera and sub-genera
are not great; in any case they do not exhibit such a wide range
of variation from each other as do two such Eudrilid genera as
_Stuhlmannia_ and _Hyperiodrilus_. We find the genera mentioned not
only in Europe but extending themselves over more or less of Asia,
even occurring in Japan; while the North American continent contains
also representatives of the same. Not only do we find this community
of genera over vast extents of country greater in diameter than
the African continent, but there are also many species which range
as widely or nearly as widely as the case may be as the genus to
which they belong. Thus the species of _Allolobophora_ (we do not
trouble about the newer sub-divisions as they hardly affect the
facts to be emphasised), _A. caliginosa_, _A. longa_, _A. rubida_,
_A. chlorotica_, _A. octaedra_, _A. constricta_, _A. beddardi_,
_Lumbricus terrestris_, _L. castaneus_, have an enormously wide range
over what is generally termed the Palaearctic region, extending
also in some cases into the Nearctic. It is true no doubt that the
majority, indeed perhaps all, of these are, like certain species of
_Dichogaster_ mentioned above, among those forms termed peregrine
which have the capability of living in every quarter of the globe to
which they have apparently been conveyed by man. But there remain
many species which have a very extended habitat in the northern
hemisphere, and in any case the genera and the species are there
truly indigenous and widely spread.

It would thus appear that the capability for independent migration
varies greatly among earthworms. Of the types selected for
consideration the Eudrilidae are the slowest movers; the genus
_Dichogaster_ comes next, while the power of migration possessed by
the genera _Allolobophora_ and _Lumbricus_ is very much greater.
Assuming for the moment the correctness of this inference it is
clear that it will influence many other propositions connected
with the relative age of the families of these worms and with many
problems of geographical distribution. It appears to us that this
simple explanation is the correct one. But to show this it will be
necessary to eliminate other possible explanations. It might be
urged that the wider range of the genus _Dichogaster_ and the still
wider range of the genus _Allolobophora_ (shown by community of
species in widely distant localities) was evidence merely of relative
age, that the older groups have had more time to travel and that
the newer groups have not had so long a time to spread themselves
over their habitat. On this hypothesis the genera of Eudrilidae
would be geologically much newer than the genus _Dichogaster_ and
similar statements might be made for the other forms here under
consideration. As already explained we cannot attempt to answer this
question in the only way in which it can be really satisfactorily
answered, by a reference to fossil forms; for there are no fossils
to refer to. So far as comparative anatomy enables us to arrive
towards a solution of the question, it would appear that the genus
_Dichogaster_ belongs to a more ancient race than either of the other
two groups considered, and that of these latter the Lumbricidae are
the most modern. Moreover we associate not only a wide, but also
a discontinuous, distribution with an archaic race; and for this
reason also we should place the genus _Dichogaster_ in the position
of being the most ancient of these Oligochaeta. For the genus occurs
in Central America and in certain parts of the East as well as in
Africa. So that we can fairly dismiss the view that the Lumbricids by
virtue of their greater range over a given area are the most ancient
type and that their range is associated merely with their antiquity.
Nor does it appear that geographical or meteorological consideration
can have had effect in the present instances. For conditions
favourable to earthworms prevail in tropical Africa, as in Europe and
much of North Asia.


CLIMATE AS AFFECTING MIGRATION.

That excessively rigorous climatic conditions affect the range of
earthworms as well as fresh-water forms is quite clear from the
conditions which obtain in the most northern climes. At any rate
in those regions where physical conditions render it impossible
for these Annelids to have their being. A perpetual mantle of snow
and a temperature far below freezing point are absolute barriers
to the extension of range. And yet there are some few Oligochaeta
which do not in the least suffer from a somewhat milder taste of
such conditions. Thus species of Enchytraeidae have been met with
on glaciers and even found in frozen water, while a few earthworms
have been brought from the island of Kolguev. These however are
quite exceptions to the general sterility as regards earthworms of
the excessively cold regions. We have already seen that there are
no general facts to be deduced as concerning the relative abundance
of terrestrial worms in the tropics and in more temperate climes.
Tropical Africa is, it is true, rich in genera and species; but on
the other hand tropical East Indies have but few genera inhabiting
their numerous islands. Temperate England has very few genera and not
a large number of species; temperate New Zealand has a considerable
number of different indigenous genera. When however we leave this
general aspect of the question and consider separate families and
genera, there seems to be some little relation between climate and
distribution and thus some effect of climate in acting as a barrier
to migration. For example, though continuity of land surface permits
of the tropical African Eudrilidae ranging southwards as far as the
Cape they are not met with so far as we know in the most southern
parts of Africa; nor are the South American Geoscolecidae found in
Patagonia or northward beyond Central America. These instances do
really look like an influence of climate upon range. On the other
hand we must be careful to eliminate the possibility of another
explanation and that is the impossibility of successful migration
owing to the previous occupation of the ground with abundant other
forms. The very same countries would appear to show that this
explanation is unnecessary. For the prevalent genus of the southern
tracts of South America _Notiodrilus_ extends its way northward as
does the same genus from temperate to tropical Africa and Madagascar.

