=Modern Science Series=

               _EDITED BY SIR JOHN LUBBOCK, BART., M. P._




                       THE FAUNA OF THE DEEP SEA


[Illustration: STOMIAS BOA. HALF NATURAL SIZE. FROM A DEPTH OF 1,900
METRES. (AFTER FILHOL.)]




                       THE FAUNA OF THE DEEP SEA


                                   BY

                        SYDNEY J. HICKSON, M. A.
                           (CANTAB. ET OXON.)
          D.SC. (LOND.), FELLOW OF DOWNING COLLEGE, CAMBRIDGE


                   _WITH TWENTY-THREE ILLUSTRATIONS_

[Illustration]

                                NEW YORK
                        D. APPLETON AND COMPANY
                                 1894.




                         _Authorized Edition._




                                PREFACE


The time may come when there will be no portion of the earth’s surface
that has not been surveyed and explored by man.

The work of enterprising travellers has now been carried on within a
measurable distance of the North Pole; the highest mountain ranges are
gradually succumbing to the geological surveyor; the heart of Africa is
giving up to us its secrets and its treasures, and plans of all the
desert places of the earth are being made and tabulated.

The bottom of the deep sea was until quite recently one of these terræ
incognitæ. It was regarded by most persons, when it entered into their
minds to consider it at all, as one of those regions about which we do
not know anything, never shall know anything, and do not want to know
anything.

But the men of science fifty years ago were not disposed to take this
view of the matter. Pushing their inquiries as to the character of the
sea-fauna into deeper and deeper water, they at length demanded
information as to the existence of forms of animal life in the greatest
depths. Unable themselves to bear the heavy expenses involved in such an
investigation, they sought for and obtained the assistance of the
Government, in the form of national ships, for the work, and then our
knowledge of the depths of the great ocean may be said to have
commenced.

We know a good deal now, and in the course of time we may know a great
deal more, about this interesting region; but it is not one which, in
our generation at any rate, any human being will ever visit.

We may be able to plant the Union Jack on the summit of Mount Everest,
we may drag our sledges to the South Pole, and we may, some day, be able
to travel with ease and safety in the Great Sahara; but we cannot
conceive that it will ever be possible for us to invent a diving-bell
that will take a party of explorers to a depth of three and a half miles
of water. We may complete our survey of the ocean beds, we may analyse
the bottom muds and name and classify the animals that compose their
fauna, but there are many things that must remain merely matters of
conjecture. We shall never know, for example, with any degree of
certainty, how _Bathypterois_ uses its long feeler-like pectoral fins,
nor the meaning of the fierce armature of _Lithodes ferox_; why the
deep-sea Crustacea are so uniformly coloured red, or the intensity of
the phosphorescent light emitted by the Alcyonaria and Echinoderms.
These and many others are and must remain among the mysteries of the
abyss.

Our present-day knowledge of the physical conditions of the bottom of
the deep sea and the animals that dwell there is by no means
inconsiderable.

It may be found in the reports of the scientific expeditions fitted out
by the English, French, German, Italian, Norwegian, and American
Governments, in numerous volumes devoted to this kind of work, and in
memoirs and notes scattered through the English and foreign scientific
journals.

It is the object of this little book to bring together in a small
compass some of the more important facts and considerations that may be
found in this great mass of literature, and to present them in such a
form that they may be of interest to those who do not possess a
specialist’s knowledge of genera and species.

When it was found that animals can and do live even at the greatest
depths of the ocean, the interest of naturalists was concentrated on the
solution of the following problems. Firstly, do the animals constituting
the fauna of the abyss exhibit any striking and constant modification in
correlation with the physical conditions of their strange habitat? And,
secondly, from what source was the fauna of the abyss derived? Was it
derived from the shallow shore waters, or from the surface of the sea?
Is it of very ancient origin, or the result of, comparatively speaking,
recent immigrations?

These questions cannot be answered in a few lines. Any views that may be
put forward regarding them require the support of a vast array of facts
and figures; but as the limits of this little book would not permit of
my giving these, I have endeavoured to select a few only of those which
bear most directly upon the points at issue.

To overburden my work with the names of genera or the lists of species
would not, it seemed to me, either clear the issues or interest the
general reader. These may be found in the ‘Challenger’ monographs, and
other books dealing with the subject.

Those who wish to pursue the subject further will find in the ‘Voyages
of the “Blake,”’ by Alexander Agassiz, an excellent and elaborate
discussion of deep-sea problems, and numerous illustrations of some of
the most interesting forms of abysmal life.

In Volume XXIII. of the ‘Bulletin of Comparative Zoology’ the same
author gives a most interesting account of the deep-sea work that has
recently been done by the ‘Albatross’ expedition.

Filhol’s ‘La Vie au Fond des Mers’ is also a book that contains a great
deal of new and interesting matter, together with some excellent
coloured plates of deep-sea animals.

                                                      SYDNEY J. HICKSON.

                      DOWNING COLLEGE, CAMBRIDGE:
                           _September, 1893_.




                                CONTENTS


 CHAPTER                                                           PAGE
      I. A SHORT HISTORY OF THE INVESTIGATIONS                        1

     II. THE PHYSICAL CONDITIONS OF THE ABYSS                        17

    III. THE RELATIONS OF THE ABYSMAL ZONE AND THE ORIGIN OF ITS
           FAUNA                                                     45

     IV. THE CHARACTERS OF THE DEEP-SEA FAUNA                        59

      V. THE PROTOZOA, CŒLENTERA, AND ECHINODERMA OF THE DEEP SEA    86

     VI. THE VERMES AND MOLLUSCA OF THE DEEP SEA                    109

    VII. THE ARTHROPODA OF THE DEEP SEA                             123

   VIII. THE FISH OF THE DEEP SEA                                   148

 INDEX                                                              167




                         LIST OF ILLUSTRATIONS


       STOMIAS BOA. AFTER FILHOL, ‘LA VIE AU FOND DES
         MERS’                                            _Frontispiece_

  FIG.                                                              PAGE
     1 DIAGRAM ILLUSTRATING THE PASSAGE OF AN OCEAN
         CURRENT ACROSS A BARRIER                                     32

     2 _Sicyonis crassa._ AFTER HERTWIG, ‘“CHALLENGER”
         REPORTS’                                                     36

     3 GLOBIGERINA OOZE. AFTER AGASSIZ, ‘VOYAGES OF THE
         “BLAKE”’                                                     38

     4 SECTION THROUGH THE EYE OF _Serolis schythei_.
         AFTER BEDDARD, ‘“CHALLENGER” REPORTS’                        74

     5 SECTION THROUGH THE EYE OF _Serolis bromleyana_.
         AFTER BEDDARD, ‘“CHALLENGER” REPORTS’                        74

     6 _Opostomias micripnus._ AFTER GÜNTHER,
         ‘“CHALLENGER” REPORTS’                                       78

     7 HEAD OF _Pachystomias microdon_. AFTER VON
         LENDENFELD, ‘“CHALLENGER” REPORTS’                           79

     8 SECTION THROUGH THE ANTERIOR SUB-ORBITAL
         PHOSPHORESCENT ORGAN OF _Pachystomias microdon_.
         AFTER VON LENDENFELD, ‘“CHALLENGER” REPORTS’                 80

     9 _Challengeria Murrayi._ AFTER HAECKEL,
         ‘“CHALLENGER” REPORTS’                                       90

    10 _Umbellula Güntheri._ AFTER AGASSIZ, ‘VOYAGES OF
         THE “BLAKE”’                                                 97

    11 _Rhizocrinus lofotensis._ AFTER CARPENTER,
         ‘“CHALLENGER” REPORTS’                                      100

    12 _Rhabdopleura normani._ AFTER LANKESTER,
         ‘CONTRIBUTIONS TO OUR KNOWLEDGE OF RHABDOPLEURA
         AND AMPHIOXUS’                                              112

    13 A SINGLE POLYPIDE OF _Rhabdopleura normani_. AFTER
         LANKESTER, _tom. cit._                                      114

    14 _Bathyteuthis abyssicola._ AFTER HOYLE,
         ‘“CHALLENGER” REPORTS’                                      121

    15 _Bathynomus giganteus._ AFTER FILHOL, _tom. cit._             131

    16 _Euphausia latifrons._ AFTER SARS, ‘“CHALLENGER”
         REPORTS’                                                    134

    17 _Bentheuphausia amblyops._ AFTER SARS,
         ‘“CHALLENGER” REPORTS’                                      134

    18 _Polycheles baccata._ AFTER SPENCE BATE,
         ‘“CHALLENGER” REPORTS’                                      136

    19 _Colossendeis arcuatus._ AFTER FILHOL, _tom. cit._            141

    20 _Hypobythius calycodes._ AFTER MOSELEY,
         ‘“CHALLENGER” REPORTS’                                      145

    21 _Melanocetus Murrayi._ AFTER GÜNTHER,
         ‘“CHALLENGER” REPORTS’                                      156

    22 _Saccopharynx ampullaceus._ AFTER GÜNTHER,
         ‘“CHALLENGER” REPORTS’                                      164




                       THE FAUNA OF THE DEEP SEA




                               CHAPTER I
                 A SHORT HISTORY OF THE INVESTIGATIONS


Our knowledge of the natural history of the deep seas may be said to
have commenced not more than fifty years ago. There are, it is true, a
few fragments of evidence of a fauna existing in depths of more than a
hundred fathoms to be found in the writings of the earlier navigators,
but the methods of deep-sea investigation were so imperfect in those
days that naturalists were disposed to believe that in the abysses of
the great oceans life was practically non-existent.

Even Edward Forbes just before his death wrote of an abyss ‘where life
is either extinguished or exhibits but a few sparks to mark its
lingering presence,’ but in justice to the distinguished naturalist it
should be remarked that he adds, ‘Its confines are yet undetermined, and
it is in the exploration of this vast deep-sea region that the finest
field for submarine discovery yet remains.’

Forbes was only expressing the general opinion of naturalists of his
time when he refers with evident hesitation to the existence of an azoic
region. His own dredging excursions in depths of over one hundred
fathoms proved the existence of many peculiar species that were
previously unknown to science. ‘They were like,’ he says, ‘the few stray
bodies of strange red men, which tradition reports to have been washed
on the shores of the Old World before the discovery of the New, and
which served to indicate the existence of unexplored realms inhabited by
unknown races, but not to supply information about their character,
habits, and extent.’

In the absence of any systematic investigation of the bottom of the deep
sea, previous to Forbes’s time the only information of deep-sea animals
was due to the accidental entanglement of certain forms in sounding
lines, or to the minute worms that were found in the mud adhering to the
lead.

As far back as 1753, Ellis described an Alcyonarian that was brought up
by a sounding line from a depth of 236 fathoms within eleven degrees of
the North Pole by a certain Captain Adriaanz of the ‘Britannia.’ The
specimen was evidently an _Umbellula_, and it is stated that the arms
(i.e. Polyps) were of a bright yellow colour and fully expanded when
first brought on deck.

In 1819 Sir John Ross published an account of his soundings in Baffin’s
Bay, and mentions the existence of certain worms in the mud brought from
a depth of 1,000 fathoms, and a fine Caput Medusæ (Astrophyton)
entangled on the sounding line at a depth of 800 fathoms.

In the narrative of the voyage of the ‘Erebus’ and ‘Terror,’ published
in 1847, Sir James Ross calls attention to the existence of a deep-sea
fauna, and makes some remarks on the subject that in the light of modern
knowledge are of extreme interest. ‘I have no doubt,’ he says, ‘that
from however great a depth we may be enabled to bring up the mud and
stones of the ocean, we shall find them teeming with animal life.’ This
firm belief in the existence of an abysmal fauna was not, as it might
appear from the immediate context of the passage I have quoted, simply
an unfounded speculation on his part, but was evidently the result of a
careful and deliberate chain of reasoning, as may be seen from the
following passage that occurs in another part of the same book:—‘It is
well known that marine animals are more susceptible of change of
temperature than land animals; indeed they may be isothermally arranged
with great accuracy. It will, however, be difficult to get naturalists
to believe that these fragile creatures could possibly exist at the
depth of nearly 2,000 fathoms below the surface; yet as we know they can
bear the pressure of 1,000 fathoms, why may they not of two? We also
know that several of the same species of creatures inhabit the Arctic
that we have fished up from great depths in the Antarctic seas. The only
way they could get from one pole to the other must have been through the
tropics; but the temperature of the sea in those regions is such that
they could not exist in it, unless at a depth of nearly 2,000 fathoms.
At that depth they might pass from the Arctic to the Antarctic Ocean
without a variation of five degrees of temperature; whilst any land
animal, at the most favourable season, must experience a difference of
fifty degrees, and, if in the winter, no less than 150 degrees of
Fahrenheit’s thermometer—a sufficient reason why there are neither
quadrupeds, nor birds, nor land insects common to both regions.’

In the year 1845, Goodsir succeeded in obtaining a good haul in Davis
Straits, at a depth of 300 fathoms. It included Mollusca, Crustacea,
Asterids, Spatangi, and Corallines.

In 1848, Lieutenant Spratt read a paper at the meeting of the British
Association at Swansea, on the influence of temperature upon the
distribution of the fauna in the Ægean seas, and at the close of this
paper we find the following passage, confirming in a remarkable way the
work of previous investigators in the same field. He says: ‘The greatest
depth at which I have procured animal life is 390 fathoms, but I believe
that it exists much lower, although the general character of the Ægean
is to limit it to 300 fathoms; but as in the deserts we have an oasis,
so in the great depths of 300, 400, and perhaps 500 fathoms we may have
an oasis of animal life amidst the barren fields of yellow clay
dependent upon favourable and perhaps accidental conditions, such as the
growth of nullipores, thus producing spots favourable for the existence
and growth of animal life.’

The next important discovery was that of the now famous Globigerina mud
by Lieutenants Craven and Maffit, of the American Coast survey, in 1853,
by the help of the sounding machine invented by Brooke. This was
reported upon by Professor Bailey.

Further light was thrown upon the deep-sea fauna by Dr. Wallich in 1860,
on board H.M.S. ‘Bulldog’, by the collection of thirteen star-fish
living at a depth of 1,260 fathoms.

Previous to this Torell, during two excursions to the Northern seas, had
proved the existence of an extensive marine fauna in 300 fathoms, and
had brought up with the ‘Bulldog’ machine many forms of marine
invertebrates from depths of over 1,000 fathoms; but it was not until
1863, when Professor Lovén read a report upon Torell’s collections, that
these interesting and valuable investigations became known to
naturalists.

Nor must mention be omitted of the remarkable investigations of Sars and
his son, the pioneers of deep-sea zoology on the coasts of Norway, who,
by laborious work commenced in 1849, failed altogether to find any
region in the deep water where animal life was non-existent, and indeed
were the first to predict an extensive abysmal fauna all over the floor
of the great oceans. One of the many remarkable discoveries made by Sars
was _Rhizocrinus_, a stalked Crinoid.

Ever since that time the Norwegians and the Swedes have been most
energetic in their investigations, and the publications of the results
of the Norske Nord-havns expeditions are regarded by all naturalists as
among the most valuable contributions to our knowledge of the deep-sea
fauna.

Notwithstanding the previous discovery of many animals that undoubtedly
came from the abysmal depths of the ocean, there were still some persons
who found a difficulty in believing that animal life could possibly
exist under the enormous pressure of these great depths. It was
considered to be more probable that these animals were caught by the
dredge or sounding lines in their ascent or descent; and that they were
merely the representatives of a floating fauna living a few fathoms
below the surface.

The first direct proof of the existence of an invertebrate fauna in deep
seas was found by the expedition that was sent to repair the submarine
cable of the Mediterranean Telegraph Company. The cable had broken in
deep water, and a ship was sent out to examine and repair the damage.
When the broken cable was brought on deck, it bore several forms of
animal life that must have become attached to it and lived at the bottom
of the sea in water extending down to a depth of 1,200 fathoms. Among
other forms a _Caryophyllia_ was found attached to the cable at 1,100
fathoms, an oyster (_Ostrea cochlear_), two species of Pecten, two
gasteropods, and several worms.

The discoveries that had been made indicating the existence of a
deep-sea fauna led to the commission of H.M. ships ‘Lightning’ and
‘Porcupine,’ and the systematic investigation that was made by the
naturalists on these vessels brought home to the minds of naturalists
the fact that there is not only an abysmal fauna, but that in places
this deep-sea fauna is very rich and extensive. The ‘Lightning’ was
despatched in the spring of 1868 and carried on its investigations in
the neighbourhood of the Faeroe Islands, but the vessel was not suitable
for the purpose and met with bad weather. The results, however, were of
extreme importance; for, besides solving many important points
concerning the distribution of ocean temperature, ‘it had been shown
beyond question that animal life is both varied and abundant at depths
in the ocean down to 650 fathoms at least, notwithstanding the
extraordinary conditions to which animals are there exposed.’

Among the remarkable animals dredged by the ‘Lightning’ were the curious
Echinoderm, _Brisinga coronata_, previously discovered by Sars, and the
Hexactinellid sponges, _Holtenia_ and _Hyalonema_, the Crinoids
_Rhizocrinus_ and _Antedon celticus_, and the Pennatulid _Bathyptilum
Carpenteri_, not to mention numerous Foraminifera new to science.

In the spring of the following year, 1869, the Lords Commissioners of
the Admiralty despatched the surveying vessel ‘Porcupine’ to carry on
the work commenced by the ‘Lightning.’

The first cruise was on the west coast of Ireland, the second cruise to
the Bay of Biscay, where dredging was satisfactorily carried on to a
depth of 2,435 fathoms, and the third in the Channel between Faeroe and
Scotland.

The dredging in 2,435 fathoms was quite successful, and the dredge
contained several Mollusca, including new species of _Dentalium_,
_Pecten_, _Dacrydium_, &c., numerous Crustacea and a few Annelids and
Gephyrea, besides Echinoderma and Protozoa. A satisfactory dredging was
also made in 1,207 fathoms.

The third cruise was also successful and brought many new species to
light, including the _Porocidaris purpurata_, and a remarkable heart
urchin, _Pourtalesia Jeffreysi_.

Concerning Pourtalesia Sir Wyville Thomson says:—

‘The remarkable point is that, while we had so far as we were aware no
living representative of this peculiar arrangement of what is called
“disjunct” ambulacra, we have long been acquainted with a fossil
family—the Dysasteridæ—possessing this character. Many species of the
genera Dysaster, Collyrites, &c., are found from the lower oolite to the
white chalk, but there the family had previously been supposed to have
become extinct.’

The discovery of two new Crinoids led to the anticipation that the
Crinoidea, the remarkable group of Echinoderma, supposed at the time to
be on the verge of extinction, probably form rather an important element
in the abysmal fauna.

One of the most interesting results was the discovery of three genera in
deep water, _Calveria_, _Neolampas_ and _Pourtalesia_, almost
immediately after they were discovered by Pourtales in deep water on the
coasts of Florida, showing thus a wide lateral distribution and
suggesting a vast abysmal fauna.

A year before the ‘Lightning’ was despatched, Count Pourtales had
commenced a series of investigations of the deep-sea fauna off the coast
of Florida. The first expedition started in 1867 from Key West for the
purpose of taking some dredgings between that port and Havana.
Unfortunately yellow fever broke out on board soon after they started,
and only a few dredgings were taken. However, the results obtained were
of such importance that they encouraged Pourtales to undertake another
expedition and enabled him to say very positively ‘that animal life
exists at great depths, in as great a diversity and as great an
abundance as in shallow water.’

In the following years, 1868 and 1869, the expeditions were more
successful, and many important new forms were found in water down to 500
fathoms. Perhaps the most interesting result obtained was the discovery
of _Bourguetticrinus_ of D’Orbigny; it may even be the species named by
him which occurs fossil in a recent formation in Guadeloupe.

By this time the interest of scientific men was thoroughly excited over
the many problems connected with this new field of work. The prospect of
obtaining a large number of new and extremely curious animals, the faint
hope that living Trilobites, Cystids, and other extinct forms might be
discovered, and lastly the desire to handle and investigate great masses
of pure protoplasm in the form of the famous but unfortunately
non-existent Bathybius, induced some men of wealth and leisure to spend
their time in deep-sea dredging, and stimulated the governments of some
civilised countries to lend their aid in the support of expeditions for
the deep-sea survey.

Mr. Marshall Hall’s yacht, the ‘Norma,’ was employed for some time in
this work, and an extensive collection of deep-sea animals was made.
About the same time Professor L. Agassiz was busy on board the American
ship, the ‘Hassler,’ in continuing the work of Count Pourtales, and
later on the Germans fitted out the ‘Gazelle,’ and the French the still
more famous ‘Travailleur’ and ‘Talisman’ expeditions. Nor must we omit
to mention in this connection the cruise of the Italian vessel, the
‘Vittor Pessani,’ nor those of the British surveying vessels, the
‘Knight Errant’ and the ‘Triton,’ and the American vessels, ‘The Blake’
and the ‘Fish Hawk.’

But of all these expeditions, by far the most complete in all the
details of equipment, and the arrangements made for the publication of
the results, was the expedition fitted out in 1873 by the British
Government. The voyage of H.M.S. ‘Challenger’ is so familiar to all who
take an interest in the progress of scientific discovery, that it is not
necessary to do more than make a passing mention of it in this place.
The excellent books that were written by Wyville Thomson, by Moseley,
and by other members of the staff, have made the general reader familiar
with the narrative of that remarkable cruise and the most striking of
the many scientific discoveries that were made; while the numerous large
monographs that have been published during the past fourteen years give
opportunities to the naturalist of obtaining all the requisite
information concerning the detailed results of the expedition.

The expenditure of the large sum of money upon this expedition and the
publication of its reports has been abundantly justified. The
information obtained by the ‘Challenger’ will be for many years to come
the nucleus of our knowledge of the deep-sea fauna, the centre around
which all new facts will cluster, and the guide for further
investigations.

To say that the ‘Challenger’ accomplished all that was expected or
required would be to over-estimate the value of this great expedition,
but nevertheless it is difficult for us, even now, thoroughly to grasp
the importance of the results obtained or to analyse and classify the
numerous and very remarkable facts that were gained during her four
years’ cruise.

It is, of course, impossible, in a few lines, to give a summary of the
more important of the Natural History results of the ‘Challenger’
expedition. Besides proving the existence of a fauna in the sea at all
depths and in all regions, the expedition further proved that the
abysmal fauna, taken as a whole, does not possess characters similar to
those of the fauna of any of the secondary or even tertiary rocks. A few
forms, it is true, known to us up to that time only as fossils, were
found to be still living in the great depths, but a large majority of
the animals of these regions were found to be new and specially modified
forms of the families and genera inhabiting shallow waters of modern
times. No Trilobites, no Blastoids, no Cystoids, no new Ganoids, and
scarcely any deep-sea Elasmobranchs were brought to light, but the fauna
was found to consist mainly of Teleosteans, Crustacea, Cœlentera, and
other creatures unlike anything known to have existed in Palæozoic
times, specially modified in structure for their life in the great
depths of the ocean.

In 1876 the S.S. ‘Vöringin’ was chartered by the Norwegian Government
and was dispatched to investigate the tract of ocean lying between
Norway, the Faeroe islands, Jan Mayen, and Spitzbergen. The
investigations extended over three years, the vessel returning to Bergen
in the winter months.

The civilian staff of the ‘Vöringin’ included Professors H. Mohn,
Danielssen, and G. O. Sars, and the expedition was successful in
obtaining a large number of animals from deep water by means of the
dredge and tangles and by the trawl.

The results of this expedition have been published in a series of large
quarto volumes under the general title of the Norske Nord-havns
Expedition.

The most interesting forms brought to light by the Norwegians are the
two genera _Fenja_ and _Aegir_, animals possessing the general form of
sea anemones but distinguished from all Cœlenterates by the presence of
a continuous and straight gut reaching from the mouth to the aboral
pores which completely shuts off the cœlenteron or general body cavity
from the stomodæum.

In more recent times the work has been by no means neglected. With the
advantage of employing many modern improvements in the dredges and
trawls in use, the American steamer, the ‘Albatross,’ has been engaged
in a careful investigation of the deep-sea fauna of the eastern slopes
of the Pacific Ocean, while at the same time Her Majesty’s surveying
vessel, the ‘Investigator,’ has been obtaining some interesting and
valuable results from a survey of the deep waters of the Indian Ocean.
But our knowledge of this vast and wonderful region is still in its
infancy. We have gathered, as it were, only a few grains from a great
unknown desert. It is true that we may not for many years, if ever,
obtain any results that will cause the same deep interest and excitement
to the scientific public as those obtained by the first great national
expeditions, but there are still many important scientific problems that
may be and will be solved by steady perseverance in this field of work,
and if we can only obtain the same generous support from public
institutions and from those in charge of national funds that we have
received in the past two decades, many more important facts will
doubtless be brought to light.




                               CHAPTER II
                  THE PHYSICAL CONDITIONS OF THE ABYSS


It is not surprising that the naturalists of the early part of the
present century could not believe in the existence of a fauna at the
bottom of the deep seas.

The extraordinary conditions of such a region—the enormous pressure, the
absolute darkness, the probable absence of any vegetable life from want
of direct sunlight—might very well have been considered sufficient to
form an impassable barrier to the animals migrating from the shallow
waters and to prevent the development of a fauna peculiarly its own.

The fragmentary accounts of animals brought up by sounding lines from
great depths might, it is true, have thrown doubts on the current views;
but they were not of sufficient importance in themselves, nor were the
observations made with such regard to the possibility of error, as to
withstand the critical remarks that were made to explain them away.

The absence of any evidence obtained by accurate systematic research,
together with the consideration of the physical character of the ocean
bed, were quite sufficient to lead scientific men of that period to
doubt the existence of any animal life in water deeper than a few
hundred fathoms.

We now know, however, that there is a very considerable fauna at
enormous depths in all the great oceans, and we have acquired, moreover,
considerable information concerning some of those peculiar physical
conditions of the abyss that fifty years ago were merely matters of
speculation among scientific men.

The relation between animals and their environment is now a question of
such great interest and importance that it is necessary in any
description of the fauna of a particular region to consider its physical
conditions and the influence that it may be supposed to have had in
producing the characteristics of the fauna.

The peculiar physical conditions of the deep seas may be briefly stated
to be these: It is absolutely dark so far as actual sunlight is
concerned, the temperature is only a few degrees above freezing point,
the pressure is enormous, there is little or no movement of the water,
the bottom is composed of a uniform fine soft mud, and there is no plant
life.

All of these physical conditions we can appreciate except the enormous
pressure. Absolute darkness we know, the temperature of the deep seas is
not an extraordinary one, the absence of movement in the water and the
fine soft mud are conditions that we can readily appreciate; but the
pressure is far greater than anything we can realise.

At a depth of 2,500 fathoms the pressure is, roughly speaking, two and a
half tons per square inch—that is to say, several times greater than the
pressure exerted by the steam upon the piston of our most powerful
engines. Or, to put the matter in other words, the pressure per square
inch upon the body of every animal that lives at the bottom of the
Atlantic Ocean is about twenty-five times greater than the pressure that
will drive a railway train.

A most beautiful experiment to illustrate the enormous force of this
pressure was made during the voyage of H.M.S. ‘Challenger.’ I give the
description of it in the words of the late Professor Moseley.

‘Mr. Buchanan hermetically sealed up at both ends a thick glass tube
full of air, several inches in length. He wrapped this sealed tube in
flannel, and placed it, so wrapped up, in a wide copper tube, which was
one of those used to protect the deep-sea thermometers when sent down
with the sounding apparatus.

‘This copper tube was closed by a lid fitting loosely, and with holes in
it, and the copper bottom of the tube similarly had holes bored through
it. The water thus had free access to the interior of the tube when it
was lowered into the sea, and the tube was necessarily constructed with
that object in view, in order that in its ordinary use the water should
freely reach the contained thermometer.

‘The copper case containing the sealed glass tube was sent down to a
depth of 2,000 fathoms and drawn up again. It was then found that the
copper wall of the case was bulged and bent inwards opposite the place
where the glass tube lay, just as if it had been crumpled inward by
being violently squeezed.

‘The glass tube itself, within its flannel wrapper, was found when
withdrawn, reduced to a fine powder, like snow almost. What had happened
was that the sealed glass tube, when sinking to gradually increasing
depths, had held out long against the pressure, but this at last had
become too great for the glass to sustain, and the tube had suddenly
given way and been crushed by the violence of the action to a fine
powder. So violent and rapid had been the collapse that the water had
not had time to rush in by means of the holes at both ends of the copper
cylinder and thus fill the empty space left behind by the collapse of
the glass tube, but had instead crushed in the copper wall and brought
equilibrium in that manner. The process is exactly the reverse of an
explosion, and is termed by Sir Wyville Thomson an “implosion.”’

It is but reasonable to suppose that the ability to sustain this
enormous pressure can only be acquired by animals after generations of
gradual migrations from shallow waters. Those forms that are brought up
by the dredge from the depths of the ocean are usually killed and
distorted by the enormous and rapid diminution of pressure in their
journey to the surface, and it is extremely probable that shallow-water
forms would be similarly killed and crushed out of shape were they
suddenly plunged into very deep water. The fish that live at these
enormous depths are in consequence of the enormous pressure liable to a
curious form of accident. If, in chasing their prey or for any other
reason, they rise to a considerable distance above the floor of the
ocean, the gases of their swimming bladder become considerably expanded
and their specific gravity very greatly reduced. Up to a certain limit
the muscles of their bodies can counteract the tendency to float upwards
and enable the fish to regain its proper sphere of life at the bottom;
but beyond that limit the muscles are not strong enough to drive the
body downwards, and the fish, becoming more and more distended as it
goes, is gradually killed on its long and involuntary journey to the
surface of the sea. The deep-sea fish, then, are exposed to a danger
that no other animals in this world are subject to, namely that of
tumbling upwards.

That such accidents do occasionally occur is evidenced by the fact that
some fish, which are now known to be true deep-sea forms, were
discovered dead and floating on the surface of the ocean long before our
modern investigations were commenced.

Until quite recently, every one agreed that no rays of sunlight could
possibly penetrate the sea to a greater depth than a few hundred
fathoms.

Moseley says that ‘probably all is dark below 200 fathoms excepting in
so far as light is given out by phosphorescent animals,’ and Wyville
Thomson speaks of the ‘utter darkness of the deep-sea bottom.’

Within the last few years a few authors have maintained that it is quite
possible that a few rays of sunlight do penetrate even to the greatest
depths of the ocean—a view mainly based on the fact that so many
deep-sea animals possess extremely perfect and complicated eyes and very
brilliant colours. Verrill says: ‘It seems to me probable that more or
less sunlight does actually penetrate to the greatest depths of the
ocean, in the form of a soft sea-green light, perhaps at 2,000 or 3,000
fathoms equal in intensity to our partially moonlight nights and
possibly at the greatest depths equal only to starlight. It must be
remembered that in the deep sea far away from land the water is far more
transparent than near the coast.’ Packard is of a similar opinion.

There seem to me to be very slight grounds for this view. The fact that,
comparatively speaking, shallow-water fish avoid nets that are rendered
phosphorescent by entangled jelly-fish does not justify us in assuming
that deep-sea fish avoid regions where there are phosphorescent
Gorgonians or Pennatulids. It is not by any means certain that fish
avoid sunken nets on account of their phosphorescence. Most fish
possess, as is well known, a very acute sense of smell, and it is very
probable that they avoid such nets on account of the putrid odours of
the dead animals that remain attached to them.

Nor is there much strength in the further argument that it can hardly be
possible that there can be an amount of phosphorescent light regularly
evolved by the few deep-sea animals, having this power, sufficient to
cause any general illumination, or powerful enough to have influenced,
over the whole ocean, the evolution of complex eyes, brilliant and
complex protective colours, and complex commensal adaptations.

We have no sound information to go upon to be able to judge of the
amount of light given off by phosphorescent animals at the bottom of the
deep sea. The faint light they show on deck after their long journey
from the depths in which they live to the surface may be extremely small
compared with the light they give in their natural home under a pressure
of 2½ tons to the square inch. The complex eyes that many deep-sea
animals exhibit were almost certainly not evolved as such, but are
simple modifications of eyes possessed by a shallow-water ancestry.

The more recent experiments that have been made, tend to show that no
sunlight whatever penetrates to a greater depth, to take an extreme
limit, than 500 fathoms.

Fol and Sarasin, experimenting with very sensitive bromo-gelatine
plates, found that there was no reaction after ten minutes’ exposure at
a depth of 400 metres on a sunny day in March.

But although it is highly probable that not a glimmer of sunlight ever
penetrates to the depths of the ocean, there is in some places,
undoubtedly, a very considerable illumination due to the phosphorescence
of the inhabitants of the deep waters.

All the Alcyonarians are, according to Moseley, brilliantly
phosphorescent when brought to the surface. Many deep-sea fish possess
phosphorescent organs, and it is quite possible that many of the
deep-sea Protozoa, Tunicates, Jelly-fish, and Crustacea are in their
native haunts capable of giving out a very considerable amount of
phosphorescent light.

If we may be allowed to compare the light of abysmal animals with that
of surface forms, we can readily imagine that some regions of the sea
may be as brightly illuminated as a European street is at night—an
illumination with many very bright centres and many dark shadows, but
quite sufficient for a vertebrate eye to distinguish readily and at a
considerable distance both form and colour.

To give an example of the extent to which the illumination due to
phosphorescent organisms may reach, I may quote a passage from the
writings of the late Sir Wyville Thomson.

‘After leaving the Cape Verde Islands the sea was a perfect blaze of
phosphorescence. There was no moon, and although the night was perfectly
clear and the stars shone brightly, the lustre of the heavens was fairly
eclipsed by that of the sea. It was easy to read the smallest print,
sitting at the after-port in my cabin, and the bows shed on either side
rapidly widening wedges of radiance so vivid as to throw the sails and
rigging into distinct lights and shadows.’

A very similar sight may frequently be seen in the Banda seas, where on
calm nights the whole surface of the ocean seems to be a sheet of milky
fire. The light is not only to be seen where the crests of waves are
breaking, or the surface disturbed by the bows of the boat, but the
phosphorescence extends as far as the eye can reach in all directions.
It is impossible, of course, to say with any degree of certainty whether
phosphorescence such as this exists at the bottom of the deep sea, but
it is quite probable that it does in some places, and hence the
well-developed eyes and brilliant colours of some of the deep-sea
animals.

On the other hand the entire absence or rudimentary condition of the
eyes of a very considerable proportion of deep-sea animals seems to
prove that the phosphorescent illumination is not universally
distributed, and that there must be some regions in which the darkness
is so absolute that it can only be compared with the darkness of the
great caves.

It is difficult to believe that the eyes of such animals as crabs and
prawns for example would undergo degeneration if there were a glimmer of
light in their habitat, a light even so faint as that of a starlight
night in shallow water. With the faintest light the eyes would be of use
to them in seeking their prey, avoiding their enemies, and finding their
mates, and any diminution in the keenness of this sense would probably
be of considerable disadvantage to them and tend to their ultimate
extinction.

It might be argued that the animals of the abysses of the ocean probably
feed chiefly upon the carcases of pelagic animals that have fallen from
the upper regions of the sea, and that the sense of smell is probably
the most important for them in searching for their food. That is quite
probable; but many shallow-water animals invariably seek their food by
their sense of smell without showing any traces of a weakness in their
sense of sight. It may be taken as an axiom of biology that unless a
particular sense is absolutely useless to an animal or a positive
disadvantage to it, that sense will be retained.

It may be stated then with some confidence that in the abysmal depths of
the ocean there is no trace of sunlight. It is highly improbable, on the
face of it, that any ray of light could penetrate through a stratum of
water four miles in thickness, even if the water were perfectly pure and
clear, but when we remember that the upper regions, at least, are
crowded with pelagic organisms provided with skeletons of lime and
silica, we may justly consider that it is impossible.

The temperature of the water in the abyss is by no means constant for a
constant depth nor does it vary with the latitude. It is true that, as a
rule, the water is colder at greater depths than in shallower ones, and
that the deeper the thermometer is lowered into the sea, the lower the
mercury sinks. This is consistent with physical laws. If there is any
difference at all in the temperature of a column of water that has had
time to settle, the thermometer will always reach its highest point at
the top of the column and its lowest at the bottom, for the colder
particles being of greater specific gravity than the warmer ones will
sink, and the warmer ones will rise.

The truth of this will be clear if we imagine a locality at the bottom
of a deep ocean with a source of great heat such as an active volcano.

Such a source of heat would, it is true, raise the temperature of the
water in its immediate vicinity, but the particles of water thus heated
would immediately commence to rise through the superjacent layers of
colder water, and colder particles would fall to take their places. Thus
the effect of an active volcano at the bottom of the deep sea would not
be apparent at any very great distance in the same plane. In fact,
unless the bottom of the ocean was closely studded with volcanoes we
should expect to find, as indeed we do find, that the temperature of the
sea rises as the water shallows.

If then we were to consider a great ocean as simply a huge basin of
water, we should expect to find the water at the surface warmer than the
water at the bottom. The temperature of the surface would vary
constantly with the temperature of the air above it. That is to say, it
would be warmer at the equator than in the temperate regions. The
temperature at the bottom would be the same as the lowest temperature of
the basin, that is, of the earth that supports it.

The great oceans however cannot be regarded as simple basins of water
such as this. The temperature of the surface water varies only
approximately with the latitude. It is generally speaking hottest at the
equator and coldest at the poles, but surface currents in the
intermediate regions produce many irregularities in the surface
temperature.

Again, although we have no means of knowing what the temperature of the
earth is at 1,000 fathoms below the surface of the ocean, it is very
probable that in the great oceans the temperature of the deepest stratum
of water is considerably lower than the true earth temperature. This is
due to currents of cold water constantly flowing from the poles towards
the equator. If these polar currents were at any time to cease, the
temperature of the lowest strata of water would rise.

Although the polar currents cannot be actually demonstrated nor their
exact rapidity be accurately determined, the deduction from the known
facts of physical geography that they do actually exist is perfectly
sound and beyond dispute. A few considerations will, I think, make this
clear.

If the ocean were a simple basin somewhat deeper at the equator than at
the poles, the cold water at the poles would gradually sink down the
slopes of the basin towards the latitude of the equator, and the bottom
temperature of the water would be constant all the world over.

A few hills here and there would not affect the general statement that
for a constant depth the temperature of the lowest stratum of water
would be constant.

But in some places ridges occur stretching across the oceans from
continent to continent, and these ridges shut off the cold water at the
bottom of the sea on the polar side from reaching the bottom of the sea
on the equator side.

If A (fig. 1) represents a ridge stretching from continent to continent
across an ocean, and the arrow represents the direction of the current,
then the water that flows across the ridge from the polar side to the
equator side will be drawn from the layers of water lying above the
level of the ridge, and consequently none of the coldest water will ever
get across it, and from the level of the ridge to the bottom of the sea
on the equatorial side the water will have the same temperature as the
water at the level of the ridge on the polar side.

It follows from this that in places where there are deep holes in the
bed of the ocean surrounded on all sides by considerable elevations, the
temperature of the water at the bottom will be the same as the
temperature of the water on the summit of the lowest ridges that
surround them.

[Illustration: FIG. 1.—Diagram illustrating the passage of an ocean
current across a barrier (A).]

This explains why it is that we find that the bottom temperature for a
given depth is frequently less in one place than it is in another, even
in places of the same parallel of latitude. One or two examples may be
taken to illustrate these points. The temperature off Rio Janeiro in
lat. 20° S. was found by the ‘Challenger’ to be 0·6° C. at a depth of
2,150 fathoms. In a similar latitude north of the equator at a depth of
2,900 fathoms the temperature was found to be 2·2° C., and at a point
near Porto Rico there is a deep hole of 4,561 fathoms with a bottom
temperature of 2·2° C.

Again it has been shown by the American expedition that the temperature
of the water at the deepest point in the Gulf of Mexico, 2,119 fathoms,
is the same as that of the bottom of the Straits of Yucatan, 1,127
fathoms, namely 4·1° C. And, passing to another part of the world
altogether, we find in the small but deep sea that lies between the
Philippines and Borneo that, at a depth of 2,550 fathoms, the
temperature is 10·2° C.

These facts then show that, although at the bottom of the deep seas the
water is always very cold, the degree of coldness is by no means
constant in the same latitude for the same depth.

We must now return to the polar currents. We have assumed above that
these currents do exist, and it is probable that by this time the reader
must have seen why they are assumed to exist.

The water at the bottom of the ocean is exceedingly cold. Where does
this coldness come from? It is obvious that in temperate and tropical
climes it does not come from the surface. Nor is it at all probable that
it comes from the earth upon which the water rests; for, if it were so,
the temperature for water of a given depth would always be the same. We
should not find the bottom temperature of 0·4° C. at 2,900 fathoms off
Rio de la Plata and a temperature of 2·2° F. in 4,561 fathoms off Porto
Rico.

In fact the only hypothesis that can with any show of reason be put
forward to account for the temperature of the bottom of the ocean is
that which derives its coldness from the Polar ice.

We have at present very little evidence to enable us to judge of the
force and direction of the polar currents in the two hemispheres, but
the researches of the ‘Challenger’ prove almost conclusively that in the
Atlantic Ocean there is a very strong predominance of the Antarctic
polar current. In fact it seems very probable that the Arctic polar
current, if it exist at all, is very small and confined to the eastern
and western shores of the North Atlantic.

It is very probable, however, that these currents at the bottom of the
ocean are extremely slow, and, as the water is never affected by tides
or storms, the general character of the deep sea is probably one of calm
repose. This is a matter of no little importance; for, in the
consideration of the characters presented by the fauna of any particular
region, it is always necessary to take into account the physical
difficulties the animals have to contend against and the modifications
of structure they present to combat these difficulties.

Thus in a region such as that presented by the deep sea, where there are
no rapid tides, we should not expect to find such a powerful set of body
muscles in the free-swimming forms nor such a firm vertebral column as
in the animals that live in more lively water.

Perhaps it is of the nature of an assumption to say that there are no
rapid currents and tides in the abysmal depths of the ocean, for we have
no means of demonstrating or even of calculating the rate of flow of
these waters. But it is a reasonable hypothesis and one that we may well
use until the contrary is proved.