It looks very much, therefore, as if certain Oligochaeta are
dependent upon climate for their range, and as if others were at
least more independent of climatic conditions. And there are other
facts which support this view. The same opinion is supported by
the phenomena of involuntary migration, a subject which has been
considered also separately under the head of 'Peregrine forms.' The
great prevalence of Lumbricidae accidentally imported into many
parts of the world shows that temperature is no real bar to their
voluntary migration. On the other hand the fact that specimens of the
East Indian genus _Pheretima_ though commonly imported accidentally
into the warmer regions of the world have not been able to make good
a footing in Europe, save in greenhouses, shows that this genus is
affected in its range by questions of climate. These facts suggest
another inference of great interest which can only be mentioned
tentatively, and not put forward as a demonstrated conclusion. Seeing
that _Lumbricus_ (_sensu lato_) can comfortably take up its home
in warm extra-European countries, but yet that it has evidently
not spread to those countries in the course of nature but by man's
interference, it seems possible that time alone has prevented this;
and that therefore this family Lumbricidae is one of the most
recently evolved families of Oligochaeta. Certain structural features
support this way of looking at the matter. The same arguments
precisely apply to the genus _Pheretima_, which is also regarded by
most systematists as a recently developed race of earthworms. Anyhow
the conclusion which the facts seem to warrant is that the effects
of climate in influencing distribution are seen to have an unequal
effect upon earthworms, some genera being debarred by climatic
conditions while others are indifferent to the same.


MOUNTAIN RANGES AND THE MIGRATION OF EARTHWORMS.

In many groups of animals the interposition of a lofty chain of
mountains presents an insuperable barrier to migration. The barrier
is effective for more than one reason. Lack of vegetation and a
differing climate are among the more obvious causes which render
Alpine chains important as affecting distribution. There is plenty
of evidence in the way of positive fact that mountains are not
necessarily barriers to the spread of earthworms. The recent
explorations of the Ruwenzori chain of mountains in Africa have
resulted in the collection of a considerable number of species, some
of which come from great altitudes (_e.g._ 4000 metres and slightly
upwards), and one species, viz. _Dichogaster duwonica_, which Dr
Cognetti de Martiis described from the foot of the glacier Elena. I
have in my temporary possession a number of examples of the eastern
genus _Pheretima_, some of which are new species from lofty areas in
the Philippine Islands. There are plenty of other examples pointing
to a like conclusion. It is noteworthy that these forms which have
been met with at lofty heights are not essentially different from
the plain living forms. One cannot exactly speak, at any rate in the
present state of our knowledge, of anything like an Alpine fauna.

It is in fact clear enough that whatever may prove to be the
case with regard to particular species, a mountain range is not
necessarily a barrier to the dispersal of generic types.


THE OCEAN AS A BARRIER TO MIGRATION.

It is very possible that further investigations into the Oligochaeta
will prove that there are more marine forms than those which are
enumerated in another chapter. Particularly is this likely to be
the case among the family Tubificidae and Naididae. For up to the
present those forms belonging to those families which are known to be
positively marine in their habit show no great difference from allies
inhabiting fresh water, and are in one case indeed (_Paranais_)
common to fresh brackish and saline waters. As to earthworms,
the number is also extremely limited, and _Pontodrilus_ is up to
the present the only genus which is known to inhabit a marine
situation almost exclusively. It has, moreover, been shown that both
earthworms and their cocoons are susceptible to salt water and are
killed thereby. Thus the facilities which these animals possess of
crossing tracts of ocean are limited by this fact alone, besides
other impediments offered by tracts of water as such. We may in fact
entirely discount the possibility of earthworms floating across
arms of the sea--of any extent at any rate. For they do not swim or
float, but sink in water. Possibly when the alimentary tract was
entirely empty of earth the worms might float; but it is always full
and even if evacuated during their passage to the bottom waters the
body thus freed would hardly rise. However the noxious qualities of
sea water to earthworms is a sufficient barrier to their traversing
even narrow straits. On the other hand it might be suggested that
torn up trees especially with the roots and clinging earth still
attached might harbour worms and thus transmit them to foreign
shores. It has been suggested that in this or in some similar way
the species of _Notiodrilus_ have been wafted from shore to shore
of those lands which are washed by the Antarctic Ocean. Dr Benham,
however, in criticising this, calls attention to the violent gales
and disturbances of the ocean surface which are so prevalent in those
stormy regions, and doubts much whether these animals could retain
a safe hold upon some travelling tree trunk. Moreover it is only
in this antarctic region where the earthworm fauna of the various
continents and islands are so very similar.


FACILITIES OF MIGRATION.

The above brief account of physical features which affect the
range in space of the terrestrial Oligochaeta seem to show that
the only really important barrier is the ocean; and even a narrow
tract of sea water would, as it appears, act fatally in preventing
the successful immigration of a race inhabiting one shore to the
opposite shore. On the other hand we do undoubtedly find in different
countries--even when separated by a large expanse of ocean--closely
related forms. The most striking instance of this is that afforded
by a consideration of the antarctic species of _Notiodrilus_ and
_Chilota_. Can this interchange of Oligochaetous faunas be explained
by any means which earthworms possess of crossing tracts of sea by
the aid of living carriers such as birds? It has been definitely
shown that these creatures actually do convey such small animals as
Mollusca attached to their feet. Is anything of the kind likely in
the case of earthworms? In the first place it may be safely asserted
that if it be possible it has not been actually proved. This however
might be perhaps put down to the lack of sufficient observation of
actual birds and the contents of such masses of soil as are found
attached to their feet. A consideration of the habits of earthworms
seems to imply that such a mode of transference from country to
country is unlikely. In the first place we remark that the general
behaviour of earthworms renders this unlikely. Even the smaller
kinds, whose bulk would allow of their being carried, are too active
in their habits to permit of a safe transference. When disturbed
they wriggle and progress with activity. It is not conceivable that
they would remain quiescent for sufficient time to allow of a long
voyage. But while the bodily transference of adult earthworms seems
highly improbable it is conceivable at the first view that their
cocoons might be so transferred. We require to know rather more
about the cocoons of earthworms before we can accept this view as a
possibility; as far as our present knowledge goes it is not likely
that these animals can be assisted to emigrate in this way.