A fact of some importance that supports this hypothesis, as regards some
parts of the ocean at least, is presented by the sea-anemones.

Many of the shallow-water Actinians are known to possess minute slits in
the tentacles and disc, affording a free communication between the
general body cavity or cœlenteron and the exterior.

In many deep-sea forms the tentacles are considerably shorter and the
apertures larger than they are in shallow-water forms. It is difficult
to believe that such forms, perforated by, comparatively speaking, large
holes, could manage to live in rapidly flowing water, for if they did so
they would soon be smothered by the fine mud that composes the floor of
all the deep seas. In fact anemones of the type presented by such forms
as _Sicyonis crassa_ are only fitted for existence in sluggish or still
water.

[Illustration: FIG. 2.—_Sicyonis crassa._ M, mouth; S, ciliated groove;
T, tentacles. Each tentacle is perforated by a single large aperture.
(After Hertwig.)]

Another character that must be taken into consideration is that
presented by the floor of the great oceans. The floor of the ocean, if
it were laid bare, would probably present a vast undulating plain of
fine mud. Not a rock, not even a stone would be visible for miles.

The mud varies in different parts of the globe according to the depth,
the proximity to land, the presence of neighbouring volcanoes or the
mouths of great rivers.

The Globigerina ooze is perhaps the best known of all the different
deep-sea deposits. It was discovered and first described by the officers
of the American Coast Survey in 1853. It is found in great abundance in
the Atlantic Ocean in regions shallower than 2,200 fathoms. Deeper than
this, it gradually merges into the ‘Red mud.’ It is mainly composed of
the shells of Foraminifera, and of these the different species of
Globigerina are the most abundant. It is probably formed partly by the
shells of the dead Foraminifera that actually live on the bottom of the
ocean and partly by the shells of those that live near the surface or in
intermediate depths and fall to the bottom when their lives are done.

So abundant are the shells of these Protozoa that nearly 95 per cent. of
the Globigerina ooze is composed of carbonate of lime. The remaining
five per cent. is composed of sulphate and phosphate of lime, carbonate
of ammonia, the oxides of iron and manganese, and argillaceous matters.
The oxides of iron and manganese are probably of meteoric origin; the
argillaceous matter may be due to the trituration of lumps of pumice
stone and to the deposits caused by dust storms.

[Illustration: FIG. 3.—Globigerina ooze. (After Agassiz.)]

Globigerina ooze may be found on the floor of the ocean at depths
ranging from 500 to 2,800 fathoms of water in equatorial and temperate
latitudes. The reason that it is not found in Arctic seas may be that
the cold surface waters of these regions do not bear such an abundant
fauna of Foraminifera. This is supported by the fact that it extends ten
degrees further north than south in the Atlantic, the warm water of the
Gulf Stream bearing a richer fauna than the waters of a corresponding
degree of latitude in the Southern Sea.

The Pteropod ooze has only twenty-five per cent. of carbonate of lime.
It contains numerous shells of various Pteropods, Heteropods, and
Foraminifera, but nearly fifty per cent. of its substance is composed of
the siliceous skeletons of Radiolaria and the frustules of diatoms.

According to Murray it is found in tropical and subtropical seas at
depths of less than 1,500 fathoms.

The Radiolarian ooze is found only in the deepest waters of the Central
and Western Pacific Ocean. In some of the typical examples, not a trace
of carbonate of lime was to be found, but in somewhat shallower waters a
few small fragments occurred.

A Diatom ooze, mainly composed of the skeletons of diatoms, has also
been found in deep water near the Antarctic Circle, but it has not
apparently a very wide range.

Of all the deep-sea deposits, however, the so-called ‘Red mud’ has by
far the widest distribution. It is supposed to extend over one-third of
the earth’s surface. It is essentially a deep-sea deposit, and one that
is found in its typical condition at some considerable distance from
continental land. Like the Globigerina ooze it is never found in
enclosed seas. To the touch it is plastic and greasy when fresh, but it
soon hardens into solid masses. When examined with the microscope it is
seen to be composed of extremely minute fragments rarely exceeding 0·05
mm. in diameter. It contains a large amount of free silica that is
probably formed by the destruction of numerous siliceous skeletons, and
a small proportion of silicate of alumina. It usually contains the
remains of diatoms, radiolaria, and sponge spicules, and occasionally
lumps of pumice stone, meteoric nodules, and, in colder regions, stones
and other materials dropped by passing icebergs.

In the great oceans, then, we find in the deepest places red mud, or,
where there is an abundant Radiolarian surface fauna, Radiolarian ooze;
in water that is not deeper than about 2,000 fathoms, we find the
Globigerina ooze; in shallower waters and in some localities only
Pteropod ooze.

It must not be supposed that sharp limits can anywhere be drawn between
these different kinds of deposits, for they pass gradually into one
another and present many intermediate forms.

It is probable that the sea water by virtue of the free carbonic acid it
contains in solution is able to exert a solvent action upon the calcium
carbonate shells of animals as they sink to the bottom, and during the
long and very slow journey from the surface to the bottom of the deepest
seas these shells are completely dissolved.

The first to be dissolved would be the thin delicate shells of the
Pteropods and Heteropods, for besides the fact that they present a wider
surface to the solvent action of the water they are probably influenced
more by tide and currents, sink more slowly and erratically, and thus
have a longer journey to perform.

Then the smaller but more solid and compact shells of the Foraminifera
are dissolved, and lastly, in the deepest water only the siliceous
skeletons of the radiolaria and diatoms are able to reach their last
resting place at the bottom of the ocean.

These four oozes then are characteristic of the floor of the deep
oceans. In the proximity of land and in inland seas where deep water
occurs, other muds are found differing from one another in accordance
with the character of the coasts in their vicinity. It is not necessary
to give a detailed account of them, but a few remarks on some of the
more pronounced forms may not be without interest.

The blue mud contains eighty per cent. of a mixture of quartz, mica,
felspar, hornblende, and other minerals, mixed with a considerable
quantity of decomposing animal and vegetable substance, the calcareous
remains of foraminifera, mollusca, worms, echinoderms, alcyonaria and
corals, and the siliceous skeletons of radiolaria and diatoms.

The green mud is characterised by a large percentage of glauconite.

The red muds characteristic of the Brazilian coast contain a large
amount of ochreous matter brought into the sea by the great rivers.

In the neighbourhood of active volcanoes there is a characteristic
volcanic mud, and in the coral seas the deep-sea deposits contain a
large percentage of the calcareous remains of dead corals.

One more character of the deep-sea region must be referred to before we
pass on, and that is the absence of vegetable life. It has not been
determined yet with any degree of accuracy where we are to place the
limit of vegetable life, but it seems probable that below a hundred
fathoms no organisms, excepting a few parasitic fungi, are to be found
that can be included in the vegetable kingdom. While then the researches
of recent times have proved beyond a doubt that there is no depth of the
ocean that can be called azoic, they have but confirmed the perfectly
just beliefs of the older naturalists that there is a limit where
vegetable life becomes extinct. It is not difficult to see the reason
for this. All plants, except a few parasites and saprophytes, are
dependent upon the influence of direct sunlight, and as it has been
shown above that the sunlight cannot penetrate more than a few hundred
fathoms of sea water, it is impossible for plants to live below that
depth.

The absence of vegetable life is an important point in the consideration
of the abysmal fauna, for it is in consequence necessary to bear in mind
that the food of deep-sea animals must be derived from the surface. It
is possible that deep-sea fish, in some cases, feed upon one another and
upon deep-sea crustacea, that deep-sea crustacea feed upon deep-sea
worms, that deep-sea echinoderms feed upon deep-sea foraminifera, and so
on through all the different combinations; but the fauna would soon
become exhausted if it had no other source of food supply. This other
source of food supply is derived from the bodies of pelagic organisms
that fall from the upper waters of the ocean, and is composed of
protozoa, floating tunicates, crustacea, fish, and other animals,
together with diatoms and fragments of sea-weed.




                              CHAPTER III
     THE RELATIONS OF THE ABYSMAL ZONE AND THE ORIGIN OF ITS FAUNA


In the study of the geographical distribution of terrestrial animals one
of the great difficulties met with is the impossibility of defining
exactly the limits of the regions into which we divide the surface of
the earth. In a general way we recognise that there is an Australian
region, an Ethiopian region, &c.; but, when we come to discuss the exact
position of the frontier lines that separate these regions from their
neighbours, we find all kinds of difficulties to overcome and
inconsistencies to meet.

For the sake of convenience it is useful to adopt certain arbitrary
limits for these regions, notwithstanding these difficulties and
inconsistencies, but we must recognise the fact that nature recognises
no such limits, that every region overlaps its neighbours to a greater
or less extent, and that there are many debateable grounds in the world
where the fauna characteristic of one region is mixed with that
characteristic of another.

But this difficulty in defining the exact limits of the terrestrial
faunistic regions is even more pronounced in the case of the regions and
zones of the marine fauna.

On the dry land we find mountain ranges, forests, deserts, and other
barriers, that to a very considerable extent prevent the mixing of one
fauna with another, but in the sea there are no barriers of anything
like the same importance, but one fauna gradually merges into the
neighbouring fauna according to the temperature, the pressure, the
amount of light, the salinity of the water or the food supply. This then
is one of the difficulties met with in the study of the geographical
distribution of the marine fauna.

But there is another that leads to almost greater complications. In
considering terrestrial life it is customary to refer only to regions of
geographical, or perhaps it would be more correct to call it—superficial
distribution. It would be quite possible, however, to subdivide the
geographical areas into zones of elevation above the sea-level, not very
clearly marked off from one another, it is true, but nevertheless each
showing a number of characteristic features. This idea is expressed, for
example, when we speak of the Alpine fauna, the Himalayan fauna, or the
fauna of the great Andes.

In the study of the marine fauna and flora we must notice, it is the
depth of the water, or in other words the depression of the habitats
below the sea-level, that forms the most important consideration.
Geographical sub-regions may be recognised and defined with a certain
amount of accuracy, especially in the case of the fauna of the shallow
waters, but by far the most important changes in the general characters
of the fauna are found when we pass from one ‘zone’ of depression to
another. Thus in describing any particular marine fauna we should
mention first of all its zone or sub-zone of depression and then its
geographical region and sub-region. For example, we may speak of the
fauna of the pelagic zone of the British sub-region of the European
region, or the fauna of the abysmal zone of the Northern sub-region of
the Atlantic region.

We can recognise three primary zones of the marine fauna which we may
call the ‘Pelagic,’ the ‘Neritic,’ and the ‘Abysmal’ zones.

The Pelagic zone includes the superficial waters of all seas extending
from the surface to a depth which cannot at present be very accurately
determined, but is probably the same as the limit of the influence of
direct sunlight.

The animals of this zone are frequently characterised by a general
transparency of their tissues, a white or sea water (i.e. blue or green)
colour, an organisation capable of prolonged swimming or floating
movement, and by giving birth to floating eggs which hatch out
transparent larvæ or embryos.

The pelagic zone may be divided into several geographical regions and
sub-regions, which it would be beyond the scope of this book to
enumerate here, but there is one that calls for a few brief remarks. In
many parts of the ocean there may be found vast areas of floating
sea-weed, which carry with them a population of crustacea and other
animals peculiarly their own. This ‘sargasso’ fauna presents so many
characteristics and so many features different from that of the ordinary
pelagic fauna, that the tracts of sea bearing this weed must be
considered to rank as a special region of the pelagic zone, which may be
called the Sargasso region.

The zone of shallow water for which we shall adopt Professor Haeckel’s
term—the Neritic zone—embraces all parts of the seas of less depth than
500 fathoms, including the inland seas, the shores of great continents
and islands, and the shallow banks in the great oceans. It does not
include the superficial waters—which belong to the pelagic zone—but
extends only from the actual bottom to a distance of a few fathoms above
it. The fauna of this zone is extremely varied, consisting of animals
that swim, crawl, or are permanently fixed to the bottom, animals of
almost every variety of colour and marking, and of every size and shape.

The exact limits of the Neritic sub-zones are not easy to define. The
distinguished naturalist Forbes, to whom the abysmal zone was unknown,
divided the seas from 0–50 fathoms in depth into three zones—the
littoral zone lying between tide-marks, the laminarian zone extending
from 0–15 fathoms, and the coralline zone from 15–50 fathoms.

The first of these will stand as a sub-zone, the animals that are able
to withstand exposure to the sun and air either in pools or upon the
rocks and sand even for a few minutes frequently possessing features
that distinguish them from those dwelling beyond low-water mark, just as
those more active creatures that migrate backwards and forwards with the
ebb and flow of every tide differ from the dwellers in the open sea.
There is, it is true, at every low tide, a migration of part of the
fauna of this sub-zone into the next, but still it is sufficiently well
defined to be allowed to remain in our category.

The second sub-zone is not so easy to define. The terms ‘laminarian’ and
‘coralline’ used by Forbes are only applicable to certain geographical
regions and must be abandoned for general use.

We can only recognise one sub-zone between the littoral sub-zone and the
abysmal zone, for notwithstanding the important varieties it exhibits in
the nature of the bottom, whether it be rocky, sandy, or weedy, the
amount of light, the temperature of the water, and the rapidity of the
currents, it is not possible at present to point to any general
characters of the fauna of its different parts to justify us in
subdividing it.

The name that may be given to this second sub-zone of the neritic zone
is the Katantic—the sub-zone of the slopes.

The last well-marked zone is the abysmal, extending from the 500–fathom
line to the greatest depths of the ocean, one of enormous superficial
area, one that it is most difficult to investigate, and one about which
we know but little.

In the present state of our knowledge we cannot divide it into any
well-marked sub-zones nor even into geographical regions or sub-regions.
It is not divided into sections by any important geographical barriers,
and the general characters presented by its fauna are practically the
same all the world over.

Professor A. Agassiz has pointed out in his ‘Challenger’ monograph that
the deep-sea echinoids of the Atlantic Ocean differ from those living in
corresponding depths in the Pacific Ocean, but it is doubtful whether
any such well-marked differences can be observed in other groups of
animals. If, in the course of time, increased knowledge of deep-sea
animals emphasises the difference between the abysmal fauna of the
Pacific and that of the Atlantic, then we can divide this zone into two
geographical regions; but at present it seems more correct to consider
the abysmal zone as one that is indivisible either bathymetrically or
geographically.

Before passing on to the consideration of the general characters of the
abysmal fauna, there are still one or two points that must be just
briefly referred to.

It is the function of every true naturalist to consider animals from
every possible point of view. Not only must he regard them as members of
a certain species belonging to a genus, a family, an order, and so on,
presenting certain peculiarities of structure and development; not only
must he regard them as inhabitants of a certain locality or zone of
depth, but he must also pay attention to their habits and mode of life.

Now amongst marine animals we can recognise three principal modes of
life. Some animals simply float or drift about with the currents of the
sea and are unable to determine for themselves, excepting, perhaps,
within very small limits, the direction in which they travel. Such are
the countless forms of protozoa, the jelly-fishes and medusæ, numerous
pelagic worms and crustacea, the pyrosomas and salps, and many other
forms well known to those who are in the habit of using the tow-net.
This portion of the fauna has recently been called the Plankton.

Then there are the animals that are capable of very considerable
swimming movements, animals that are able to stem the tide and migrate
at will from one part of the sea to another, such as the cetacea, most
fishes, and perhaps also many cephalopods. This portion of the fauna has
been called the Nekton.

And lastly we have those animals that remain perfectly fixed to the
bottom or are capable only of creeping or crawling over the rocks and
sand, such as the sponges, hydroids, sedentary tunicates, gasteropods,
most lamellibranchs, and many crustacea. This portion of the fauna has
been called the Benthos.

Although it will not be necessary to use these terms very frequently in
this little book, it may be advisable for the reader to bear in mind
that in any exhaustive treatise on the marine fauna such terms would be
employed, and that in the chapters dealing with the fauna of the abysmal
zone we should find accounts of the ‘bathybial plankton,’ the ‘bathybial
nekton,’ and the ‘bathybial benthos.’

Lastly we must consider quite briefly the views that have been held
concerning the origin of the abysmal fauna.

As soon as it became clear to naturalists that there is no part of the
ocean, however deep it may be, that deserves the name ‘azoic,’ but that
almost every part has a fauna of greater or less density, the problem of
the origin of this fauna presented itself.

Whence came the curious creatures that live mostly in total darkness and
can sustain without injury to their delicate and complicated
organisation the enormous pressure of the great depths? Are they the
remnants of the fauna of shallow prehistoric seas that have reached
their present position by the gradual sinking of the ocean basins? Or,
are we to look upon the abysmal region as the nursery of the marine
fauna, the place whence the population of the shallow waters was
derived? Neither of these answers is supported by the facts with which
we are now well acquainted. The fauna of the abysmal region does not
show a close resemblance to that of any of the past epochs as revealed
to us by geology, nor are we justified in assuming without much stronger
evidence than we now possess, that the oceans have undergone any such
great depression as this first theory presupposes.

Nor can we consider for a moment that the abyss was the original source
of the shallow-water fauna; for not only do we find but few types that
can be considered to be, in any sense of the word, ancestral in
character; but on the contrary most of the animals of the deep sea seem
to be specially modified types of shallow-water forms. The most probable
explanation of the origin of the deep-sea fauna is the one that was put
forward by Moseley and has been since supported by almost every
authority on the subject, namely, that the fauna of the deep sea has
been derived from successive immigrations of the animals from the
shallow water.

This view is supported by the fact that the deep-sea fauna is much
richer in the neighbourhood of land than it is in regions more remote
from it. Many examples could be given to illustrate this point. The
extraordinary richness of the deep-sea fauna on the western slopes of
the floor of the Atlantic has been frequently commented on by the
naturalists connected with the expeditions of the American vessels, the
‘Blake,’ the ‘Fish Hawk,’ and the ‘Albatross.’ Moseley called attention
several years ago to a few localities in the neighbourhood of the land
especially rich in deep-sea forms in comparatively shallow waters, such
as one near the island of Sombrero in the Danish West Indies, where
within sight of the lighthouse a haul of the dredge in 450 fathoms
brought up a rich fauna of blind crustacea, corals, echinoderms,
sponges, &c. Another off Kermadec in 630 fathoms brought up numerous
curious blind fishes, ascidians, cuttlefishes, crustacea, _Pentacrinus_,
and large vitreous sponges, and there are similar localities lying
between Aru and Ke and between the Nanusa archipelago and the Talaut
islands. The deep water off the Norwegian, Scotch, Irish, and Portuguese
coasts also seems to be particularly rich in various forms of animal
life. The same is probably true of the deep sea of many other regions in
the neighbourhood of land, and, although it cannot be taken to be a rule
without exceptions—the abysmal fauna off the western coasts of the
Panama region being, according to the recent researches of Alexander
Agassiz in the ‘Albatross,’ particularly poor—yet we can assert as a
statement of very general application that the further removed from
continental land, the poorer is the abysmal fauna.

Another argument that has been brought forward by Moseley in support of
his view is that there is a certain relationship between the deep-sea
fauna of any particular region and the shallow-water fauna of the
nearest coasts. This is a point that is not easy to illustrate by
examples, but as Moseley’s argument has not, so far as I am aware, been
disputed by any of the naturalists who have followed him in this line of
work, and the recent results of the ‘Albatross’ in comparing the
deep-sea fauna of the eastern and western sides of the isthmus of Panama
seem if anything to support it, we can take it as a point in favour of
his view of the origin of the abysmal fauna.

It is impossible to say at present at what time in the world’s history
these migrations commenced, but, as Agassiz points out, none of the
palæozoic forms are found in the deep sea, and this seems to indicate
that the fauna did not commence its existence earlier than the
cretaceous period.

It is quite possible, however, that part of the fauna of the deep sea
has been derived directly from the pelagic zone. The occurrence of
bathybial Radiolaria, Foraminifera and Siphonophora, and among fishes
genera and species of the pelagic families Sternoptychidæ and Scopelidæ,
suggest that this zone may have contributed very largely to the fauna of
the abyss.

Much of course still remains to be done before we can consider any of
these interesting problems connected with the deep-sea fauna to be
definitely solved. All we can do at present is to speculate upon the
direction in which the facts at our disposal seem to point, and by
following up one clue after another hope that we may eventually arrive
at the truth. The task may be a difficult one, but it will reward our
efforts. If truth is hard to find when it lies at the bottom of a well,
how much more inaccessible must it be when it lies hidden in the
darkness of the sea’s abyss!




                               CHAPTER IV
                  THE CHARACTERS OF THE DEEP-SEA FAUNA


The general characters presented by animals living in deep water may be
considered under several headings. The most important are those that are
directly or indirectly related to the fact that the animals live either
in total darkness or in the faint and probably intermittent light
emitted by phosphorescent animals; namely, the colour of the skin and
the peculiarities of the eyes.

The colours of the skin of the deep-sea animals vary to a very
remarkable extent in the different groups. It cannot be said that there
is any one colour at all predominant, and it is only in certain classes
that black, white, or dull-coloured animals are more numerous than
others. The colours are however usually very evenly distributed, and we
find but few examples of animals with spots, stripes, or other
pronounced markings.