For the cocoons are rather bulky for this kind of porterage.
Moreover they are apt to be deposited rather deep down and among the
roots of grasses, and in situations where they are not so likely to
become entangled in the feet of drinking birds. Assuming, however,
that these difficulties can be got over there remains another
difficulty. A single cocoon among the terrestrial Oligochaeta does
not contain a large number of embryos, as has been pointed out on a
previous page. It is true that _Allolobophora foetida_ has six within
one cocoon, but most of our indigenous forms have but from one to
three embryos in a single cocoon. Thus, if successfully imported, it
is hardly likely that the developed embryos scattered after their
emergence would come together for breeding purposes; and in cocoons
with but one embryo the accidental importation in this way would have
to be very frequent to produce any result.

The case here is exactly the reverse of that afforded by the aquatic
families (or many of them). In these Annelids the attachment of the
cocoon to water plants, which are liable to be entangled in the feet
of shore-frequenting birds, would tend to favour migration. And in
addition to this the cocoons are naturally smaller and often contain
a considerable number of embryos. We are to note that the aquatic
forms are on the whole distinctly wider in their range than are the
earthworms.




CHAPTER IX

THE GEOGRAPHICAL DISTRIBUTION OF EARTHWORMS


The facts referred to and considered in the last chapter lead to
further observations upon the geographical distribution of this group
of animals and suggest problems for solution.

It is not the place here to give a general sketch of the division of
Biology termed Zoogeography; but a few general conclusions must be
laid before the reader in order to render what follows intelligible.
It is universally agreed that the range in space (and in time also)
of a given species of animal (or plant) is as much a part of its
scientific definition as are its anatomical characters. A description
for instance of _Acanthodrilus ungulatus_ is incomplete without a
reference to the fact that it occurs in, and is confined to, the
island of New Caledonia.

Each continent or island or part of a continent and part of an
island has its own peculiar inhabitants as well as some others
which range beyond its confines. Thus as we have seen the genus
_Hyperiodrilus_ is confined to the tropical West of Africa while the
genus _Dichogaster_ also found in that region is also met with in
other parts of Africa as well as in certain parts of America and of
the East. In this way the entire globe may be mapped out into regions
characterised by their inhabitants and these regions may also be
further subdivided. The commonly accepted regions were originally
devised by Mr Sclater and are known as the Palaearctic, Nearctic,
Neotropic, Ethiopian, Oriental (Mr Sclater's name was 'Indian'),
and Australian. These regions were originally formed to convey the
facts relative to the distribution of Passerine birds only; but
it is generally held that they apply also to the distribution of
vertebrates generally. The science of zoogeography does not however
end with the display of maps conveying graphically the mere facts of
distribution of this group and that. Its business is also to enquire
into the causes of the affinities between the faunas of different
regions or the varying degree of remoteness which those faunas may
show. On the one hand the varying powers of dispersal and the means
of extending their range possessed by different animals have to be
considered, and on the other hand geological changes in the relative
position of land masses have to be taken into account.

The specific identity between the earthworms of Great Britain and
the adjacent part of the continent of Europe would be very difficult
to understand were we only acquainted with the fact that salt water
is fatal to these animals. But we also know from geology that it
was only at a very recent date that England was cut off from union
with the continent. Thus an identity of fauna was to be expected.
On the other hand we are confronted with a very great difference
between the earthworms of eastern tropical Africa and of the adjacent
island of Madagascar. In the latter we have as a prevalent form the
genus _Kynotus_; in the former continent many Geoscolecidae but no
_Kynotus_. It is believed that the separation of Madagascar from
the mainland was at an earlier date than that of Great Britain from
Europe. We must however be cautious before slipping into what might
seem a case of arguing in a circle. It will however probably not be
disputed that Madagascar was severed earlier than England.

We will now attempt to map out the world into a series of regions
characterised by their earthworm inhabitants and see how far these
regions agree with those rendered necessary by the distribution of
some other animals.

We can to begin with accept the Palaearctic region. The region
however will be a little different from that usually accepted. For
we must probably exclude Japan, whose earthworm fauna contains the
characteristically Eastern genus _Pheretima_. Otherwise we have a
region characterised by the family Lumbricidae, which is really
limited to it, and by just a few traces of other genera such as
_Hormogaster_ among the Geoscolecidae and _Sparganophilus_ which
however is possibly an accidental immigrant. This region is certainly
quite clear. Now according to some persons such as Prof. Heilprin the
northern part of America should be joined with Europe and Asia to
form an Holarctic region; while by most authors, the separate name of
Nearctic is given to the north of the New World. With regard to the
terrestrial Oligochaeta it appears to me that this part of the world
is possibly to be excluded altogether as possessing no indigenous
worms.

In considering the distribution of the Mammalia Sir Ray Lankester
excluded New Zealand from his view as never having possessed any
indigenous mammalian fauna, and termed this part of the world
Atheriogaea. In the same way it is possible that the northern part
of the United States and Canada, whose earthworm fauna consists of
species of Lumbricidae identical with those of Europe, may possibly
be also a region to be excluded in the present survey and spoken of
as 'Ascolecogaea.' In the southern part of the United States we shall
find genera which will be considered presently. On the other hand it
is equally conceivable that this part of the world lost its earthworm
fauna through excessive glaciation in the ice age, the forms having
been driven south and are now only gradually making their way
northwards again. In this case the modern earthworm population which
appears to be absent from large tracts of Canada will be simply due
to involuntary migration. These two views must be left for further
development.

In any case the southern parts of the United States seem to
be separable as a distinct region from South America and to
be characterised by the sub-family Diplocardiinae, the genus
_Diplocardia_ extending as far northwards as the state of Illinois.
The distinctness of such a region however from Central America and
the West Indies is marred by the abundance of _Ocnerodrilus_ of
which Dr Eisen has described so many forms. On the other hand the
West Indies are closely allied in their earthworm fauna to tropical
South America, sharing with that region several forms of Geoscolecids
belonging in both cases invariably to the sub-family Geoscolecinae.
The bulk of the latter are undoubtedly tropical South American in
range and there is no doubt whatever about the distinctness of
this part of the world as a separate region. There is moreover a
further puzzle which confronts us who are trying to delimit an
American region or regions. In North America are species of the
genus _Argilophilus_ which is referred by Michaelsen to the genus
_Plutellus_ which comes from the East and at least one species of
_Megascolides_, also an Eastern genus.

There is at present no doubt to be thrown upon the indigeneity
of _Plutellus_. The species according to Dr Eisen show every sign
of being genuine inhabitants of California and like certain New
Zealand species such as the _Tokea esculenta_ of Benham (referred by
Michaelsen to the genus _Megascolides_) were eaten by the natives. If
these genera were forms restricted to North America, that is not only
with reference to the rest of America but to the world generally,
there would be as I think no doubt about the practicability of
making a Nearctic region. As it is, it seems to me to suit the
facts of distribution better to regard the whole of the land under
consideration as forming one great Neogaean region with three
sub-regions, the North American, Central American and West Indian,
and tropical South American. This region however will not as I take
it include the southernmost extremity of South America. Here in
Patagonia and in neighbouring islands we have a different earthworm
fauna. It is in fact characterised by the sub-family Acanthodrilinae
of which it is true some members of the genus _Notiodrilus_ extend
further north. I shall however defer this part of the subject until
the more easy delimitations of regions are disposed of.

Tropical Africa is evidently to be included in a third region
which will be defined by the Eudrilidae, Microchaetinae among the
Geoscolecidae, and by the great prevalence of _Dichogaster_, a genus
whose occurrence in other parts of the tropics is perhaps not yet
explained satisfactorily. Also we may record as characteristic of
this Ethiopian region a few peculiar genera such as _Nannodrilus_ and
_Gordiodrilus_. _Alma_ being a partly aquatic genus is perhaps less
distinctive and as a matter of fact it strays into the Palaearctic
region, being found in the lower waters of the Nile. It will be
observed that with this exception the limits of the Ethiopian region
according to earthworms agrees with that delimitation afforded by a
consideration of other groups since it stops short at the Sahara,
leaving northern Africa to be referred to the Palaearctic region.
At the same time we have an analogy with South America as concerns
the southern extremity of the African continent; here we meet with
_Notiodrilus_ and allied Acanthodrilinae just as in Patagonia and--as
also in that quarter of the world--these forms just stray into the
Ethiopian region above--specimens of _Notiodrilus_ being met with
in Madagascar as well as in tropical Africa. This bit of Africa as
it appears to me must also be cut off from the Ethiopian region and
included in an Antarctic region. Madagascar offers a further problem.
Are we to include this in Ethiopia or speak of a Malagasy region?
Apart from a few forms which are at least possibly to be looked upon
as accidental immigrants, such as members of the genera _Pheretima_
and _Gordiodrilus_, the fauna of Madagascar consists mainly of
many species of _Kynotus_. This genus, a member of the sub-family
Microchaetinae, of the family Geoscolecidae, affines Madagascar to
Ethiopia and leads me to place both in the same region though we may
doubtless speak of a Malagasy sub-region.

We have now to consider the eastern region of the world comprising
the two regions known generally to zoogeographers as the Oriental
and Australian. Taking a large view of the range of sub-families
and genera, and endeavouring to make the great regions of the globe
more or less equal, it seems difficult to divide further a region
which shall include all of this vast territory, and which may
therefore be termed Indo-Australian. For we find as characteristic
of the entire stretch of country the great majority of the genera
of the huge family Megascolecidae. Indeed the largest sub-family of
this family, _i.e._ the Megascolecinae, is, save for the mysterious
occurrence of the genera _Plutellus_ and _Megascolides_ in America,
absolutely limited to this area. Another sub-family, that of the
Octochaetinae, is limited to it. So far as concerns the others of
the sub-families of Megascolecidae it is only the Trigastrinae
which occur here (the genus _Eudichogaster_ and a few possibly
introduced species of _Dichogaster_) and a scattered species or two
of _Notiodrilus_ of the sub-family Acanthodrilinae. Again there are a
few and probably introduced species of the sub-family Ocnerodrilinae.
More important still this region has confined to itself the family
Moniligastridae; for a species described some years ago by myself
from the Bahamas is doubtless an introduced form. We have a complete
absence of indigenous Lumbricidae and Geoscolecidae excepting the
aquatic _Glyphidrilus_ of the sub-family Microchaetinae. It is true
that by taking isolated tracts, even large tracts, of this great
regional expanse a sub-division into well characterised regions can
be apparently recognised. But in taking such a step we shall be
confronted with the curious fact that it is rather neighbouring than
widely remote sub-divisions which present the greater differences.