The majority of the fish are dark brown or black, but many other colours
are represented. Thus _Ipnops Murrayi_, a typical deep-sea fish, is
yellowish brown with colourless fins, and it exhibits a further
character not uncommon in these abysmal forms, namely black buccal and
branchial cavities. _Typhlonus nasus_, again, is said to be of a light
brownish colour, with black fins. Many other examples could be given to
show the prevalence in these regions of these black, dull, and pale
uniform colours. But there are many exceptional cases. _Neoscopelus
macrolepidotus_, for example—a form that according to Günther
undoubtedly belongs to the bathybial region—is distinguished by its
brilliant colours. It is bright red mixed with azure blue, the whole
relieved by silver spots with circles of black on the abdomen.

_Prorogadus nudus_ is of a pale rose colour, with the under and lateral
sides of the head bluish black.

_Rhodichthys regina_, found in 1,280 fathoms of water, is uniformly
bright red in colour.

A. Agassiz says in his reports on the dredging operations on the west
coast of America: ‘The coloration of the deep-sea fishes is
comparatively monotonous. The tints are all a light violet base, tending
more or less to brownish or brownish yellow, or even to a greenish tint,
especially among the Macruridæ. Some of the Liparidæ were of a dark
violet, and one species was characterised by a brilliant blue band. The
Ophidiidæ, _Nemichthys_, and the like, are usually of an ashy violet
tint, while in _Ipnops_ and _Bathypterois_ the tints were of a decidedly
yellowish brown.’

That the deep-sea fish are usually devoid of any pronounced spots,
stripes, and other markings, is now well recognised. It may not be
altogether out of place, however, to refer briefly to a few exceptions.

The black circles on the abdomen of _Neoscopelus macrolepidotus_ have
already been referred to.

_Halosaurus johnsonianus_, has a black spot on the tail.

_Aulostoma longipes_ has three pairs of large black spots on the ventral
side, but the specimen taken in 1,163 metres of water by the ‘Talisman’
was probably a young one.

It is very probable that in all the exceptional cases, when fish taken
in deep-sea water have exhibited such spots and markings, they are
examples either of fish that have quite recently adopted an abysmal
habitat or of young specimens exhibiting ancestral inherited characters.

In referring to a specimen of _Raja circularis_, taken by the ‘Triton’
in 516 fathoms, Günther says: ‘It is notable that the spot on each side
of the back which in littoral specimens is variegated with yellow is
much smaller in the deep-sea specimen and uniformly black without
yellow.’

It seems to be then a very general rule among fishes that as they
migrate into deeper water the spots and stripes, so conspicuous among
many forms living on the surface and in shallow water, disappear, and
the coloration of the body becomes more evenly distributed and uniform.

Among the Mollusca, the deep-sea Cephalopods seem to be usually violet,
but an _Opisthoteuthis Agassizii_ caught by the ‘Blake’ is stated to be
of a dark chocolate colour, a _Nectoteuthis Pourtalesii_ reddish-brown,
and a _Mastigoteuthis_ orange brown, while of the specimens brought home
by the ‘Challenger,’ _Cirroteuthis magna_ was said to be ‘rose’ when
captured, and the spirit specimens of _Cirroteuthis pacifica_ and
_Bathyteuthis abyssicola_ were purplish madder and purplish brown
respectively.

The shells of the Gasteropods and Lamellibranchs living in the abyss are
frequently so thin as to be almost transparent, and are, with very few
exceptions, white or pale straw coloured. The colour of the only
specimen of nudibranchiate Mollusca that has been found in the abysmal
zone, namely, _Bathydoris abyssorum_, is described by Mr. Murray as
follows: ‘The body of the living animal was gelatinous and transparent,
the tentacles brown, the gills and protruding external generative organ
orange, the foot dark purple.’

Among the Crustacea various shades of red are the prevailing colours.
‘The deep-sea types, like _Gnathophausia_, _Notostomus_, and
_Glyphocrangon_,’ says Agassiz, ‘are of a brilliant scarlet; in some
types, as in the Munidæ and Willemoesiæ, the coloration tends to pinkish
or yellowish pink, while in _Nephrops_ and _Heterocarpus_ the scarlet
passes more into greenish tints and patches.’[1] But perhaps the most
remarkable point in the colour of the crustacea is that which
immediately follows the paragraph I have just quoted. ‘The large eggs of
some of the deep-sea genera are of a brilliant light blue, and in one
genus of Macrura we found a dark metallic blue patch on the dorsal part
of the carapace in marked contrast to the brilliant crimson of the rest
of the body.’

Footnote 1:

  In the recent researches of the ‘Investigator’ a few crustacea of
  rather exceptional colour were found. Whilst the great majority of
  them are described to be pink or red in colour when alive,
  _Gnathophausia bengalensis_ is deep purple lake, _Haliporus neptunus_
  lurid orange, and _Aristaeus coruscans_ bright orange.

The occurrence of this blue colour in Crustaceans of the deep sea is
very remarkable, for blue is a colour, as Moseley pointed out many years
ago, that is rarely met with in the fauna of the abyss, and it is
certainly very exceptional in the crustacea of that zone.

Among the deep-sea Echinoderma we find a wonderful variety of
coloration. Moseley says that many deep-sea Holothurians, for example,
are deep purple, and Agassiz reports that in one species the colour was
of a delicate green tinge. ‘We obtained,’ he adds, ‘a white _Cucumaria_
and some species of _Benthodytes_ of the same colour,’ while others vary
from transparent milky white to yellow and light yellowish brown and
even pinkish colours. The Crinoids are described by the authorities to
be white, purple, yellow and brownish-chestnut, and of the other groups
of the echinoderms we read that the star-fishes are, as a rule, of
duller colours than the crustacea, but all more or less pink or red.
‘The Hymenasteridæ, on the contrary, vary from light bluish violet to
deep reddish chestnut colours.’ The brittle stars are red and orange or
dullish grey, while the urchins may be deep violet, claret coloured,
brownish, or of a delicate pink.

It is impossible to account for this extraordinary variety of colour in
the deep-sea echinoderms. It is hardly probable that it can be
protective or warning in function, and it is difficult to suppose that
it is due to any peculiar excretory process. Whether it is due in any
way to the influence of the environment, or, like the colour of autumn
leaves, to the chemical degeneration of colours that in the
shallow-water ancestry were functional, are problems that must be left
for the future to decide.

The colour of the deep-sea Cœlenterates has unfortunately not been
recorded in all cases, but still the few observations that we have show
that in this group, as in the last, almost every tint and shade are
represented.

The colouring of the deep-sea jelly-fishes is said to be usually deep
violet or yellowish red. However ‘a species of _Stomobrachium_,’ says
Agassiz, ‘is remarkable for its light carmine colour, a tint hitherto
not observed among Acalephs.’

Moseley records most minutely the colour of some of the deep-sea
anemones and corals, and calls attention to the very general presence of
madder brown in the soft parts. Agassiz says: ‘Among deep-sea Actiniæ, a
species of a new _Cereanthus_ was of a dark brick-red, while other
actinians allied to _Bunodes_ were of a deep violet. Actinauge-like
forms with tentacles of a pinkish-violet tinge frequently have the
column of a yellow shade. The Zoanthidæ were greyish-green.’ And again,
in his narrative of the voyage of the ‘Blake,’ he records that ‘some of
the deep-sea corals are scarlet, deep flesh-coloured, pinkish orange,
and of other colours,’ and in referring to the Gorgonian _Iridogorgia_
he says: ‘The species are remarkable for their elegance of form and for
the brilliant lustre and iridescent colours of the axis, in some of a
bright emerald green, in others like burnished gold or mother-of-pearl.’

The fauna of the deep sea then, taken as a whole, is not characterised
by the predominance of any one colour. The shades of red occur rather
more frequently than they do in the fauna of any other zone or region,
but whether this is in any way connected with the fact that red is the
complementary colour to that of the phosphorescent light, in which many
of these animals live, it is at present difficult to say; it is possible
that, when we have further information concerning the colours of the
animals living in the deeper parts of the Neritic zone, another
explanation may be forthcoming.

Moseley points out that there are no blue animals known to live in deep
water, and it might be added that green is extremely rare as a colouring
matter in abysmal animals, although the phosphorescent light given out
by some of the echinoderms is green.

Blue, as a colouring matter of marine animals, living on the surface or
in shallow water, is not uncommonly met with, distributed in the form of
bands or stripes, but green is extremely common in fishes, crustacea and
cœlenterates, and it is a point of very considerable importance that in
this respect there is a very great difference between the deep-sea and
the shallow-sea faunas.

If a considerable collection of living abysmal forms could be placed
upon one table and a similar collection of shallow-water forms upon
another, I believe that the first general impression upon the mind of
one who saw them both for the first time would be the presence of green
colours in the last-named collection, and the absence of it in the
other.

The eyes of the animals that live in deep-sea water undergo curious
modifications. If the fauna of the abysmal region were confined to
conditions of absolute darkness, we should expect to find either a total
absence of eyes or mere rudiments of them only in those forms that have
recently migrated from the shallow water. This is the case with the
fauna of the great caves. There is probably total darkness in these
underground lakes and streams, and there is only the remotest
possibility of the animals living in them ever seeing, even temporarily,
a ray of sunlight or even a glimmer of phosphorescence during the whole
of their life-time. We find then that the cave fauna is totally blind.

The conditions in the deep sea are not quite the same. In some regions
there is probably a very considerable illumination by phosphorescent
light, and it is quite possible that many of the characteristic deep-sea
forms may occasionally wander into shallower regions where faint rays of
sunlight penetrate, or even that the young stages of some species may be
passed at or near the surface of the sea. Taking these points into
consideration, then, it is not surprising to find that, in the deep
seas, there are very few animals, belonging to families usually provided
with eyes, that are quite blind.

In the majority of cases we find that the eyes are either very large or
very small. Only in a small minority of cases do we find that the eyes
are recorded to be moderate in size. The relation between the large-eyed
forms and the small-eyed forms is not the same in all the regions of
deep seas. In depths of 300 to 600 fathoms the majority are large-eyed
forms. This is as we should expect, for it is more than probable that
many of these forms occasionally wander into shallower waters where
there is a certain amount of sunlight.

In depths of over 1,000 fathoms, the small-eyed and blind forms are in a
majority, although many large-eyed forms are to be found.

Among fishes, for example, we find the species of _Haloporphyrus_ found
in depths of 300–600 fathoms with large eyes; and so with _Dicrolene_,
_Cyttus abbreviatus_, and many other forms that are known to live in
water of less depth than 700 fathoms; while on the other hand in
_Melanocetus Murrayi_, _Ipnops Murrayi_, many deep-sea eels and other
fish that are truly abysmal and live chiefly in depths of over 1,000
fathoms, the eyes are either very small or absent.

Some interesting examples may be found in the species of widely
distributed genera to illustrate these points. Thus in _Neobythites
grandis_, from 1,875 fathoms, the eye is small, only one-eleventh the
length of the head, but in _Neobythites macrops_, _N. ocellatus_, and
_N. gillii_ from shallower water it is much larger.

         N. grandis   1,785 fms. Eye 1⁄11th length of the head
         N. macrops     375 fms. Eye  2⁄9   length of the head
         N. ocellatus   350 fms. Eye   ¼    length of the head
         N. gillii      111 fms. Eye 1⁄(3⅔) length of the head

Similarly in the species of the widely distributed deep-sea genus
_Macrurus_: the species _M. parallelus_, _M. japonicus_, _M. fasciatus_,
&c., usually living in water less than 1,000 fathoms deep, have large
and in some cases very large (_M. fasciatus_) eyes, but _Macrurus
filicauda_, _M. fernandezianus_, _M. liocephalus_, _M. Murrayi_, _M.
armatus_ have small eyes.

Some deep-sea fish have their eyes reduced to a mere rudiment; such as
_Ceratias uranoscopus_, _C. carunculatus_, _Melanocetus Murrayi_,
_Typhlonus nasus_, and _Aphyonus gelatinosus_, but not even a rudiment
of an eye is to be found in _Ipnops Murrayi_.

But the fish of the greatest depths are by no means always characterised
by small eyes. _Malacosteus_, a typical deep-sea form, has very large
eyes, and so have _Bathylagus_, living in the enormous depth of 3,000
fathoms, and _Bathytroctes_, in 1,090 and 2,150 fathoms.

The result of recent deep-sea work, then, has been to show that as we
proceed from shallow shore water to depths of 500 to 900 fathoms the
eyes of the fish become larger, but in greater depths than 1,000 fathoms
the eyes of some fish become considerably reduced, but those of others
become still more enlarged. In the greatest depths of the ocean in fact
it seems very probable that nearly all the fish are characterised by
either very large eyes or very small ones.

We cannot expect to learn very much at present from the study of the
eyes of deep-sea mollusca. The Cephalopods form the only class of this
Phylum whose genera invariably possess large and well-developed eyes,
and there does not seem to be any very marked increase or decrease in
the size of the eyes of the few deep-sea cuttlefish that are known to
us.

The eye of _Nautilus_ is certainly remarkably interesting, but as this
genus is the only representative of its order, and is known at times to
float upon the surface of the ocean, it would certainly be erroneous to
attribute the peculiarity of the structure of its eye to its ‘temporary’
deep-sea habits. We are still ignorant of the usual habitat of the
remarkable genus _Spirula_, notwithstanding the fact that many of the
tropical beaches are very largely composed of its empty shells. Whether
it is a deep-sea dweller or not, we know nothing at present of the
character of its eye, so that it can throw no light upon the problems we
are now discussing.

Among the deep-sea gasteropods we find the same irregularity in the
possession of eyes that we have just described among fishes. Thus a
species of _Pleurotoma_, dredged by the ‘Porcupine,’ in 2,090 fathoms,
has a pair of well-developed eyes on short footstalks, but _Pleurotoma
nivalis_, obtained by the ‘Talisman,’ is blind. Again a species of
_Fusus_, obtained by the ‘Porcupine,’ in 1,207 fathoms, is provided with
well-developed eyes, but _Fusus abyssorum_, obtained by the ‘Talisman,’
is blind. Among the Lamellibranchs there are very few genera that
possess well-marked eyes. The genus _Pecten_ is one of those that in
shallow waters possess numerous highly complicated visual organs
situated on the edge of the mantle. In the deep-sea species, _Pecten
fragilis_, these eyes are wanting, but we have not sufficient evidence
at present to enable us to assert that all the deep-sea species of this
genus are blind.

Among the Crustacea there is a very general tendency to lose the eyes at
a depth of a few hundred fathoms of water.

In _Ethusa granulata_, for example, the eyes disappear at 500 fathoms
and the eye-stalks become firmly fixed, greater in length, and take the
place of the rostrum which disappears. In some forms—such as
_Thaumastocheles zaleuca_ and _Willemoesia_—the eye-stalks themselves
have completely disappeared.

In the deep-sea Isopoda some forms lose their eyes entirely, but
_Bathynomus giganteus_ possesses a pair of enormous eyes, each provided
with 4,000 facets.

To illustrate the distribution of eyes in this group, we may take as an
example the genus _Serolis_. All the species of this genus are provided
with eyes except _Serolis antarctica_—a species that extends from 600 to
1,600 fathoms.

The eyes of all the deep-sea species are relatively larger than those of
the shallow-water ones, except _Serolis gracilis_, whose eyes seem to be
disappearing.

But these large eyes of the deep-sea species of _Serolis_ are not
capable of any greater perceptive power. In fact, the evidence of
degeneration they show, both in minute structure and in the diminution
of pigment, proves that they can be of very little use to these animals
for perception (see Figs. 4 and 5).

This increase in size, accompanied by degeneration of structure, is just
what we should expect to find in the eyes of deep-sea animals, and it is
difficult to explain why it is that we do not find more examples of it.

If the animals that now live in the depths of the sea are descended from
the shallow-water forms of bygone epochs, they must have passed through
many different habitats with diminished light until they reached their
present dark abode in the abyss.

[Illustration: FIG. 4.—Semi-diagrammatic section through the eye of
_Serolis schythei_, a shallow-water species (4–70 fathoms). C, lens; V,
crystalline cone; R, rhabdom; N, nerve. (After Beddard.)]

[Illustration: FIG. 5.—Diagrammatic section of the eye of _Serolis
bromleyana_, a deep-sea species (400–1,975 fathoms), showing the
degenerate character of the eye. The corneal facets C, and the
crystalline cones V, are the only structures that can be recognised.
(After Beddard.)]

In every new region they came to, the forms with larger and better eyes
would be at an advantage in the fainter light, and would be more likely
to survive and transmit their favourable variation in this respect to
their offspring than their less fortunate neighbours. Thus down to the
depth of the limit of sunlight we should expect to find, as we do find
in fishes, large-eyed species.

Beyond the limit of direct sunlight the eyes would be of very little use
to them, the pigment would disappear and the tissues become degenerate.
This is precisely what has occurred in the genus _Serolis_.

The disappearance of the sense of sight in the animals of the deep sea
is sometimes accompanied by an enormous development of tactile organs.

Thus, among fishes we find _Bathypterois_, a form that possesses
extremely small eyes, provided with enormously long pectoral fin rays
that most probably possess the functions of organs of touch.

Among the Crustacea we find the blind form, _Galathodes Antonii_, with
an extraordinary development in length of the antennæ, and
_Nematocarcinus_, with enormously long antennæ and legs.

The subject of the power of emitting phosphorescent light possessed by
some deep-sea animals is much more difficult to deal with.

The presence of distinct organs in many of the deep-sea fish that can
only be reasonably interpreted as phosphorescent organs, the presence of
well-developed and evidently functional eyes in many deep-sea animals,
and many other considerations render it very highly probable that some,
if not many, forms emit a phosphorescent light.

The power and constancy of the light emitted, however, must for the
present remain a matter of conjecture. We cannot judge at all of the
amount of light given out by an animal in deep water by its appearance
when thrown out of a dredge upon the deck. Whether the phosphorescent
light given out by an Alcyonarian or a Crustacean is more or less at a
temperature of 40° Fahr. and a pressure of one ton per square inch than
it is at 60° Fahr. and the ordinary barometric pressure of the
sea-level, is a question that has not yet been brought to an
experimental test.

Whatever the answer to this question may be, the fact remains that a
greater percentage of animals from the deep sea exhibit some sort of
phosphorescent light when brought on deck than animals that live in
shallow water.

The curious organs possessed by some fishes that are supposed to be
organs for the emission of phosphorescent light have recently been
subjected to a minute examination by von Lendenfeld.

It has been known for some years now, that the slime secreted by the
skin glands of certain sharks is highly phosphorescent. It is not
difficult, then, to understand how it came about that certain fish
developed complicated phosphorescent organs.

From the phosphorescent slime secreted by a simple skin gland to the
most complicated eye-like phosphorescent organ, we have a series of
intermediate forms that are quite sufficient, even in the imperfect
state of our knowledge at the present day, to enable us to understand
the outlines of the evolution of these peculiar and interesting organs.

We can distinguish two kinds of phosphorescent organs in the deep-sea
fish. There are the curious eye-like or ocellar organs situated usually
in one or more rows down the sides of the fish’s body, forming as it
were a series of miniature bull’s-eye lanterns to illuminate the
surrounding sea (fig. 6); and various glandular organs that may be
situated at the extremity of the barbels or in broad patches behind the
eyes or in other prominent places on the head and shoulders.

Ocellar organs have been known for many years to occur on the sides of
the interesting pelagic fish, _Scopelus_. Most of the species of this
genus live in the open sea at moderate depths, coming to the surface
only at night, but other species are found in almost every depth down to
2,000 fathoms of water.

[Illustration: FIG. 6.—_Opostomias micripnus_; 2,150 fathoms. (After
Günther.)]

In _Opostomias micripnus_, a dark black fish living at a depth of over
2,000 fathoms, there are two rows of ocellar organs running down the
sides of the body from the head to the tail. In the living animal they
are said to shine with a reddish lustre. In addition to these, the
conspicuous organs, there are groups of fifty, a hundred, or even more
very much smaller organs situated on the sides and back of the fish,
each of which is lenticular in shape and consists of a number of short
polygonal tubes containing a granular substance with rounded bases
resting on the subjacent tissue. The whole organ is covered by a simple
continuation of the cuticle of the body-wall. The granular substance
contained in the tubes is most probably the seat of luminosity.

[Illustration: FIG. 7.—Head of _Pachystomias microdon_ (after Von
Lendenfeld). A, anterior sub-orbital phosphorescent organ; B, posterior
sub-orbital phosphorescent organ.]

As a type of the glandular organs we may take one of the sub-orbital
organs found on the head of _Pachystomias microdon_.

In this fish there are two very conspicuous white organs immediately
below the eye. The anterior one, which lies below and in front of the
eye, is oval, with its upper margin slightly concave. In section it is
seen to be surrounded by a thin layer of black pigment, and to consist
of a reticular glandular substance in which is embedded a hammer-shaped
lens-like body. Between these two structures there is interposed a thick
layer of light reflecting spicules.

The exact part that is played by the different components of these
curious phosphorescent organs is not yet known, but sufficient has been
said to indicate to the reader the degree of complexity that these
organs may reach in the fish of the great depths of the ocean.

[Illustration: FIG. 8.—Section of the anterior sub-orbital
phosphorescent organ of _Pachystomias microdon_ (after Von Lendenfeld).
L, lens; O, phosphorescent gland; P, pigment sheath.]