If we compare for example India and New Zealand we find in common
such striking genera as _Octochaetus_, _Hoplochaetella_ and
_Diporochaeta_; whereas these genera are absent from the intervening
islands of the great Malay archipelago. On the other hand Australia
differs from the comparatively neighbouring islands of Borneo
and others by the absence in those islands of the characteristic
Australian genera such as _Megascolex_, _Notoscolex_, _Plutellus_
etc. which are in their turn found in India. It is facts like these
which render very difficult the apportioning of the tracts of country
forming the eastern hemisphere into separate regions.

There is no doubt that the Malay archipelago and the adjacent coasts
of Asia up to Japan differ from both India and Australia by the
almost entire limitation of the genus _Pheretima_ to them; but we
cannot intercalate a region in the middle of another geographical
area in this fashion!

The limitations of this great Indo-Australian region now demand
consideration. The chief difficulty is offered by the islands of New
Zealand and by some of the smaller islands lying far from but still
in the neighbourhood of New Zealand. Are we to include New Zealand
in this region? There is no doubt that the northern island of New
Zealand is much nearer to Australia in its earthworm fauna than
is the southern island. There are, it is true, a number of genera
peculiar to New Zealand, which are _Rhododrilus_, _Leptodrilus_,
_Maoridrilus_, _Neodrilus_, _Plagiochaeta_, _Pereiodrilus_,
_Dinodrilus_, _Dinodriloides_, but these do not represent the whole
of any family or even sub-family and they have all of them near
relations in other parts of the region as has been pointed out--even
to the peninsula of India itself. Again New Zealand contains members
of the genus _Notiodrilus_, that characteristic Antarctic form. In
fact New Zealand would appear to be a transitional zone between an
Indo-Australian and an Antarctic region.

The last region into which the world can be divided according to its
fauna of earthworms is an Antarctic. I am of distinct opinion that
this region is quite necessary in spite of the views of some others.
Although the genus _Notiodrilus_ certainly, and _Microscolex_
possibly, extend into the tropical regions of America, Africa, and
Australia, these species are but few, and the bulk of the species
and of the allied genus _Chilota_ are restricted to the antarctic
quarter of the globe; they also extend all over it, that is to say
in the southernmost parts of South America, in the Cape region of
Africa, in Kerguelen and the Crozet Islands, and in New Zealand, as
well as in the Auckland Islands and other neighbouring islands. It is
true that I have excluded New Zealand from this region on the grounds
that it forms a debateable ground between it and the Indo-Australian.
But apart from this part of the world the rest of the territories
mentioned should be combined to form the antarctic region.

Having therefore arrived at a mapping out of the world into regions
in accord with its earthworm fauna, it is desirable to ascertain
what light the facts throw upon the geological and evolutionary
questions with which the study of zoogeography deals. The existence
of an antarctic region binding together such distant points as South
Georgia, the Cape of Good Hope and Kerguelen Island, seems to argue
strongly for the former extensions northwards of the antarctic
continent so far north as to embrace these several regions of that
hemisphere. In view of the facts relating to the danger of sea water
to earthworms, to their lack of facilities for migration, other than
unassisted locomotion, points which have been dealt with earlier, it
is difficult to explain their range in the antarctic hemisphere on
other grounds. The very fact that the actual earthworm fauna of New
Zealand has led us on the whole to assign it to the Indo-Australian
regions shows the inherent uselessness of the current view of
zoogeography. For were we to leave the matter here the relationship
of New Zealand to the regions of the world which lie to the south of
it would not be apparent. However, here as in so many cases there is
an antagonism between cut and dried systems and the indications of
evolution.

This assumed existence of a former antarctic continent which
connected Southern Africa and Southern America as well as various
islands has perhaps a further justification in the distribution of
the Geoscolecidae. This family is divisible into two well-marked
sub-families of which one as has already been mentioned is limited
to South America and another practically to Africa (the exceptions
being species of the largely aquatic _Glyphidrilus_), while a third
sub-family the Criodrilinae is more widely distributed--again in
accordance, one may perhaps assume, with its largely aquatic mode of
life. It is also conceivable that the genus _Dichogaster_ is another
example pointing the same way. The arguments for regarding this genus
as an indigene of the East are not strong. But there is on the
other hand no doubt that the Indian _Eudichogaster_ is very closely
allied to it. But it is by no means excluded from this argument to
suppose that these Trigastrinae owe their likeness to convergence.
At any rate there are examples of equally marked convergence which
seem to be as nearly proved as can be in another though allied
group. The New Zealand _Neodrilus_ is to all intents and purposes a
_Maoridrilus_ in which one of the two pairs of spermiducal glands
and spermathecae has disappeared. It retains the characteristic
alternation in the position of the nephridia of _Maoridrilus_, and
other structural similarities unite the two genera. In the same way
species of _Microscolex_ seem as easily derivable from _Notiodrilus_.
_Microscolex_ and _Neodrilus_ are so near that had we no such hint of
their origin it would be reasonable to place them in the same genus.
They at least show a marked convergence.

It will be noticed therefore that the facts of their distribution
agree, as it would appear, with the structure of the terrestrial
Oligochaeta. The primitive characters of the genus _Notiodrilus_ are
to be seen in the double spermaries and glands appended to the duct,
and the corresponding spermatheca, in the absence, or very slight
development, of the papillae, so frequent in more specialised genera
such as _Pheretima_, and in the general simplicity of many organs of
the body which are more complicated elsewhere. As one would expect
with an archaic form this genus is widely ranging, being found in all
the principal land masses of the globe except in the Euro-Asiatic
continent.

Furthermore geographical facts would at least be not contradictory to
the view that this genus, and therefore the terrestrial Oligochaeta
generally, originated in the Antarctic hemisphere and that in pushing
northwards it has given off various descendants which survive in the
various regions of the world. Basing our views of the possibilities
of range among earthworms on the actual facts already dealt with, it
would seem that the peopling of America from Africa or of Africa from
America, if it has occurred, has not taken place through Europe and
the north generally. For otherwise we should expect traces of the
passage. It is true that we actually have _Hormogaster_ as a possible
sign that the Geoscolecidae have passed this way. But that is an
isolated case and may be referred to the extension northwards of this
particular genus rather than as an indication of a whole migration
through those territories. Another conclusion which a collocation of
the various facts brought together in this book appears to lead to is
that the group of the terrestrial Oligochaeta is relatively speaking
a modern one.

Convinced as we must be of the fact that range is only possible by
unaided locomotion through continuous land areas, the fact that but
few gaps occur in the range of a particular sub-family or lesser
group seems to indicate that no great time has elapsed since the
specialisation of these different forms. The dependence of earthworms
upon vegetable mould also points in the same direction and furnishes
an argument for the belief that these animals only greatly increased
on the advent of abundant dicotyledonous plants, and perhaps indeed
were actually contemporaneous with them.




LIST OF LITERATURE REFERRING TO EARTHWORMS


In the list given below I am only able to mention a few of the larger
works relating to this group. To give anything like a complete list
would demand many pages of titles. From the works selected the reader
can, if it be desired, find his way to the remaining literature of
the group.


A. GENERAL WORKS

  Vejdovsky. System und Morphologie der Oligochaeten. Prag, 1884.

  Beddard. A Monograph of the Oligochaeta. Oxford, 1895.

  Michaelsen. Oligochaeten in 'Das Thierreich.' Berlin, 1900.

  Michaelsen. Die Geographische Verbreitung der Oligochaeten, 1903.

  Vaillant. Annelès in Suites à Buffon. Paris, 1886.


B. EARTHWORMS OF (1) AUSTRALIA

  Fletcher. A series of papers in Journ. Linn. Soc. New South
  Wales, 1886-90.

  Spencer. A series of papers in Proc. Roy. Soc. Victoria, 1892-5.

  Michaelsen. In Die Fauna Südwest-Australiens. Jena, 1907.


(2) NEW ZEALAND AND ANTARCTIC ISLANDS

  Benham. Report on Oligochaeta of the Subarctic Islands of New
  Zealand. Wellington, N. Z., 1909.

  Benham. A series of papers in Quart. Journ. Micr. Sci. 1904,
  Proc. Zool. Soc. 1904, 1905 and Trans. N. Z. Inst., 1901-10.

  Beddard. In Trans. Roy. Soc. Edinb. 1891 and Proc. Zool. Soc.
  1889.


(3) ASIA

  Michaelsen. The Oligochaeta of India etc. in Memoirs Indian Mus.,
  1909.


(4) EUROPE

  Rosa. Revisione dei Lumbricidi. Mem. Acc. Torino, 1893.


(5) AFRICA

  Michaelsen. A series of papers in Mitth. Naturhist. Museum.
  Hamburg, 1891-1911.

  Beddard. Quart. Journ. Micr. Sci., 1890-95.


(6) AMERICA

  Eisen. Mem. Calif. Acad., 1894-96.

  Cognetti de Martiis. Mem. Acc. Torino, 1905-6.

  Rosa. Ibid., 1895.

  Beddard. In Hamburg. Magalh. Reise, 1895 and Nachtrag to same by
  Michaelsen.

Also numerous other works by the above-named authors and by Perrier,
Horst, Ude, Lankester, Stole, Pierantoni, Friend, Stephenson,
Southern, Goodrich, etc., etc.