But the power of emitting phosphorescent light is by no means confined
to the group of fishes. Some of the Macrurous Decapoda among the
Crustacea are known to be phosphorescent. In the case of _Heterocarpus
Alphonsi_, for example, the naturalists of the ‘Investigator’ found that
‘clouds of a pale blue highly luminous substance, which not only
illuminated the observers’ hands and surrounding objects in the vessel
in which the creature was confined, but also finally communicated a
luminosity to the water itself, were poured out apparently from the
bases of the antennæ.’

‘The _Willemoesia_, too, was luminous at two circumscribed points
somewhere near the orifices of the genital glands.’

Again, all the Alcyonarians dredged by the ‘Challenger’ in deep water
were found to be brilliantly phosphorescent when brought to the surface,
the light consisting, according to Moseley, of red, yellow, and green
rays only.

Among the Echinoderms we have not many recorded instances of a
phosphorescent light being emitted, but it is quite possible that many,
if not all of them, may possess this power. The curious deep-sea form
_Brisinga_, that was first discovered by Ch. Asbjörnsen, is known to be
so brilliantly phosphorescent that it has been called a veritable
_gloria maris_, and writing of the curious brittle-star _Ophiacantha
spinulosa_ (dredged by the ‘Porcupine’ in 584 fathoms of water),
Professor Wyville Thomson remarks that the light was of a ‘brilliant
green, coruscating from the centre of the disc, now along one arm, now
along another, and sometimes vividly illuminating the whole outline of
the star-fish.’

According to Filhol many of the abysmal Annelid worms are in the habit
of emitting a vivid phosphorescent light, and capable thereby of
illuminating the medium in which they live.

We have now considered all those characters exhibited by deep-sea
animals that may be associated with the absence of direct sunlight. To
run through them again briefly we may say: that the deep-sea species,
belonging to classes of animals that usually possess eyes, show some
modification in the size of their eyes, in that they are either very
large, very small, or altogether wanting. That deep-sea animals are
nearly always uniformly coloured. Very frequently they are black or grey
or white, less frequently bright red, purple, or blue. But whatever the
colour may be, spots, stripes, bands, and other markings of the body are
very rarely seen. That deep-sea animals are brilliantly phosphorescent,
the light being emitted either by special organs locally situated on the
head, body, or appendages, or by the general surface of the body.

But there are some other characters that cannot be thus associated with
the absence of sunlight.

In the first place bathybial fish, mollusca, crustacea, and other
animals usually possess a remarkably small amount of lime in their bones
and shells.

In fishes we are told that the bones have a fibrous, fissured, and
cavernous texture, are light, with scarcely any calcareous matter, so
that the point of a fine needle will readily penetrate them without
breaking. In some the primordial cartilage is persistent in a degree
rarely met with in surface fishes, and the membrane bones remain more or
less membranous or are reduced in extent, like the operculum, which is
frequently too small to cover the gills.

This cannot be due in all cases to a deficiency of carbonate of lime in
the sea water, for we find these characters well marked in some of the
fish, such as _Melanocetus Murrayi_, _Chiasmodus niger_, and _Osmodus
Lowii_, that are found on the Globigerina mud.

Then again, the shells of the deep-sea Lamellibranchs, Gasteropods,
Brachiopods, and Crustacea are very frequently remarkably thin and
transparent, a character that is probably more generally due to a
weakness in absorptive or secretive activity than to a deficiency in the
supply of lime.

There are one or two characters of the deep-sea fish that it is not easy
to account for, and it is necessary only to mention their occurrence
without attempting to offer any explanation of them.

One of the most common of these is the very dark pigment occurring in
certain parts of the epithelium of the mouth and respiratory passages
and the endothelium of the peritoneum. For example, in _Bathysaurus
mollis_, living at a depth of 2,000 fathoms, the mouth and buccal
cavities are black. The same thing occurs in _Ipnops Murrayi_, and
indeed in all the strictly deep-sea forms.

Another important character of very frequent occurrence is the reduction
in size, length, and number of the gill laminæ.

Among invertebrates we may mention as a fact of some interest, dependent
perhaps on the soft character of the bottom, the preponderance of
stalked forms over those of more sessile habits.

Thus among the Alcyonaria the characteristic forms of the deep water are
the Pennatulids, and more particularly the genus _Umbellula_ with its
long graceful stem and terminal tuft of polyps. Among the Echinoderma we
find many forms of stalked Crinoids. Among the Tunicates several curious
genera characterised by their long peduncles.

Taking the fauna as a whole, Moseley regarded it as similar in some
respects to the flora of the high mountains. Some forms are dwarfed in
size, such as the species of Radiolaria, Cerianthus, some of the
Cephalopods, &c., while others are very much larger than their
shallow-water allies, such as the Pycnogonids, nearly all the Crustacea,
Alcyonarians (as regards the size of the polypes), Siphonophora, and
many others.




                               CHAPTER V
        THE PROTOZOA, CŒLENTERA, AND ECHINODERMA OF THE DEEP SEA


The most important, but perhaps somewhat disappointing, result of the
deep-sea researches of recent years has been to prove that the abysmal
fauna does not possess many very extraordinary forms.

It seemed probable, before the dispatch of the ‘Challenger’ expedition,
that when the dredge and the trawl should be successfully employed in
depths of over 2,000 fathoms, a new and remarkable fauna would be
brought to light. Some naturalists thought it even possible that, not
only would many genera be found alive that are known to us only by their
fossilised skeletons in the secondary and tertiary rocks, but that there
might be many other new creatures whose anatomy would throw much light
on the theories of the evolution of the animal series.

But none of the great expeditions that have sailed since the year 1874
have yet succeeded in showing that the hopes and wishes of these
naturalists were really justified. Although thousands of species of
animals have been described in the volumes that have been devoted to
deep-sea work, the number of the sub-kingdoms and classes remains the
same, and indeed the number of new families and genera has not been
increased in any very unprecedented manner.

We have found no animals in the depths of the sea of such interest and
importance as Ornithorhynchus, Amphioxus, Balanoglossus, Peripatus,
Millepora, or Volvox among the living, or Hipparion, Archæopteryx,
Ammonites, Slimonia, and the Trilobites among extinct animals.

The abysmal fauna is not in fact remarkable for possessing a large
number of primitive or archaic forms. It is mainly composed of a number
of species belonging to the families and genera of our shallow-water
fauna that have, from time to time, migrated into greater depths and
become modified in their structure in accordance with the extraordinary
conditions of their new habitat.

There is very good reason to believe that this migration has been going
on from time immemorial, and consequently we find a few forms typical of
the bygone times, left to struggle for existence with the more recent
immigrants from shallow waters. But after all the proportion of ancient
forms to modern ones in the fauna of the abyss is not larger than it is
in the fauna of fresh-water lakes and streams or even of the dry land.
Nor is there any reason why it should be. The land and the fresh water
have been peopled by migrations from the shallow water of the sea from
generation to generation in precisely the same way, and they each can
show a certain number of archaic forms.

We must now consider briefly some of the most interesting deep-sea
representatives of the various classes of the animal kingdom, referring
as we pass on to the extent to which these classes contribute to the
fauna of the abyss.

We find a great difficulty in determining with any degree of certainty
the actual depths at which the supposed abysmal forms of Protozoa
actually live. All the Radiolaria and Foraminifera—the only Protozoa
that are largely represented in the fauna of the open seas—are
planktonic in habit; that is to say, they float or drift about in the
water without ever becoming attached to the sea bottom; and when the
contents of a dredge, that has been hauled up from a great depth, are
examined, it is impossible to say at what points in its long journey
from the bottom the Protozoa it contains were caught. Even if dredges
and nets are used which can be closed by a messenger at any particular
depth, the problem cannot be very easily settled; for even if the
protozoa shells that are captured are found to contain a certain amount
of protoplasm, it must be proved that that protoplasm is actually alive
when brought on deck before we know for certain that the species
actually live on the bottom. When the pelagic Foraminifera and
Radiolaria die and sink to the bottom, their protoplasm probably
disintegrates very slowly, and it is quite probable that the floor of
the ocean is littered with the shells of truly pelagic protozoa, each
containing a greater or smaller amount of undecomposed protoplasm.

However, there is little doubt that there are some truly abysmal
Protozoa. Among the Radiolaria, for example, it seems extremely probable
that the majority of the Phæodaria and many Spumellaria live only in
very deep water. ‘A character common to these abyssal forms,’ says
Haeckel, ‘and not found in those from the surface or slight depths, is
found in their small size and massive heavy skeletons, in which respects
they strikingly resemble the fossil Radiolaria of Barbadoes and Nicobar
islands.’ The Phæodaria are very widely distributed over the floor of
the ocean, and occur in some districts in such numbers that the
‘Challenger’ was able to bring home some hundreds of thousands of
specimens. They are distinguished from other Radiolaria by the thick
outer and thin inner capsule, by the typical main opening or atropyle
placed on the oral pole of the main axis with a radiate operculum
provided with a tubular proboscis, and lastly by the presence of the
phæodium, a voluminous pigment body which lies invariably on the oral
half of the calymma and is composed of numerous singular pigment
granules of green, olive, brown, or black colour.

[Illustration: FIG. 9.—_Challengeria Murrayi_, one of the Phæodaria
(2,250 fathoms). A, phæodium; B, central capsule; C, strands of
protoplasm in the calymma. After Haeckel.]

There are many genera belonging to the Foraminifera that are very
probably inhabitants of abysmal depths, but they do not seem to possess
any special characters, unless it be a greater thickness and density of
their shells, to distinguish them from their shallow-water allies.

Passing now to the group of the sponges or Porifera, we find that the
calcareous sponges are not represented at all in the abysmal zone. Two
species are found at a depth of 450 fathoms, but none are truly
bathybial in habit. The same remark applies to the horny sponges.
These forms chiefly belong to the littoral or very shallow-water
fauna, and never descend to greater depths than 400 fathoms. Of the
other groups of Porifera—the Monaxonia, the Tetractinellidæ, and the
Hexactinellidæ—several genera are known to extend down to some of the
greatest depths at which trawling operations have been successfully
carried on. It is difficult to point to any characters in these
sponges that can be attributed in any way to the conditions of
deep-sea life, but nevertheless we do find in deep water some of the
most remarkable and beautiful forms of sponge skeleton that can be
found anywhere.

Amongst the Cœlentera we find in the deep water a remarkable sub-family
of Medusæ, which has been named by Haeckel the Pectyllidæ. It is
distinguished from the other jelly-fish by the curious sucking cups
situated at the ends of the tentacles. It seems probable that they are
used for purposes of locomotion, the animal walking over the muddy
bottom as on a series of stilts.

Like most of the deep-sea Hydroids, the Pectyllidæ are usually devoid of
sense organs, but a single specimen of _Periphylla mirabilis_, captured
by the naturalists of the ‘Challenger,’ possessed well-marked eyes.

There is also a peculiar family of the Siphonophora, called the
Auronectæ, consisting of a few specimens that have been hitherto found
only in very deep water. Like the well-known Portuguese man-of-war
_Physalia_ of the surface waters, the Auronectæ possess a large swimming
bladder or pneumatophore, but they have in addition another peculiar
bladder-like cavity, called the aurophore, communicating with it, which
may be an organ for secreting gas.

A very interesting genus allied to Velella was also found in depths of
over 2,000 fathoms by the ‘Challenger’ expedition. It is supposed to be
a survival of the ancestral form of the Disconectæ, or, at any rate, to
be a link connecting the Siphonophora with the Medusæ. The very well
marked octoradial arrangement of the parts of _Discalia_, as this genus
has been termed, is certainly a point of great interest and importance.

There is no large family of the sea anemones that is peculiar to deep
water, but several genera that occur only in the abyss exhibit some
curious modifications. The manner in which the tentacular pores have
become enlarged, and the tentacles themselves diminished in size and
flexibility, has already been referred to in a previous chapter (p. 36).

The family of sea anemones that has been named the Corallimorphidæ,
characterised by the stiffness and slight contractility of the body, the
knobbed nature of the tentacles, and their distribution in several
series, was, until quite recently, considered to be a true abysmal
family. The two species, _Corallimorphus rigidus_ and _C. profundus_,
are known to occur only in very deep waters, and present some curious
modifications of structure in relation to their habit; but it seems
probable that to this family should be added the remarkable littoral
form _Thelaceros rhizophoræ_ found on the coast of Celebes attached to
the roots of the mangrove trees in the swamps.

The fact that all the principal groups of the Actiniaria, except,
perhaps, the group that includes those forms with only eight
mesenteries, the Edwardsiæ, have representative genera or species in the
great depths of the ocean, points to the conclusion that the sea
anemones have migrated from the shallow waters in comparatively recent
times, and that the migrations have been successive, each period of
their history sending some specimens to survive or to become extinct in
the struggle for life in the deep sea.

Of the Madreporarian corals, several genera are now known to inhabit
very deep water, but they do not present many very remarkable points of
divergence from the shallow-water forms.

It is true that as we pass from the shallow waters, of those parts of
the world where the great colonial madrepores build up the greater part
of the vast coral reefs, into the deeper water beyond them, the solitary
forms become relatively more abundant, but no new groups characterised
by any special deep-sea attributes make their appearance. We must
remember, not only that a great many solitary corals occur in shallow
water in different parts of the world, but that some colonial forms,
such as _Lophohelia prolifera_ for example, are found only in very deep
water.

Until quite recently it was usually stated in works dealing with the
structures of coral reefs that the so-called reef-building corals, that
is to say the large madrepores, astræids, and others, are confined to
water not deeper than thirty fathoms. This limit must now be somewhat
extended, in consequence of the discovery by Captain Moore of an
abundance of growing coral at a depth of forty-four fathoms in the China
seas; but, nevertheless, it is perfectly true that the corals do not
grow in such profusion in very deep water as to form anything that can
be compared with the reefs of the shores. It is quite possible that the
advantages afforded by the light, warmth, and abundance of food of the
shallow water may account for the luxuriance and vigour of the reef
corals, and that where the food is scarce, and the water cold and dark
as it is below fifty fathoms, the power of continuous gemmation is lost,
and the rapidity of the growth and reproduction of the individual polyps
is considerably diminished.

The fact remains, however, that, as with the sea anemones, so with the
madrepores, nearly all the great divisions have a few isolated
representatives in the abyss, and that no great family occurring in
large numbers has yet been discovered peculiar to this zone.

The Alcyonaria, on the other hand, do present us with at least one
example of a true deep-sea family. This great class of Anthozoa,
distinguished from the Zoantharia by the presence of not more than eight
tentacles and mesenteries and by the pinnate character of the former,
falls into four principal divisions. The Stolonifera, the Alcyonidæ, the
Gorgonidæ, and the Pennatulidæ. The first three of these divisions
principally inhabit the shallow water. Each of them sends a few
representatives into the great depths, but by far the greater number of
the genera and species are to be found between tide-marks or in depths
of less than fifty fathoms.

The Pennatulids, on the other hand, are rarely found in very shallow
water, and nearly half the known genera live in deep water. At least two
families may be said to be characteristically abysmal. These are the
Umbellulidæ and the Protoptilidæ.

The Pennatulidæ are regarded by naturalists as the most complicated or
highly organised group of the Alcyonaria. Three different forms of
polype build up the colony or sea-pen as it is called. There is a single
very much modified and enormously large polype, without tentacles,
forming the axis, a large number of ordinary Alcyonarian polypes
(autozooids) arranged in the form of leaves, or simply scattered
irregularly on the surface of the central polype, and a number of very
small undeveloped polypes (Siphonozoids) without tentacles, whose
function seems to be to pump water into the canals of the colony, and
thus to keep up the circulation of water.

[Illustration: FIG. 10.—Umbellula Güntheri. Nat. size. After Agassiz.]

The deep-sea genus _Umbellula_ possesses a very long and delicate axial
polype, and the Autozooids and Siphonozooids form a little cluster only
at its extreme summit. The small number of these polypes and the very
limited area over which they extend are the two most characteristic
features of the genus. It would take me too far into the anatomy of the
group if I were to add any further details; but I cannot pass on without
noting that the whole structure of Umbellula shows that it is far more
primitive and simple than the shallow-water genera. And, generally
speaking, this holds good for all the deep-sea Pennatulids. In fact, we
have here one of the rare examples of a series of genera, that can be
regarded as a slightly modified ancestry of the shallow-water genera,
that has been brought to light by the exploration of the abysmal depths
of the ocean.

We have seen, then, that of the Cœlentera, the only order that has a
large proportion of its genera living in deep water, is the only one
whose members all possess a stalk by which they fix themselves into the
mud or sand at the bottom of the sea.

It is not uninteresting to note, then, in passing on to the Echinoderma,
that the stalked Crinoids, the only Echinoderms that can permanently fix
themselves to the bottom, are nearly all found in deep water.

Several years before the ‘Lightning’ was despatched on her memorable
pioneering voyage, Vaughan Thomson had proved that the common feather
star of the shallow waters of the British coasts passes through a stage
in its development which resembles the fossil genera of the order in
being provided with a stalk for attachment.

But it was left for the naturalists of the ‘Porcupine,’ the
‘Challenger,’ the ‘Talisman,’ and other vessels employed in deep-sea
researches to prove that adult stalked Crinoids are still living in
nearly all parts of the world at the great depths of the sea.

The genera of stalked Crinoids now living are the remains of a family
that at one time had many representatives in all parts of the world.
Nearly all the marine deposits of bygone epochs, including even those of
such remote periods as the Cambrian and Sub-Silurian, contain the
fossilised skeletons of these Crinoids. In some strata they are
represented by only a few genera, but in others they are found in such
enormous numbers that the seabeds of those early times must have been
literally carpeted with them.

At the present day the few genera that survive have been driven from the
shore waters, and are chiefly found at depths of more than 200 fathoms,
a few only extending into 140 and even 70 fathoms.

[Illustration: FIG. 11.—_Rhizocrinus lofotensis_, one of the deep-sea
stalked Crinoids. (After Carpenter.)]

There are six genera known, and of these, two, _Hyocrinus_ and
_Bathycrinus_, are not found in less than 1,000 fathoms of water.

There can be no doubt that these modern stalked Crinoids are closely
related to many of those that flourished in bygone periods of the
history of the earth. As Carpenter has pointed out, the family
Pentacrinidæ are remarkable for their long geological history. The genus
_Pentacrinus_ itself first appeared in the Trias and persisted through
the Secondary and Tertiary times to the present day.

The general character of the fossil Pentacrinidæ is essentially the same
as that of their recent representatives, except that they often had much
longer stems which reached to a length of as much as 50 or even 70 feet;
while the number of arms was frequently limited to ten, which is not the
case in any recent species but _Pentacrinus naresianus_.

But the deep-sea Echinoids, or sea-urchins, also present some features
of particular interest. Professor Agassiz in his report says, ‘One of
the very first results clearly indicated by the deep-sea dredgings of
Count Pourtales and the subsequent investigations of the “Porcupine”
expedition was the antique character of the new genera discovered in
deep water, and especially their resemblance to the cretaceous genera;
and the study of the “Challenger” Echinoids has brought this out still
more clearly.’

No fewer than twenty-four genera extend into the abysmal regions; of
these no less than sixteen, nearly all belonging to a new group of
Spatangoids, do not live at all in shallow water.

The most interesting forms among these are the Pourtalesiæ, a group that
has existed since the Chalk. These are heart-shaped urchins with a very
peculiar test. ‘They all have large coronal plates, recalling the
Echini, with a disconnected apical system characteristic of many
cainozoic spatangoids; they have a sunken anal system, some of them a
most remarkable anal beak and a very striking pouch in which the mouth
is placed.’ They are found only in very deep water, and have no allies
among the modern littoral fauna.

The genera _Calveria_ and _Phormosoma_ are two of the most abundant
Echinoids found in deep water, and they are both representatives of
forms that were very abundant in cretaceous times. They are remarkable
for the extreme flexibility of their shells. In shallow-water
sea-urchins the shells are composed of a great number of little plates
that fit so closely to one another that no movement is possible between
them. When the animal dies all the soft tissues decay and the shell
remains, to be tossed about by the waves until crunched or dashed to
pieces. In _Phormosoma_, however, the tiny plates of which the shell is
composed are freely movable on one another, and when the animal is alive
very considerable contractions and expansions can take place.

None of the modern shallow-water Echinoids present this peculiarity, and
it is a very interesting and surprising fact that in this respect the
fossils of the chalk should resemble so closely the living urchins of
the abyss.

But before leaving the Echinoids reference must be made to two more
points that have been made by the illustrious American naturalist.
Agassiz points out that all those genera that have the greatest
bathymetrical range, extending from the littoral to the abysmal region,
are at the same time genera which date back to the Cretaceous period,
while those having a somewhat more limited range go back to the
tertiaries, and those that extend only slightly beyond the littoral area
go back only to the later tertiaries.

This interesting generalisation brings home to our minds the enormous
length of time that it must have taken these animals to migrate from the
shallow to the deep sea. In the struggles for existence between marine
animals it must always have been the last resort of those unable to
compete with the younger generations in shallow water to migrate into
the deeps.

The scarcity of food, the darkness, and the pressure of these regions
can never be so favourable for the support of animals as the conditions
of the shores. We can well imagine that a species would take every
opportunity that is afforded to return from such inhospitable habitats,
and that only when, as it were, every door is closed, when no island,
continent, or cape can afford it a free scope for life in shallow water,
does it become a true deep-sea species.

Steps taken towards the darkness in one period may be retrieved in the
next. The competing species may itself have become extinct or have moved
to another locality. Organs may have become modified or a new source of
food supply tapped which enable them to return once more to shallower
waters. No wonder that the steps in the progress, or rather retreat, to
the abyss have been the work of a time that can be counted only by
geological periods; and no wonder then at the remark made by many
deep-sea naturalists that the abysmal fauna becomes poorer the farther
it is from shallow water.

The group of the Asteroidea, or star-fishes, contributes largely to the
fauna of the abyss.

During the voyage of the ‘Challenger’ no fewer than 109 different
species were found in depths of over 500 fathoms, and in some localities
a very large number of star-fish were taken in one haul of the dredge.

Nevertheless, there are not many abysmal genera that differ to any
remarkable degree from the littoral ones; and indeed it may be said that
the recent work on deep-sea Asteroids does not throw much new light
either on the phylogeny of the group or on their palæontological
history.

The genus _Brisinga_, at one time supposed to be a connecting link
between the star-fishes and the brittle stars (Ophiurids), has recently
been shown to be closely related to the families Heliasteridea,
Echinasteridea, and others typical of the class Asteroidea; and, as
Sladen has pointed out, the peculiarities of structure that it exhibits
are probably due to its extreme isolation and the influence of its
abysmal habitats.

But no work on the deep-sea fauna would be complete without some
reference to Brisinga. Discovered by Asbjörnsen in 1853, in 200 fathoms
of water in the Hardanger fjord, and described in a splendid memoir by
the elder Sars, it excited great interest among naturalists. The great
brilliancy of the phosphorescent light that it gave out on being brought
on deck, the remarkable tendency that it had to cast off some of its
numerous long, thin, ophiurid-like arms, and some of the general
features of its internal anatomy were points that were considered at the
time to be sufficient to justify the establishment of a separate
sub-order for the family Brisingidæ.

The more recent discovery, however, of genera allied to Brisinga has
bridged over the gap separating it from other star-fish, and it is now
considered simply as the type of a family of the order.

The numerous species of the genus that have been found since
Asbjörnsen’s original discovery are all inhabitants of deep water, some
of them going down to the enormous depth of 2,000 fathoms; indeed there
are very few genera in the animal kingdom, containing so many species as
the genus Brisinga, that have such a uniform deep-sea habitat.

The last group of Echinoderms that we have to consider is the
Holothurians. It contains one order—the Elasipoda—that may be considered
to be truly bathybial, as there is only one species belonging to it,
_Elpidia glacialis_, that extends into water as shallow as fifty
fathoms.

The Elasipoda are remarkable for their strongly-developed bilateral
symmetry. Adult Echinoderms as a rule possess a well-marked radial
symmetry, as we see exemplified in the feather-star, star-fish, and
sea-urchin, but this radial symmetry is only adopted when they undergo
their metamorphosis from the free swimming and bilaterally symmetrical
larval stage. They are not born radially symmetrical, but become so as
they grow up. Moreover, we must bear in mind that the radial symmetry of
the adult only obscures, it does not obliterate, the bilateral symmetry
of the larva.

In the Holothurian, however, we can always discover a clear bilateral
symmetry even in the adult. That is to say, we can recognise an anterior
and a posterior end, a right and a left side of the body. It is an
organisation which emphasises, as it were, the anterior and posterior
ends, the right and left sides and the dorsal and ventral surfaces that
characterise this interesting deep-sea order, the Elasipoda.

Here, then, we have an example of a character common to all the larvæ of
the sub-kingdom and exceptionally well marked in the adults of a family
confined to deep-sea habitats.

Now we know that there is a tendency for some of the peculiar characters
of the ancestors of animals to be recapitulated in the course of their
development from the egg, and accordingly most naturalists are agreed
that all the Echinoderms have descended from some form of bilaterally
symmetrical ancestor. Are we, then, to believe that the Elasipoda
brought from the depths of the sea are more closely related to these
ancestral forms than the shallow-water families?

The state of our knowledge at the present day hardly allows us to answer
this question very definitely. However nearly they are related to such
ancestral Echinoderms in general form, they are probably profoundly
modified by a deep-sea life. Nevertheless, in the simple shape of the
calcareous corpuscles of the skin, the simple form of the calcareous
ring, the communication of the water-vascular system with the exterior
by one or several pores, and in some other anatomical characters, they
give evidence of their primitive characters.




                               CHAPTER VI
                THE VERMES AND MOLLUSCA OF THE DEEP SEA


It has not been my intention in this volume to confine my attention to
the truly abysmal forms, but rather to consider all those animals living
in deep water that show any characters strikingly different from their
relatives living in shallow water.

The term deep water is, after all, only a relative one.

To one accustomed only to shore collecting, ten fathoms is deep water,
while on the other hand, to such naturalists as those on board the
‘Challenger,’ who are accustomed to dredge in all seas, nothing under
1,000 fathoms is considered deep water.

We must bear in mind, however, that at a depth of only 200 fathoms, the
conditions of life are very different to those of the shore waters. We
find a very great diminution in the amount of light, for instance, that
can penetrate through sea water teeming with floating organisms of all
kinds to reach the fauna attached to the bottom at such a depth. The
diminution in the amount of light must mean a diminution in the rapidity
of growth of chlorophyll-bearing plants, and consequently a diminution
in the food supplies of animals drawn from that source.

We might expect then to find, even in such shallow water as this, some
forms of particular interest. It is true that the greater part of the
fauna is made up of ordinary shallow-water forms that have migrated
quite recently, and perhaps only temporarily, into the depths, but we
expect to find, and actually do find, the outposts of a new fauna.

These remarks lead me to the consideration of one or two very remarkable
animals that have recently been brought to light.

In that strange assembly of animals which, for want of a better word,
the authorities call the Vermes, there are three groups whose relations
to one another and to the other groups of Vermes have been and still
remain a puzzle to naturalists.

These three groups are the Gephyrea, the Polyzoa, and the Brachiopoda.
In external form they are as different from one another as possible.

The Gephyrea are solitary worm-like forms burrowing in the sand or
perforating rocks; the Polyzoa are minute creatures that frequently
build up by budding large colonies which assume in some cases dendritic
forms like corals, and the Brachiopoda are protected by thick bivalve
shells simulating in a striking manner the shells of the
Lamellibranchiate mollusca.

But external form is not the only character that can be relied upon for
purposes of classification. The general and minute anatomy, together
with the story of the development of these animals, teach us that they
are in some way closely related.

It is not within the scope of this book to enter into the discussion of
what these relations are; suffice it to say that the controversy has
within recent years to a great extent turned upon the position in our
classification of three interesting genera. These are Phoronis,
Rhabdopleura and Cephalodiscus.

Phoronis occurs only in shallow water, Rhabdopleura has been found in
water from 40 to 200 fathoms deep off the Shetlands and on the Norwegian
coasts, while Cephalodiscus was discovered by the ‘Challenger’ at a
depth of 245 fathoms off Magellan Straits.

Rhabdopleura forms colonies consisting of branched tubes growing upon
the tests of Ascidians, on sea-weeds, corals, or other objects fixed to
the sea bottom. In the open, free extremity of each of the branches may
be found the polypide attached to a filament or stalk which connects it
with the other polypides of the colony (fig. 12).

[Illustration: FIG. 12.—A portion of a colony of _Rhabdopleura normani_.
(After Lankester.)]

Each polypide is provided with a single pair of large pinnate arms,
resembling the arms of a Brachiopod, and a broad muscular epistome by
means of which it is able to creep up or down the tube.

The affinities of this interesting creature are by no means sufficiently
well understood. It is one of those forms that, without being, strictly
speaking, a connecting link between large and well-known groups of
animals, indicates to us some of the lines of evolution that these
groups may have passed through; and, in so far as it does this, it has
its value and importance.

Cephalodiscus, though related to Rhabdopleura in the presence of a
structure corresponding to the arms, and a broad epistome, seems to be
more closely connected with such a form as Balanoglossus in the presence
of a single pair of gill-slits, a small rudimentary notochord and the
position of the central nervous system.[2]

Footnote 2:

  A rudimentary notochord projecting forward from the buccal cavity into
  the epistome has quite recently been discovered in Rhabdopleura.

[Illustration: FIG. 13.—A single polypide of _Rhabdopleura normani_. M,
mouth; B, epistome; S, polypide stalk. (After Lankester.)]

Whatever position these genera may ultimately occupy in our systems of
classification, there can be little doubt that much valuable information
will be obtained by a further study of their structure and
development—information that will probably shed much light on the
relationships to one another of the many groups of Vermes. Their
occurrence in water of moderate depths only indicates perhaps that they
are gradually being crowded out from the more favourable localities of
shallow water, and are tending towards extinction on the one hand, or a
deep-sea habitat on the other.

The Brachiopoda need not detain us long. Some species are capable of
existing at a great variety of depths without any observable
modification of shape or characters. Thus _Terebratulina caput
serpentis_ has the extraordinary bathymetrical distribution of 0–1,180
fathoms, and _Terebratula vitrea_ 5–1,456 fathoms. _Atretia_ is the only
genus peculiar to deep water. It is a noteworthy fact in connection with
this order that the two genera, _Lingula_ and _Glottidia_, which compose
the sub-order Ecardines, are both confined to shallow water. Now the
Ecardines are anatomically, at any rate, the most primitive of the
Brachiopoda, and Lingula has the most ancient geological history of any
living genus of the animal kingdom, shells almost identical with those
of the living species being found abundantly in the Cambrian strata. Why
it is that Lingula has been able to maintain itself almost unchanged
through all the countless generations that have elapsed since Cambrian
times, and can now flourish amid the desperate struggle for existence in
the shallow waters of the tropics, while its companions, the corals,
mollusks, arthropods, &c., have changed or passed away, is one of those
problems in natural history that seem to us impossible of solution. The
time may come when we shall be able to appreciate better than we do now
the complicated relations between animals and their environment, and
then perhaps the peculiar fitness of Lingula will be made manifest; but
at present we can but mention the fact as a fact, and leave the solution
of the problem to the future.

The order Gephyrea is probably another very ancient group of animals,
although in the absence of any hard calcareous, siliceous, or horny
skeleton the geological record can give us no confirmation of their
antiquity. As with the Brachiopods so with the Gephyrea, some of the
species have a very wide bathymetrical distribution. _Sipunculus nudus_,
for example, the commonest and best known of all the Gephyrea, extends
from quite shallow water to a depth of over 1,500 fathoms. As Selenka
has pointed out, it is those Gephyreans that live in holes in stones, or
in shells such as _Phascolion_ and _Phascolosoma_, that are more
frequently found at the greater depths; but, apart from this, there are
no characters that exclusively belong to the abysmal Gephyrea or are
more frequently found in them than in the shallow-water forms. Nor are
there any genera, at present brought to light, that are confined to
those regions of the sea.

The group of the Annelida is not very well represented in the deep-sea
fauna. The genera _Serpula_ and _Terebella_ have been found very widely
distributed over the earth, at all depths from the shore to the abyss,
but there do not seem to be many genera that are confined to deep water.
In some cases, where there is a scarcity of lime in the water, the thin
protecting tubes of the sedentary forms are strengthened by the adhesion
of foreign particles, such as sponge spicules and arenaceous
foraminifera, but in others, the tubes are formed of successive layers
of a transparent quill-like substance (_Nothria Willemoesii_) which is
frequently armed with spiny projections.

Most of the errant Polychætes found at great depths are said to be most
brilliantly coloured, and some of these, such as _Eunice amphiheliæ_,
have the power of emitting a bright phosphorescent light; but there seem
to be no very definite and constant characters separating these forms
from the Polychætes of shallow waters.

As is the case with many other orders of animals, the species of
Annelida living in deep water are either blind or possess eyes of a
remarkably large size. _Genityllis oculata_ may be taken as an example
of a deep-sea annelid with large eyes. This annelid, belonging to the
family Phyllodocidæ, was found at a depth of 500 fathoms in the Celebes
sea. It possesses two enormous eyes which cover almost the whole of the
head, and there can be no doubt, from the investigations of Dr. Gunn on
their minute anatomy, that they are perfectly functional.

Before leaving the Annelida a brief notice must be made of the very
extraordinary form _Syllis ramosa_, found parasitic on a hexactinellid
sponge at depths of about 100 fathoms. It is chiefly remarkable for the
very complicated manner it has of producing buds which do not
immediately become detached from the parent, but form a compound network
which ramifies through the interstices of the sponge like the colony of
a Hydromedusan.

Passing now to the sub-kingdom Mollusca, we shall find that all the
classes are represented in the abysmal fauna.

The Lamellibranchiata, or bivalves, occur in almost all depths of the
ocean, _Callocardia pacifica_ and _Callocardia atlantica_ having been
found at the enormous depth of 2,900 fathoms. Some species, such as
_Venus mesodesma_, have a very wide bathymetrical distribution, but
others are only known to occur in deep water.

Concerning the characters of the deep-sea Lamellibranchiates, Mr. Smith,
in his report on the Lamellibranchia of the ‘Challenger’ expedition,
says ‘very deep-water benthal species certainly have a tendency to be
without colour, and of thin structure, no doubt resulting from the
absence of light the difficulty of secreting lime, the scarcity of food
and other unfavourable conditions of existence.’ But notwithstanding
this, the same author continues: ‘The species are apparently few in
number in comparison with those of shallow water; and new and peculiar
generic forms which we naturally expected would have been discovered are
of even still rarer occurrence.’

As in the group of the Lamellibranchia, so in the Gasteropoda, no very
remarkable new genera or species have been found in deep sea. Some
shallow-water genera, such as _Fusus_ for example, have representative
species in the abyss; but, with the exception of a want of brilliant
coloration and marking and a thinness of the shell, the deep-sea forms
do not exhibit any characteristic features. One of the most brightly
coloured shells found at great depths is that of _Scalaria mirifica_,
which is tinted rose and white, but this seems to be quite an
exceptional character among the deep-sea Gasteropods. Several new genera
were found in the deep water, but their general characters do not call
for any special remark.

Among the Cephalopoda there seems to be little doubt that the genera
_Cirroteuthis_, _Bathyteuthis_, and _Mastigoteuthis_ are entirely
abysmal, and the same applies probably to one or two species of octopus;
but as Hoyle remarks, ‘apart from the single fact that _Bathyteuthis_
and _Mastigoteuthis_ both have slender filiform tentacles with minute
suckers, no structural features have been discovered which will serve to
diagnose a deep-sea form from a shallow-water one.’

The exact habitat of the interesting genus _Spirula_ is unfortunately
still unknown. In some parts of the tropics the shores are covered with
spirula shells, and yet the animals that secrete them are still to be
reckoned amongst the greatest rarities of our museums. The numerous
dredgings of the ‘Challenger’ only brought to light one specimen of this
animal, and that from a depth of 360 fathoms, and the ‘Blake’ caught one
at a depth of 950 fathoms, so that there can be little doubt that
_Spirula_ lives in deep water.

[Illustration: FIG. 14.—_Bathyteuthis abyssicola._ (After Hoyle.)]

It seems to be very probable that some day, when the right place and
depth are discovered, _Spirula_ may be discovered in great abundance,
but we have at present no means of judging whether this will be in very
deep water or not.

Almost precisely similar remarks apply to the distribution of the pearly
Nautilus. The shells of this Cephalopod are sometimes found in great
number on the shores of some of the islands of the Southern Pacific
Ocean and the Malay Archipelago, but the living animal is but rarely
captured. It has been asserted by some travellers that the pearly
Nautilus floats on the surface of the ocean and possesses the power of
suddenly diving to great depths on being disturbed; but it must be
remembered that Rumphius originally caught his Amboyna specimens of
Nautilus in traps set at a depth of 200 fathoms baited with sea-urchins,
and that the ‘Challenger’ captured a single living specimen off Matuku
island in 300 fathoms.

The probability, then, is that both _Nautilus_ and _Spirula_ should be
included in the deep-sea fauna, but we are still in want of a great deal
more information concerning their life and habits before this point can
be definitely determined.




                              CHAPTER VII
                     THE ARTHROPODA OF THE DEEP SEA


The deep-sea fauna seems to be particularly rich in marine Arthropoda,
many curious and interesting forms being brought up with almost every
haul of the dredge. The Arthropoda, too, being very highly organised
animals, afford interesting and instructive examples of the effect of
abysmal life in the modification of the sense organs and the production
of varieties specially modified for the conditions of the struggle for
existence in their strange habitat.

Concerning the groups of Ostracoda and Copepoda it may be said that the
evidence is not yet conclusive that they include any truly deep-sea
species. The largest known Ostracod, measuring somewhat more than an
inch in length and probably allied to the genus _Crossophorus_, has
quite recently been captured by Professor Agassiz at depths of less than
200 fathoms, but he could obtain no evidence that it descended into much
deeper water than this.

Mr. Brady, in writing the report of the ‘Challenger’ Ostracoda, came to
the conclusion that they do exist in very limited numbers in the most
profound depths of the sea; but it is nevertheless quite possible that
all the Ostracods brought on deck by the trawl or dredge were really
captured either on the way down or on the way up, and are, strictly
speaking, pelagic in habit.

Similar caution must be taken in dealing with the Copepoda, an order of
Crustacea that is essentially pelagic in habit. The only species that
has been regarded as undoubtedly abysmal is _Pontostratiotes
abyssicola_, a form whose carapace and antennæ are armed with
exceedingly long and strongly toothed spines, and was found in the mud
brought up by the trawl from a depth of 2,200 fathoms.

_Calamus princeps_, the largest species of its genus of a deep reddish
brown colour, may also belong to the fauna of the deep sea, but we have
less evidence concerning the habitat of _Hemicalamus aculeatus_,
_Phyllopus bidentatus_, and some of the Euchætæ.

The Amphipoda seem to be but poorly represented in the fauna of the
abyss; in fact it may be considered to be still an open question whether
any Amphipods habitually live in very deep water.

In the reports on the ‘Challenger’ Amphipoda, the Rev. T. R. Stebbing
states that thirty-one specimens are known to come from great depths,
but it would be more correct to say that these specimens were found in
the dredges and trawls that had been lowered into the great depths. It
should be noticed, however, that some of these specimens do show
characters that suggest, at any rate, that they come from deep water.
Thus the genus _Lanceola_, for example, is characterised by the
smallness of the eyes and a soft membranous integument, while _Cystisoma
spinosum_, found in a dredge that had been at work at a depth of over a
thousand fathoms, has very large eyes.

In his report on the Crustacea of the ‘Norske Nord-havns’ expedition,
Professor Sars gives a full description of many species of Amphipoda
brought by the dredge from depths of over 1,000 fathoms, and nearly all
of these were found to be quite blind.

The form that seems to be most peculiar to the great depths of the
Northern Ocean is _Harpinia abyssi_. It was found at no less than
fifteen different stations at depths ranging from 350 to 2,215 fathoms,
and is characterised by its large size and the total absence of eyes.

Another point that should be considered in coming to any conclusion on
the supposed habitat of such forms, is the similarity or dissimilarity
of widely distributed species.

I have had occasion to point out in a previous chapter the general
similarity of the abysmal fauna all over the world, a very striking
phenomenon, commented on by almost every naturalist who has had a wide
experience of this kind of investigation.

Among the Amphipoda we have a very striking example of this. The species
_Orchomene musculosus_ was taken by the ‘Challenger’ off the southern
part of Japan at a depth of 2,425 fathoms, the bottom being red clay and
the temperature 35·5° Fahr. The species _Orchomene abyssorum_ was taken
off the east coast of Buenos Ayres at a depth of 1,900 fathoms, the
bottom being blue mud and the temperature 33·1° Fahr. To the description
of this last-named species Mr. Stebbing adds, ‘had this species been
taken within reasonable distance of _O. musculosus_, the resemblance is
so great that one might have been tempted to disregard the points of
difference as due to some other cause than difference of species.’

Such a striking similarity between two species living so far apart from
one another may, when we take into consideration the depth, the
character of the bottom, and the temperature from which they are
supposed to have been dredged, be taken to support very strongly the
view that these species are really abysmal in habit.

Among the Isopoda we have several very characteristic forms—no fewer
than nine distinct genera peculiar to the abysmal zone have been
described by Beddard—and of these two, _Bathynomus_ and _Anuropus_, are
to be regarded as types of sub-families. They seem to be very unevenly
distributed over the floor of the ocean, some regions, such as the whole
of the Central and Southern Atlantic and the Central and Western
Pacific, produce none; whilst the waters of the east coast of New
Zealand, the Crozets, and others, produce a great many varieties. Many
of the deep-sea Isopoda exhibit characters that are usually associated
with the bathybial life. Thus, according to Beddard, thirty-four of the
deep-sea species are totally blind, and eighteen have well-developed
eyes. In four species there are eyes which are evidently degenerating.
If we compare, for instance, the structure of the eye of _Serolis
schythei_, a species found in shallow water ranging from 4 to 70
fathoms, with the eyes of _Serolis bromleyana_, a species living in deep
water ranging from 400 to 1,975 fathoms, we cannot fail to see that the
latter are undergoing a process of degeneration; the retinulæ and
pigment being absent, and nothing left of the complicated structure of
the Isopod eye but the remnants of the crystalline cones and corneal
facets (see figs. 4 and 5, p. 74).