INDEX


    Aeanthodrilinae, 14, 78, 80, 82, 84, 87, 102

    Acanthodrilus, 129

    Aeolosoma, 34-36

    Aeolosomatidae, 33, 34

    African earthworms, 74

    Agriodrilus, 31

    Allolobophora, 62, 85, 111, 117-119, 128

    Alluroides, 40, 41

    Alluroididae, 40

    Allurus, 45

    Alma, 24, 49, 50, 60, 61, 95, 135

    Alpine earthworms, 123

    Alvania, 75, 115

    American earthworms, 71

    Amphichaeta, 37

    Anachaeta, 42

    Anatomy of earthworms, 1

    Andiodrilus, 72, 74

    Andiorrhinus, 72

    Anteoides, 72

    Anteus, 72

    Antarctic earthworms, 86, 138

    Aptodrilus, 72

    Aquatic earthworms, 44

    Aquatic families of worms, 30

    Arctic earthworms, 88

    Argilophilus, 93, 133

    Asiatic earthworms, 80

    Astacopsidrilus, 37, 38

    Athecospermia, 39

    Aulophorus, 36

    Aurantina, 39

    Australian earthworms, 83


    Barriers to migration, 124

    Beddardiella, 76, 95, 115

    Benhamia, 72, 92

    Bettonia, 76, 115

    Bimastos, 117

    Bogertia, 75, 114

    Books and Memoirs upon earthworms, 144

    Bothrioneuron, 38, 39

    Branchiura, 38, 39, 70

    Branchiodrilus, 36

    Buttneriodrilus, 75, 115


    Callidrilus, 48, 77

    Cape of Good Hope, worms of, 76

    Chaetogaster, 37

    Chilota, 14, 72, 76, 77, 93, 126, 139

    Claparedilla, 39

    Climate and distribution, 120

    Clitellio, 38, 39, 53

    Cocoons of worms, 62, 127

    Criodrilinae, 24, 62, 85, 140

    Criodrilus, 24, 48, 49, 62, 72, 85, 92, 98


    Dendrobaena, 85

    Dero, 36, 37

    Desmogaster, 81, 94

    _Diachaeta_, 73

    Diaphorodrilus, 76

    Dichogaster, 15, 22, 28, 56, 72, 74, 76, 80, 82, 83, 84, 92, 99,
    104, 116-120, 129, 134, 140

    Didymogaster, 84

    Digaster, 84

    Dinodriloides, 87, 138

    Dinodrilus, 16, 55, 87, 138

    Diplocardia, 85, 133

    Diplotrema, 83

    Diporochaeta, 19, 80, 84, 87, 94, 111, 137

    Dispersal of earthworms, 113

    Drawida, 57, 81


    Eclipidrilus, 39

    Eisenia, 85, 108

    Eiseniella, 41, 45, 46, 49, 66, 85

    Eminoscolex, 75, 115

    Enantiodrilus, 72

    Enchytraeidae, 31, 41

    Environment, 59

    Eudichogaster, 80, 136, 141

    Eudrilidae, 25, 26, 28, 29, 61, 66, 75, 78, 86, 116, 134

    Eudrilus, 66, 75, 78, 79, 82, 83, 86, 97, 98, 101, 104, 105

    Eudriloides, 27, 75, 114

    Eupolygaster, 57, 81, 94

    Eupolytoreutus, 76

    Euscolex, 76, 95, 115

    Eutyphoeus, 16, 17, 80


    Families of worms, 14

    Fimoscolex, 72

    Fletcherodrilus, 26, 84

    Fresh-water worms, 30, 44


    Gardullaria, 76, 115

    Genera, range of, 90

    Geographical distribution, 129

    Geoscolecidae, 20, 22, 23, 46, 59, 61, 71, 76, 79, 86, 104, 132,
    134, 136, 137

    Geoscolecinae, 22, 23, 71

    Geoscolex (= Glossoscolex)