Taking the genus _Serolis_ alone, it has been said ‘that in all the
shallow-water forms the eye is relatively small but very conspicuous
from the abundant deposition of pigment; in all the deep-sea forms, with
the exception of _S. gracilis_, where the eye seems to be disappearing,
it is relatively larger but not so conspicuous, owing to the fact that
little or no pigment is present.’

In many groups of animals it has been shown that some of the deep-sea
species are relatively much larger in size than the shallow-water
species, and that others, more rarely, are much smaller, the abysmal
fauna reminding us in this respect of the characters of the alpine
flora.

The Isopoda show many examples of this largeness in size, thus
_Bathynomus giganteus_, dredged by Professor Agassiz off the Tortugas at
a depth of over 900 fathoms, reaches the enormous size, for an Isopod,
of 9 inches (fig. 15). _Stenetrium haswelli_, again, is larger than any
of the shallow-water species of the genus, and the same remark applies
to the deep-sea species of the genus _Ichnosoma_, while _Iolanthe
acanthonotus_, from a depth of nearly 2,000 fathoms, is considerably
larger than most of the shallow-water Asellidæ.

There is another very common character of deep-sea Crustacea that is
also well exemplified in the group of the Isopods, and that is the
extraordinary length and number of the spines covering the body.

I have already referred to this character in the supposed deep-sea
Copepod _Pontostratiotes abyssicola_, and I shall have again to refer to
it in treating of the Decapoda and other groups of the Crustacea.

Besides its enormous size _Bathynomus_ possesses some other characters
that may be correlated with its deep-sea environment. The respiratory
organs are quite different from those of other Isopods; instead of being
borne by the abdominal appendages, they are in the form of branched
outgrowths from the body-wall containing numerous blood-lacunæ, and the
appendages simply act as opercula to cover and protect them. The eyes of
the Bathynomus too are remarkably well developed, each one bearing 4,000
facets, and they are directed not dorsally as in the Cymothoadæ, but
ventrally. The cause of these curious modifications of structure in
Bathynomus is by no means clear, but it is quite probable that they are
connected with the conditions of pressure and light in the deep sea. It
is a remarkable fact that the other deep-sea Isopods do not exhibit
precisely these modifications, and it might be supposed that the same
causes would produce the same or similar effects on the structure of
animals belonging to the same order. That is perfectly true, but we
cannot yet determine how long ago any one species has taken to a
deep-sea life, or what length of time, in other words, these conditions
have been at work in modifying the structure of the organism. A recent
immigrant into the abyss will naturally exhibit closer affinities with
its shallow-water allies than those that have dwelt in the region since
secondary or tertiary times. If we take this into consideration we
should expect to find considerable differences occurring between
deep-sea species of the same order, which is precisely what we do find.

[Illustration: FIG. 15.—_Bathynomus giganteus._ From a depth of 1,740
metres. (From Filhol.)]

Concerning the Cirripedia, that curious group of profoundly modified
Crustacea that includes the barnacles and acorn shells, Dr. Hoek writes
in the ‘Challenger’ monograph:—

‘Though unquestionably by far the greater part of the known Cirripedia
are shallow-water species, and though some of the species are capable of
living at a considerable variety of depths, as, for instance,
_Scalpellum stroemii_, yet it must be granted that the number of true
deep-sea species of Cirripedia is very considerable.’ Only two genera,
however, occur in depths of over 1,000 fathoms, and these—_Scalpellum_
and _Verruca_—occur also as fossils in secondary and tertiary deposits.
The oldest of all fossil cirripedes, however, namely, _Pollicipes_,
never occurs, at the present day, in deep water, but is purely littoral
or neritic in habit. But what is perhaps more interesting still is the
fact, that, when we come to compare the living and the fossil species,
we find that in the one genus (_Scalpellum_) the deep-sea forms have
preserved the more archaic characters, and in the other (_Pollicipes_)
the shallow-water forms.

Here then we are presented with a veritable puzzle for which we can at
present frame no manner of answer. Pollicipes on the one hand—like
Lingula among the brachiopods—has been able to maintain itself almost
unchanged amid the tremendous struggle for life of the shallow water of
the tropics ever since the Lower Oolite epoch; while Scalpellum, on the
other hand, has either become profoundly modified, or been driven into
the abysmal depths of the ocean.

The group of the Thoracostraca, or stalk-eyed Crustacea, including
lobsters, crabs, hermit crabs, prawns, and shrimps, is well represented
in the deep sea. Most of them are characterised by being quite blind (in
many cases even the eye-stalks are obliterated), by being protected with
a dense covering of spines, by the thinness of their shells, and by
their bright red or carmine colour.

The order Stomatopoda is almost entirely confined to the shallow waters
of the tropical or temperate shores. Not a single species is known to
inhabit the deep sea, and only a very few specimens have been captured
in more than a few fathoms of water.

The Schizopoda, however, present us with many curious abysmal forms.
Most of the genera of this order belong to the pelagic plankton, and
many of them are known to possess the power of emitting a very strong
phosphorescent light. Several genera, however, such as _Gnathophausia_,
_Chlaraspis_, _Eucopia_, _Bentheuphausia_, &c., never seem to leave the
great depths of the ocean, and nearly all of these genera are
distinguished by being quite blind or possessing very much reduced or
rudimentary eyes.

[Illustration: FIG. 16.—_Euphausia latifrons_, from the surface of the
sea. (After Sars.)]

[Illustration: FIG. 17.—_Bentheuphausia amblyops_, from 1,000 fathoms.
(After Sars.)]

If we compare, for example, the pelagic _Euphausia latifrons_ (fig. 16)
with the nearly allied but abysmal _Bentheuphausia amblyops_ (fig. 17),
the difference in this respect between a Schizopod living in the
sunlight and one living in the darkness of the deep-sea is very
apparent.

The pelagic Schizopoda are usually quite pale and transparent; the
deep-sea forms on the other hand are frequently if not invariably of a
bright red colour, as is the case with many other deep-sea Crustacea to
which reference will be made later on.

Passing on to the group of the Decapoda, we find that the most
interesting of all the abysmal cray-fish is the family of the Eryonidæ;
indeed, in some respects the discovery of these curious forms may be
reckoned among the most valuable results of the ‘Challenger’ Expedition.
They are characterised by the dorsal depression of the anterior part of
the cephalothorax, the absence of a rostrum, and the absence or very
rudimentary condition of the eyes (fig. 18).

Their nearest relations seem to be certain genera of Crustacea that are
found in jurassic strata, in the lias, and more particularly in the
lithographic slates of Solenhofen.

They have a very wide bathymetrical range extending from a depth of 250
fathoms (_Polycheles crucifera_) to a depth of 2,000 fathoms
(_Willemoesia_).

[Illustration: FIG. 18.—_Polycheles baccata_, one of the Eryonidæ. The
eyes and eye-stalks are absent, and the margin and sides of the carapace
armed with spines. (After Spence Bate.)]

But there are many other curious forms of the macrurous crustacea that
deserve a passing mention. The graceful _Nematocarcinus gracilipes_,
distinguished by the extraordinary length of the antennæ and last four
pairs of legs, these appendages being three or four times the length of
the body, is by no means rarely met with in depths of over 400 fathoms.

The genus _Glyphus_ captured by the ‘Talisman’ is remarkable for the
development of a peculiar pouchlike arrangement on the abdomen for the
protection of the larvæ during the younger stages of their existence.

The proof of the existence of a peculiar cray-fish, _Thaumastocheles
zaleuca_, at a depth of 450 fathoms, was one of the most important
contributions to carcinology made by the ‘Challenger’ Expedition. The
chelæ of this remarkable form are of great but unequal length and armed
with long tooth-like spines giving it an appearance not unlike that of
the jaws of some carnivorous fish. The shell is soft and the abdomen
broad and flattened. There are no eyes nor even eye-stalks, but ‘in
front of the carapace,’ as Sir Wyville Thomson remarks, ‘between the
anterior and upper edge and the insertions of the antennæ, in the
position of the eyes in such forms as _Astacus fluviatilis_, there are
two round vacant spaces, which look as if the eye-stalks and eyes had
been carefully extirpated and the space they occupied closed with a
chitinous membrane.’ The deep-sea prawn, _Psalidopus_, recently taken in
500 fathoms of water by the ‘Investigator,’ affords us an example of a
common bathybial character, the whole body being covered with an
extraordinary array of sharp needle-like spines.

Among the crabs many curious forms have been found in deep water
extending down to depths of over 2,500 fathoms. They are nearly all
characterised by blindness and a remarkable development of tooth-like
spines covering the carapace and limbs.

The remarkable _Lithodes ferox_, from a depth of from 450 to 800
fathoms, is perhaps the most perfectly armed crab—in the way of
spines—that exists. Every part of the body and limbs is so covered with
spines that one has to be extremely careful in handling even a dead
specimen.

This is only one of the many examples that might be given to illustrate
this curious feature of the deep-sea Crustacea. Among the crabs alone we
have such forms as _Galathodes Antonii_, _Pachygaster formosus_,
_Dicranodromia mahyeuxii_ covered with a fierce armature of spines or
bristles; but there are nevertheless some species in which this
character is not particularly noticeable, and in these we usually find
some other protection against their enemies. An interesting example of
this has been described by A. Agassiz in a crab allied to the Maiadæ,
‘in which the dorsal face appears like a bit of muddy area covered by
corals, with a huge white arm resembling a fragment of an Isis-like
gorgonian.’ It is evident that this is a case in which the animal is
protected by its resemblance to the surroundings.

The hermit crabs of the abyss, too, are not usually characterised by any
very great development of spines. They find their protection in the
shells they inhabit. Some of the deep-sea hermit crabs carry about with
them on their shells a sea anemone, as we find to be frequently the case
among the shallow-water species. _Pagurus abyssorum_, from a depth of
3,000 fathoms, is an example of this.

In cases where there is a scarcity of gasteropod shells the hermit crabs
are obliged to find some other form of protection for their bodies. The
‘Blake’ found in the West Indies a hermit crab that had formed for
itself a case of tightly compressed sand, and another curious form,
named _Xylopagurus rectus_, makes its home in pieces of bamboo or in the
holes in lumps of water-logged wood.

The last group of the Arthropoda we need refer to is that of the
Pycnogonida, those curious creatures seemingly made up entirely of legs,
and by some naturalists considered to be related to the Crustacea and by
others to the scorpions and spiders.

Like the Brachiopoda the Pycnogonida are not usually found in greater
depths than 500 fathoms. Out of the twenty-seven known genera, only five
extend into the abyss, and not one of these can be called a true
deep-sea genus.

There are three genera, _Nymphon_, _Collosendeis_, and _Phoxichilidium_,
that show a very wide distribution over the floor of the ocean, and are
capable of existing at the greatest depths, and of these the species of
the genus Nymphon have a truly remarkable range extending from the shore
to a depth of 2,225 fathoms.

‘As a rule,’ says Hoek, ‘the deep-sea species are slender, the legs very
long and brittle, and the surface of the body smooth.’ They have
further, either no eyes at all or rudimentary eyes without pigment, and
in many cases—as, for example, _Collosendeis_—they are distinguished for
reaching to a gigantic size compared with their shallow-water relatives.

The Tunicata is the group of animals that includes all those curious
vegetable-like organisms found upon our coasts that are familiarly known
as sea-squirts, or Ascidians, besides the salps, pyrosomas, and the
microscopic appendicularias of the pelagic plankton.

[Illustration: FIG. 19.—_Collosendeis arcuatus_, from a depth of 1,500
metres. (After Filhol.)]

Notwithstanding the apparent simplicity of their adult structure,
naturalists are now agreed that they must be removed from the Mollusca,
with which they have hitherto been most frequently associated, and
placed in the group of the Vertebrata. It is the study of embryology
that has led to this unexpected conclusion, for we find, when we study
the larval forms, that they possess both a notochord and gill-slits, two
features that are characteristic of the group of the Vertebrata.

The species of the group Perennichordata, which includes all those
Tunicates that possess a notochord persistent through life, are chiefly
pelagic in habit, the little creatures, rarely more than two or three
millimetres in length, swimming or drifting about with the sagittas,
copepods, ctenophores, and medusæ that compose the pelagic plankton. Fol
has recently described a gigantic form belonging to this group, reaching
a size of thirty millimetres in length, called _Megalocercus abyssorum_,
which he dredged from a depth of 492 fathoms; and other species have
been recorded down to a depth of 710 fathoms in the Mediterranean Sea.

Among the simple Ascidians we find no family that is peculiar to deep
water; but the Cynthiidæ and Ascidiidæ both contain genera that are
abysmal, and the Molgulidæ have one species, _Molgula pyriformis_, that
extends into the abysmal zone to a depth of 600 fathoms.

In the genus _Culeolus_ and in _Fungulus cinereus_ and _Bathyoncus_, all
deep-water Ascidians, there is a very curious modification of the
branchial sac, the stigmata being apparently not formed, in consequence
of the suppression of the fine inter-stigmatic vessels. This peculiar
feature is only found in the deep-sea simple Ascidians and, as we shall
see presently, in one species of the deep sea compound Ascidians, but it
is not apparently an essential character of those living in the abysmal
zone, notwithstanding the fact that it is found in such widely separated
genera; for _Corynascidia_, _Abyssascidia_, and _Hypobythius_, living in
depths lying between 2,000 and 3,000 fathoms below the surface, have
branchial sacs of the ordinary type. Professor Herdman is of opinion
that this simple form of branchial sac is not a primitive form, but most
probably a modification of a more complicated type.

In _Culeolus Murrayi_ there is a remarkably abundant supply of
blood-vessels to the tunic, and these send special branches to a number
of small papilliform processes on its outer surface. This system of
highly vascular processes probably constitutes, as Professor Herdman
suggests, an additional or complementary respiratory apparatus. All
these modifications of the branchial system are of particular interest,
for we find so many instances of a similar kind among the inhabitants of
very deep water. I need only refer here to the modifications of this
system in the Isopod _Bathynomus_ already referred to (p. 129), and to
the reduction in the number of the gills of many of the deep-sea fishes.
Why there should be such modifications is a question upon which the
physical and natural history investigations of the conditions of life in
the great depths of the ocean at present throw no light.

In a previous chapter I have referred to the fact that many of the
bathybial animals are characterised by being stalked. Among the simple
Ascidia we find many examples of stalked kinds living in deep water,
such as _Culeolus_ and _Fungulus_, but also several exceptions, such as
_Bathyoncus_, _Styela bythii_, and _Abyssascidia_, that are sessile. It
is a noteworthy fact, however, that the genus that has the most deep-sea
species—namely, _Culeolus_—is a genus that is provided with a very long
stalk. Furthermore, the only known stalked forms of the very large
family Ascidiidæ are the abysmal genera _Corynascidia_ and
_Hypobythius_.

[Illustration: FIG. 20.—_Hypobythius calycodes._ G, nerve ganglion; H,
heart; M, the position of the atriopore. The large opening on the upper
side is the mouth. (From a drawing by Professor Moseley in Herdman’s
‘Tunicata of the “Challenger” Expedition.’)]

The most remarkable character of the genus _Hypobythius_ is the simple
condition of its branchial sac, reminding one of the structure of this
organ in the shallow-water genus _Clavelina_. ‘There are no folds and
there are no internal bars,’ to quote the description given by Professor
Herdman; ‘only a single system of vessels can be recognised, branching
and anastomosing so as to form a close network, the small rounded meshes
of which are the stigmata. The tentacles and dorsal lamina cannot be
made out.’

Among the compound Ascidians only four families extend into the abysmal
zone, namely, the Botryllidæ, Polyclinidæ, Didemnidæ, and Cœlocormidæ,
and of these only one species, _Pharyngodictyon mirabile_, of the family
Polyclinidæ, extends into water of greater depth than 1,000 fathoms. In
_Pharyngodictyon_ we find the same curious simplification of the
branchial sac that we have just referred to in the genera of simple
Ascidians, _Culeolus_, _Fungulus_, and _Bathyoncus_. _Cœlocormus
Huxleyi_ from a depth of 600 fathoms is a very peculiar form and the
type of a separate family, the Cœlocormidæ.

The free-swimming Tunicata included in the group _Ascidiæ salpiformes_,
which contains the genus _Pyrosoma_, and the order Thaliacea containing
the salps, are in all probability mainly confined to the surface waters.
A few specimens of _Pyrosoma_ were captured by the ‘Challenger’ dredges
which came up from very deep water, but it is doubtful at what point in
the journey to the surface the specimens entered the net.

The most remarkable form of free-swimming Tunicate that has come to
light is _Octacnemus bythius_, a form that is probably allied to
_Salpa_. It was found twice, once in the dredge that came from a depth
of 1,070 fathoms, and once from 2,160 fathoms. The tunic of the animal
is gelatinous and hyaline, but the most important feature it possesses
is an imperforate membrane separating the branchial sac from the
peribranchial cavity. Octacnemus, in other words, possesses no true
stigmata, these structures being represented only by little pits in the
walls of the branchial sac. This curious and extremely interesting
modification of the respiratory organs points very strongly to the
conclusion that Octacnemus is truly a deep-sea animal.




                              CHAPTER VIII
                        THE FISH OF THE DEEP SEA


Of all the groups of animals that constitute the deep-sea fauna, the
fish show the greatest number of peculiarly abysmal characters. Being
much more highly differentiated than the invertebrates, they possess
more organs liable to undergo modifications of colour, size, and
structure, and consequently we are able to point to a great many more
features characteristic of deep-sea fish than we can do in any other
group of animals.

The first point that calls for remark in the consideration of the fish
fauna of the deep sea is the almost entire absence of ancient and
primitive types. The Elasmobranchii, including the Sharks, Rays, and
Chimæra, constituting the order that from anatomical embryological
grounds is always regarded by naturalists as the most primitive order of
this class, is represented in very deep water by only one species. _Raia
hyperborea_ and _Chimæra monstrosa_, it is true, just enter into the
abysmal zone, but _Chimæra affinis_ is the only Elasmobranch that
extends to depths of over 1,000 fathoms.

The Ganoidei too, the order that in palæozoic and mesozoic times was so
rich in genera and species, is entirely absent from the abysmal zone,
not a single representative having been found at any time by any of the
deep-sea expeditions.

The Dipnoi, that remarkable order including the three fresh-water
genera, _Ceratodus_ from Australia, _Lepidosiren_ from Brazil, and
_Protopterus_ from West Africa, has no representative and no ally in the
deep waters of the ocean.

The fishes of the deep sea, in fact, with only one or two exceptions,
all belong to the Order Teleostei, the most modern and most highly
differentiated order of the class, the families that are most fully
represented being the Macruridæ and then the Ophidiidæ and Gadidæ, and
the Berycidæ.

At the limits of the katantic and abysmal zones, a large number of
families of Teleosteans entirely disappear, and as we approach the
deepest parts of the ocean, the number of fish that are found is
considerably reduced. As Dr. Günther very wisely remarks, ‘this
diminution in the number may be due to the difficulty of capturing
fishes at great depths, a difficulty which increases in proportion to
the depths at which the dredge is worked. But it must also be regarded
as evidence of the actually diminished variety of fishes.’

It may be interesting to the reader to give Dr. Günther’s table of the
number of species found at different depths, as it shows, among other
things, the marked change that occurs in the character of the fauna in
passing from the katantic to the abysmal zone.

                Between   100–300   fathoms, 232 species
                Between   300–500   fathoms, 142 species
                Between   500–700   fathoms,  76 species
                Between  700–1,500  fathoms,  56 species
                Between 1,500–2,000 fathoms,  24 species
                Between 2,000–2,900 fathoms,  23 species

As regards the general character presented by the deep-sea fishes, I
have already pointed out in the chapter dealing with the general
characters of the deep-sea fauna, the peculiarities in the size of the
eyes, the colours and markings of the body, and the texture of the bones
and muscles. There are, however, a few more characters of which mention
must be made.

Notwithstanding the fact that all the abysmal fishes are carnivorous and
must consequently be capable—in the great number of cases—of rapid and
vigorous movement, the muscles of the trunk and tail are usually thin,
and the fascicles loosely connected with one another.

Deep-sea fish are not characterised by an absence of the swimming
bladder. This organ occurs just as frequently and in the same families
as in the shallow-water fauna, but we do not know whether it possesses
any special peculiarities or not, as it is usually so ruptured and
destroyed by the change of pressure it undergoes in being brought to the
surface, that it is impossible to make any thoroughly accurate
investigation of its anatomy and relations.

The extraordinary development of glands in the skin which secrete mucus,
and the presence in many forms of very highly specialised organs for
emitting phosphorescent light, are characters of the deep-sea fish
fauna, to which I have referred in a previous chapter.

As with the Tunicates, some of the Crustacea and other groups, the fish
of the abysmal zone show curious modifications of the respiratory
system. The gill laminæ of these animals are not only reduced in number,
but appear to be short and shrunken. It is possible, of course, that
during life they may end in fine delicate points which are broken off or
ruptured during their capture, but still the horny rods that support
them are shorter than they are in shallow-water forms, and the general
evidence of their structure tends to show that they have undergone
profound modifications in the change to the conditions of deep-sea life.