    Glossoscolex, 23, 72-74

    Glyphidrilus, 48, 49, 77, 79, 83, 98, 137, 140

    Gordiodrilus, 76, 78, 81, 92, 99, 106, 135


    Haplodrilus, 73

    Haplotaxidae, 42

    Haplotaxis, 31, 42

    Heliodrilus, 75, 115

    Helodrilus, 79, 85, 90, 91, 103, 108, 112, 117

    Henlea, 41

    Hesperodrilus, 38

    Hesperoscolex, 23, 58, 72, 73

    Holoscolex, 72, 76

    Hoplochaetella, 80, 87, 93, 137

    Hormogaster, 24, 85, 131, 142

    Hormogastrinae, 24, 85

    Howascolex, 78

    Hyperiodrilus, 30, 75, 115, 117, 129

    Hypogaeon, 111


    Ilyodrilus, 38

    Ilyogenia, 22, 73

    Iridodrilus, 76, 115

    India, earthworms of, 79


    Japan, earthworms of, 131


    Kerguelen, earthworms of, 139

    Kaffania, 75, 115

    Kerria, 18, 73, 99

    Kynotus, 23, 24, 29, 78, 95, 97, 131, 136


    Lampito, 78, 80

    Lamprodrilus, 39, 40

    Leptodrilus, 86, 111, 112, 138

    Libyodrilus, 28, 75, 115

    Light, influence of, 67

    Limicolae, 30, 47, 58

    Limnodriloides, 31, 38

    Limnodrilus, 38, 44

    Liodrilus, 73

    Lophochaeta, 38

    Lumbricidae, 38, 57, 61, 62, 83-86, 90, 103, 104, 107

    Lumbriculidae, 37, 41, 48

    Lumbriculus, 39

    Lumbricus, 85, 90, 108, 120, 122

    Lycodrilus, 38


    Macrochaetina, 37

    Madagascar, earthworms of, 77

    Maheina, 14, 78

    Malay region, earthworms of, 82, 137

    Malodrilus, 76, 115

    Man, his influence in distribution, 106, 123

    Maoridrilus, 11, 14, 87, 95, 138, 141

    Marine worms, 50

    Megachaetina, 75, 114

    Megadrili, 44, 50

    Megascolecidae, 14, 22, 29, 61, 75, 76, 79, 82, 87, 92, 99, 103, 136

    Megascolecinae, 19, 22, 80, 82, 84, 87, 136

    Megascolex, 19, 55, 62, 64, 80, 84, 93, 99, 137

    Megascolides, 80, 84, 87, 93, 133, 134, 136

    Mesoporodrilus, 39

    Metadrilus, 75, 114

    Metascolex, 76, 115

    Metschaina, 75, 114

    Microchaetinae, 24, 48, 76, 134, 136, 137

    Microchaetus, 23, 77, 95

    Microdrili, 31

    Microdrilus, 92

    Microscolex, 14, 72, 74, 83, 84, 94, 99, 102, 103, 106, 107, 139, 141

    Migration of earthworms, 113

    Millsonia, 92

    Moniligaster, 57, 81, 99

    Moniligastridae, 29, 41, 57, 61, 81, 83, 86, 94, 98, 137


    Naididae, 33-35

    Naidium, 37

    Nais, 37

    Nannodrilus, 18, 76, 135

    Nemertodrilus, 75, 115

    Nematogenia, 76, 81, 104

    Neodrilus, 87, 95, 138, 141

    Neumanniella, 76, 115

    New Zealand, earthworms of, 86

    Nile, worm of, 50

    Notiodrilus, 14-16, 20, 22, 37, 47, 52, 72, 74, 76, 77, 83, 84,
    86, 91-94, 111, 122, 126, 134-139, 141

    Notoscolex, 80, 84, 93, 137

    Notykus, 75, 114


    Ocnerodrilinae, 16, 32, 73, 76, 81, 136

    Ocnerodrilus, 18, 22, 73, 76, 78, 81, 84, 92, 99, 133

    Octochaetinae, 16, 80, 84

    Octochaetus, 16, 62, 80, 87, 93, 94, 137

    Octolasium, 85, 117

    Oceanic islands, earthworms of, 109

    Onychochaeta, 23, 72, 73

    Opisthodrilus, 72


    Papillae, 64

    Paranais, 36, 125

    Parascolex, 75, 115

    Pareudrilus, 75, 115

    Patagonia, worms of, 139

    Pelodrilus, 42, 111, 112

    Peloryctes, 39

    Peregrine species, 96

    Pereiodrilus, 87, 138

    Perichaetidae, 104

    Perionyx, 19, 22, 82, 84, 95, 99

    Perionychella, 84

    Periscolex, 23, 56, 72, 73

    Perissogaster, 84

    Pheretima, 19, 23, 55, 60-64, 73, 78, 80, 82, 84, 90, 94, 99, 102,
    104, 105, 110, 111, 122-124, 131, 135, 138, 141

    Phreatothrix, 39

    Phreodrilidae, 37

    Phreodrilus, 37, 38, 70, 111

    Phreodriloides, 38

    Phreoryctes (= Haplotaxis)

    Plagiochaeta, 15, 55, 87, 111, 138

    Platydrilus, 75, 114

    Pleurophleps, 35

    Plionogaster, 19, 29, 82

    Plutellus, 80, 84, 87, 93, 95, 111, 133, 134, 136, 137

    Polytoreutus, 25, 26, 58, 76

    Pontodrilus, 50, 51, 125

    Pontoscolex, 20, 22, 23, 66, 72-74, 78, 79, 82, 83, 96, 97, 98,
    101, 102, 104, 105, 111

    Preussiella, 75, 115

    Pristina, 37

    Psammoryctes, 38

    Pygmæodrilus, 76


    Reithrodrilus, 75, 114

    Rhinodrilus, 72, 73

    Rhizodrilus, 39

    Rhododrilus, 86, 111, 138

    Rhynchelmis, 39

    Ripistes, 86

    Rosadrilus, 75, 115


    Sea-shore, worms of, 50

    Sea as a barrier, 124

    Schmardaella, 36

    Senses of worms, 65

    Sparganophilus, 24, 47, 85, 92, 132

    Spenceriella, 80, 84, 87

    Structure (_see_ Anatomy)

    Stuhlmannia, 75, 114, 117

    Stylaria, 37

    Stylodrilus, 39

    Sutroa, 39

    Systematic arrangement, 14


    Teleudrilus, 76, 115

    Teleutoscolex, 39, 40

    Teleutoreutus, 76, 115

    Telmatodrilus, 38

    Temperature and distribution, 120

    Thamnodrilus (_see_ Rhinodrilus)

    Titanus, 23

    Tokea, 87, 134

    Trichochaeta, 23, 53

    Trichodrilus, 39

    Trigaster, 80, 84, 85

    Trigastrinae, 15, 72, 80, 82, 84, 136

    Tritogenia, 77

    Tubifex, 38, 44

    Tubificidae, 33, 37, 38, 48

    Tykonus, 72


    Urobenus, 72

    Urochaeta, 20


    Vejdovskyella, 37

    Vermiculus, 39


    Woodwardia, 80, 82, 84, 93


    Yagansia, 14, 72, 74, 76, 93


    Zapotecia, 85

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