An extremely common and almost general character of deep-sea fishes is
the black coloration of some of the body cavities; this is limited to
the pharynx in many of the fishes that live about the hundred fathoms
limit, but the colour is more intense and spread all over the oral,
branchial, and peritoneal cavities in typical deep-sea forms. It may
seem very difficult at first to account for this remarkable development
of black pigment in parts of the body that are not usually, and, in some
cases, cannot at any time be exposed to view. It is obvious that it
cannot be functional as a hiding colour, either in offence or defence.
But it is quite possible that it is due to some modification of the
function of excretion. It is well known that in many cases of disease or
injury to the kidneys in vertebrates, the colour of the skin is
affected, and every one recognises now the fact that in many
invertebrates the colour of the skin is greatly dependent upon the
function of the secretion of the urates.

It would at least be interesting to know if this dark coloration of the
mucous membranes is in any way correlated with any modification of the
structure or function of the kidneys. At present we have no recorded
observations on this point, but it is to be hoped that, when we have a
sufficient number of specimens brought home from the deep water, a
systematic investigation of this subject will be made.

Lastly, it should be pointed out that our knowledge of the abysmal fauna
has not, at present, brought to light any evidence that the fish are of
an extraordinarily large size. In many groups of animals, as I have
frequently pointed out in the last few chapters, the large and gigantic
species or specimens are only found in the abyss. This may also be the
case with fishes, but we have no evidence that it is so. The only
methods that have been used at present for the investigation of the
fauna living on or near the floor of the deep oceans, are not of a kind
to lead to the capture of really large fish. That they may exist is
highly probable, but all that we know at present is, that the fish with
which we are acquainted living at great depths are not in any way
remarkable for their great size.

Of the only two Elasmobranchs, one, namely _Raia hyperborea_, has been
found in water extending from 400 to 608 fathoms in depth. Only four
specimens have yet been taken, one by the Norwegian expedition off
Spitzbergen and three by the ‘Knight Errant’ off the northern coasts of
Scotland. It is interesting to find that this, the only deep-sea species
of the Rays, shows some striking peculiarities. ‘The teeth are
remarkably slender,’ says Günther, ‘small, irregularly and widely set,
different from those of other British Rays. In young specimens at any
rate those of the male do not differ from those of the female. The
mucous membrane behind the upper jaw forms a pad with a lobulated
surface. The mucous cavities of the head are extremely wide, and finally
the accessory copulatory organs have a spongy appearance, and are
flexible, the cartilage by which they are supported being a simple
slender rod.’

The other Elasmobranch, that extends into very deep water, is _Chimæra
affinis_, a species which can hardly be distinguished from the better
known _Chimæra monstrosa_, a fish that itself very frequently wanders
within the limits of the abysmal zone.

Among the Teleostei, the family Berycidæ has several representatives in
the deep water. They are small fish rarely exceeding four inches in
length, with large heavy heads, with functional but small eyes, and an
abundant supply of large mucous glands on the skin.

_Melamphaes beanii_, belonging to this family, has been captured at the
enormous depth of 2,949 fathoms.

_Bathydraco antarcticus_, belonging to the family Trachinidæ, from a
depth of 1,260 fathoms, is an example of a true abysmal fish possessing
very large eyes.

The Pediculati, the family of the anglers, is represented at depths of
over 2,000 fathoms by the interesting form _Melanocetus Murrayi_. The
eyes are very small indeed, the mouth huge and armed with long uneven
rasp-like teeth. At the end of the fishing-rod tentacle hanging over the
mouth, there is an organ that has been supposed to be capable of
emitting a phosphorescent light. This curious modification of the red
worm-like bait of the common shallow-water angler into a will-o-the-wisp
lantern attracting little fishes to their destruction in the deadly jaws
of the _Melanocetus_ is one of the most interesting adaptations that
have been brought to light by our study of the deep-sea fauna.

Several species of the family Lycodidæ occur in the abysmal zone, but
they do not possess any features that call for special mention in this
place.

[Illustration: FIG. 21.—_Melanocetus Murrayi_, 1,850–2,450 fathoms.
(After Günther.)]

The family Ophidiidæ contributes very largely to the fish fauna of the
abyss. Some of the deep-water genera, such as _Neobythites_, have a wide
bathymetrical distribution extending from 100 fathoms to depths of over
2,000 fathoms, but others, such as _Bathyonus_, _Typhonus_, and
_Aphyonus_, only occur in depths of over 1,000 fathoms.

The body is usually elongate and slender, ending in a pointed tail, the
head large and heavy, and the eyes, in the genera confined to the
abysmal zone, usually so far degenerated that they are not visible at
all from the outside.

The Macruridæ form a family that contributes very largely to the
deep-sea fauna; no fewer than twenty-six different species are known to
occur within the limits of the abysmal zone. Not only do the Macruridæ
contribute a large number of different species, but they probably occur,
in some districts at any rate, in vast numbers.

During the voyage of the ‘Talisman,’ for example, the French naturalists
caught in one haul of the dredge off the coast of Morocco in 500 fathoms
of water no fewer than 134 fish, of which number 95 belonged to the
family Macruridæ.

They are usually small fish, measuring from a few inches to two feet in
length, with a body terminating in a long compressed tapering tail and
covered with spiny, keeled, or striated scales.

The Pleuronectidæ or flat fish are not, as a rule, found in the abysmal
zone; one species, however, _Pleuronectes cynoglossus_, was found by the
American ship ‘Blake’ to extend into 732 fathoms of water.

The families Sternoptychidæ and Scopelidæ are of particular interest to
us, as almost all the genera they contain belong either to the pelagic
or abysmal zones, and lend support to the view enunciated by Moseley,
that the deep-sea fauna has, partly at any rate, been derived from the
fauna of the pelagic zone. They are nearly all small slender fish with
delicate and frequently semi-transparent bodies, large gaping mouths
armed with numerous long irregular teeth, and frequently provided upon
the head and sides of the trunk with rows of eye-like phosphorescent
organs.

These families, and others that have still to be referred to, belong to
the group of Teleostei that is called Physostomi, the name referring to
the open communication that usually exists in all these families between
the swimming bladder and the alimentary canal. It is a remarkable fact
that in none of the deep-sea representatives has this open communication
been discovered. It is true that many specimens are, when examined, so
lacerated by the diminution in pressure as to render anatomical study a
matter of difficulty, but still a fair number of uninjured
well-preserved specimens have now been examined and the duct has not
been found.

Of the family Sternoptychidæ, _Gonostoma microdon_ has a most remarkable
distribution. It has been found at numerous stations in both the Pacific
and Atlantic Oceans at depths ranging from 500 to nearly 3,000 fathoms
of water.

The Scopelidæ are represented by some very extraordinary types. The
genus _Bathypterois_, for example, occurring in depths ranging from 500
to 2,500 fathoms, is characterised by the development of enormously long
pectoral fins to serve probably as organs of touch. ‘The rays of the
pectoral fin,’ says Dr. Günther, ‘are much elongated. The ventral fins
abdominal, with the outer rays prolonged, eight-rayed.... Gill rakers
long.’ They are further characterised by the absence of any true
phosphorescent organs and the smallness of their eyes.

There can be little doubt, I think, that in these fishes the sense of
touch or taste to a great extent takes the place of the sense of sight
in other Scopelids. Not being provided with well-developed eyes or
phosphorescent organs to attract their prey, the pectoral fins and the
outer rays of the pelvic fins have become elongated and provided with
special sense organs for searching for their food in the fine mud of the
floor of the ocean.

These long pectoral rays must have a very curious appearance in the
living fish. Mr. Murray observes: ‘When taken from the trawl they were
always dead, and the long pectoral rays were erected like an arch over
the head, requiring considerable pressure to make them lie along the
side of the body; when erected they resembled the Pennatulids like
Umbellula.’ Filhol considers that when the fish is progressing through
the obscurity of the abyss it probably carries these organs directed
forward, seeking with them in the mud for any worms or other animals
upon which it preys, or receiving through them warning of the approach
of an enemy from whom it is necessary to make an immediate escape. One
of the most remarkable of the deep-sea fish is closely related to
Bathypterois, namely _Ipnops Murrayi_, living in depths of over 1,000
fathoms. It is about five inches long, of a yellowish brown colour, with
an elongated subcylindrical body covered with large thin deciduous
scales. There are no phosphorescent organs of the ordinary type met with
in the Scopelidæ, but upon the upper surface of the head there is found
a pair of organs somewhat resembling the ordinary eyes of fishes but
devoid of retina and optic nerve, that, from the researches of Moseley,
seem to be undoubtedly organs for emitting light. ‘The organs are paired
expanses, completely symmetrical in outline, placed on either side of
the median line of the upper flattened surface of the head of the fish,
extending from a line a little posterior to the nasal capsules nearly to
a point above the posterior extremity of the cranial cavity.’ They are
covered by the upper walls of the skull, which is extremely thin and
completely transparent in the region lying over them. ‘They are
membranous structures 0·4 mm. in thickness marked by hexagonal areas
about 0·04 mm. in diameter. When their surface is viewed by reflected
light the appearance is that of a number of glistening white isolated
short columns standing up in relief from its basal membrane.’ Each
hexagonal column is composed of a number of transparent rods disposed
side by side at right angles to the outer surface of the organ, with
their bases applied against the concave surface of a large hexagonal
pigment cell, one of which forms the basis of each hexagonal column. It
is still very doubtful what are the true homologies of this
extraordinary phosphorescent organ, but Moseley was of opinion that, ‘on
the whole, it seems not unlikely that the remarkable head organs of
Ipnops may be regarded as highly specialised and enormously enlarged
representatives of the phosphorescent organs on the heads of such allied
Scopelidæ as _Scopelus rafinesquii_ and _Scopelus metopoclampus_. It may
be conceived that in _Ipnops_ the supra-nasal and sub-ocular
phosphorescent organs of these species on either side have united and
become one with the result of the total obliteration of the eye.’

Most of the species of the genus _Scopelus_ are undoubtedly pelagic in
habit, descending during the day to depths of semi-darkness but rising
at night to the surface waters. It is not certain how many of the known
species occasionally or habitually dwell in very deep water, but there
seems to be no doubt that two species at least—_S. macrolepidotus_ and
_S. glacialis_—belong to the abysmal zone. Both of these species were
found in dredges that had been at work in depths of over 1,000 fathoms
and showed signs when examined of having been brought from the abyss.

The Stomiatidæ are almost entirely confined to water from 450 to nearly
2,000 fathoms in depth. They may be distinguished from the Scopelidæ by
the long hyoid barbel close to the symphysis of the lower jaw, but like
many of the genera of that family they have wide gaping mouths armed
with a profusion of vicious looking teeth and a series of luminous spots
on the sides of the head and body. (_See_ Frontispiece.)

In _Eustomias obscurus_, found in depths of over 1,000 fathoms in the
Atlantic by the ‘Talisman,’ the barbel is provided with a terminal
swelling, shaped like a dumb-bell, which may be capable of emitting a
phosphorescent light and serve the animal as a lure for the attraction
of its prey. The genus _Malacosteus_, too, presents us with some of the
most remarkable forms that are found in the abysmal zone. The mouth is
of enormous size and the integuments of the abdomen present very
definite longitudinal folds, leaving no doubt that this fish is able,
like several others living in deep water, to swallow prey of an enormous
size.

But a perfectly unique structure in this fish ‘is a thin cylindrical
muscular band which connects the back part of the mandibular symphysis
with the hyoid bone. It is probably the homologue of a muscular band
which, in other Stomiatids, stretches on each side from the mandible to
the side of the hyoid, the two bands coalescing into an unpaired one in
_Malacosteus_. It is, in the present state of preservation, much
elongated, like a barbel, but during life it is notably contractile, and
serves to give to the extremity of the mandible the requisite power of
resistance when the fish has seized its prey, as without such a
contrivance so long and slender a bone would yield to the force of its
struggling victim.’

[Illustration: FIG. 22.—_Saccopharynx ampullaceus_; a deep-sea eel, with
the head of a large fish, which it has swallowed, showing through the
thin integuments of the body. (From Günther.)]

Belonging to the family of the Salmons we find one genus _Bathylagus_
that is undoubtedly an abysmal form. Although there may be some doubt as
to the exact depth at which the specimens were captured, the thinness of
the bones, the enormous size of the eyes, and other bathybial characters
prove that they must live in very deep water. Closely allied to the
salmon and the herrings is the family of the Alepocephalidæ, a family
that contributes several forms to the fauna of the deep sea, but they do
not possess any characters that call for special comment. Their vertical
distribution varies between 345 and 2,150 fathoms.

The family Halosauridæ contains five species all confined to the abysmal
zone. They have long bodies tapering to a finely pointed tail, and the
head is provided with a snout that projects considerably in front of the
mouth.

Of the family of the Eels there are several representatives in the deep
sea. They are characterised by a combination of true eel characters with
special modifications due to a bathybial existence. ‘To enable them to
seize upon prey more powerful than themselves certain organs have
undergone a degree of specialisation, as is observed in bathybial
members of other families with a similar mode of life; the jaws are
exceedingly elongate and the whole gape, the pharynx and stomach capable
of enormous distension.’ The head is very large, the eye very small and
the tail long and tapering (fig. 22).

The lessons we learn from the study of the fishes of the deep sea are
particularly instructive. It would take far more space than can be
afforded here to fully illustrate all of the points that seem clear to
us, but I hope I have said sufficient to show that the fish fauna is
made up of genera and species belonging to several widely separate
families of the Teleostei; that some of them show, in a very marked way,
what may be looked upon as peculiarly bathybial characters, whilst
others are but slightly modified from their shallow-water
representatives. These facts by themselves lend support to the view that
the fauna of the deep sea has been derived from the fauna of shallower
water by successive migrations at different periods of the world’s
history. Those that exhibit in a most marked degree the special
bathybial characters are probably those whose immigration took place
long ago, whilst those more closely related to shallow-water forms are,
comparatively speaking, recent importations. The occurrence of Scopelidæ
and Sternoptychidæ in deep water suggests, as Moseley pointed out many
years ago, that the fauna is partly derived from the pelagic plankton.
But while these points may seem clear to us, there are others that still
require much more investigation and consideration. The whole question of
the function and use of the phosphorescent organs, the mucous glands,
the barbels and elongated fin rays, the mode of life, the deposition of
ova and their development, afford problems which in the present state of
our knowledge must remain unsolved. Let us hope that in the future there
may be a new stimulus given to deep-sea research, and these problems may
be again seriously studied and eventually solved.




                                 INDEX


 Actiniaria, 36, 93

 —— two remarkable genera of, 15

 _Aegir_, 15

 Agassiz, A.,

 —— on colour of Cœlentera, 65

 —— on Echinoidea, 101, 103

 Agassiz, L., on board the ‘Hassler,’ 12

 ‘Albatross,’ American vessel, 15

 Alcyonaria, 95

 —— phosphorescence of, 81

 Amphipoda, 124

 Anemones

 —— of deep water, 36, 92

 —— two remarkable genera of, 15

 Annelida, 117

 Ascidia compositæ, 146

 —— salpiformes, 147

 —— simplices, 142

 Asteroidea, 104


 Barriers of temperature, 32

 _Bathynomus_, 129

 Beddard, F. E., on Isopoda, 127

 Benthos, 53

 Berycidæ, 154

 ‘Blake,’ American vessel, 12

 Blue mud, 42

 Brachiopoda, 115

 Brachyura, 138

 _Brisinga_, 9, 105

 Buchanan’s experiment, 19


 Carpenter, P. H., on Crinoidea, 100

 _Cephalodiscus_, 113

 Cephalopoda, 120

 ‘Challenger,’ H.M.S., voyage of, 12

 Cirripedia, 130

 Cœlentera,

 —— colour of, 65

 —— of deep sea, 91

 Colour

 —— of the deep-sea fauna, 59, 66

 —— of the deep-sea fish, 60

 Copepoda, 124

 Corals, 94

 Crinoidea, 99

 Crustacea, 123

 —— colour of, 63


 Darkness of the abyss, 22

 Diatom ooze, 39

 Dipnoi, 149


 Echinoderma, 99

 —— colour of, 64

 Echinoidea, 101

 Eels, 165

 Elasipoda, 106

 Elasmobranchii, 148, 153

 Eryonidæ, 135

 Eyes of abysmal animals, 67

 —— of deep-sea crustacea, 72
 —— of deep-sea fish, 69

 —— of deep-sea mollusca, 71

 —— of _Genityllis_, 118

 —— of _Neobythites_, 69

 —— of _Serolis_, 73


 _Fenja_, 15

 ‘Fish Hawk,’ American vessel, 12

 Fol and Sarasin’s experiments, 25

 Foraminifera, 90

 Forbes, on the probable existence of a deep-sea fauna, 2

 —— on zones of distribution, 49


 Ganoidei, 149

 Gasteropoda, 119

 Gephyrea, 116

 Gills of deep-sea fish, 151

 Globerigina ooze, discovery of, 5

 —— distribution and composition of, 37

 Green mud, 42

 Gunn, Dr., on the eyes of _Genityllis_, 118

 Günther, Dr., on deep-sea fish, 150


 Hall, Marshall, 12

 Halosauridæ, 164

 ‘Hassler,’ American ship, 12

 Herdman, on Ascidians, 143, 146

 Hermit crabs, 139

 Hoek, Dr., on Cirripedia, 132

 —— on Pycnogonida, 140

 Holothuridea, 106

 Hoyle, on Cephalopoda, 120

 Hydroids, 92


 ‘Investigator,’ H.M.S., 16

 _Ipnops Murrayi_, colour of, 60, 61

 _Ipnops Murrayi_, phosphorescent organs of, 160

 Isopoda, 127


 Katantic sub-zone, 50

 ‘Knight Errant,’ H.M.S., 12


 Lamellibranchia, 119

 ‘Lightning,’ H.M.S., 8, 9

 Lime, scarcity of, in bones of bathybial fish, 83

 —— in shells of mollusca, 83

 Littoral sub-zone, 49

 Lycodidæ, 155


 Macrura, 135

 Macruridæ, 156

 Madreporaria, 94

 Medusæ, 91

 Mollusca, 119

 —— colour of, 62

 Moore, Capt., 94

 Moseley, H. N., on colour of Cœlentera, 65

 —— on phosphorescent organs of _Ipnops_, 160

 —— on the darkness of the abyss, 22

 —— on the phosphorescence of Alcyonarians, 25

 Murray, on _Bathypterois_, 159


 Nekton, 53

 Neritic zone, 48

 ‘Norma,’ Mr. Hall’s yacht, 12

 Norske Nord-havns expedition, 7, 15


 Ophidiidæ, 156

 Ostracoda, 123


 Packard, on the illumination of the abyss, 23

 Pediculati, 155

 Pelagic zone, 47

 Pennatulidæ, 96

 _Phoronis_, 111

 Phosphorescence of Alcyonarians, 81
 — of deep-sea Crustacea, 80
 — of Echinoderma, 81

 Phosphorescent light in the abyss, 24
 — organs of deep-sea fish, 77

 Pigment in mucous membranes of deep-sea fish, 84, 152

 Plankton, 52

 Pleuronectidæ, 157

 Polar currents, 30, 33

 Polychæta, 118

 ‘Porcupine,’ H.M.S., 8, 9

 Porifera, 91

 Pourtales, Count, 10

 Pressure in the abyss, 19

 Protozoa, 88

 Pteropod ooze, 39

 Pycnogonida, 139


 Radiolaria, 89

 Radiolarian ooze, 39

 Red mud, 37
 — — off the Brazilian coasts, 42

 _Rhabdopleura_, 111

 Ross, Sir James, on the fauna of the deep sea, 3


 Salmonidæ, 163

 Sargasso sub-zone, 48

 Sars, 6, 9,
 — on Amphipoda, 125
 — on _Brisinga_, 105

 Schizopoda, 133

 Scopelidæ, 158

 _Serolis_, 127

 Siphonophora, 92

 Size of deep-sea animals, 85
 — of fish, 153

 Smith, Mr., on Lamellibranchia, 119

 Spatangoids, 101

 Sponges, 91

 Stebbing, Rev. T. R., on Amphipoda, 125

 Sternoptychidæ, 158

 Stomatopoda, 133

 Stomiatidæ, 162


 ‘Talisman,’ French vessel, 12

 Teleostei, 149, 154

 Temperature of the abyss, 28

 Thomson, Sir Wyville, on _Pourtalesia_, 10
 — on _Thaumastocheles_, 137
 — on the darkness of the abyss, 22
 — on the phosphorescence of the sea, 26

 Thoracostraca, 133

 ‘Travailleur,’ French vessel, 12

 ‘Triton,’ H.M.S., 12

 Tunicata, 140


 Vegetable life, absence of, 42

 Verrill, on the illumination of the abyss, 23

 ‘Vittor Pessani,’ Italian vessel, 12

 ‘Vöringin,’ Norwegian vessel, 14

------------------------------------------------------------------------




                         MODERN SCIENCE SERIES.


               _Edited by Sir JOHN LUBBOCK, Bart., M. P._


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II. =The Horse:=

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preliminary topics, as the history of the Greek and the Latin alphabets,
writing materials, the forms of books, punctuation, measurement of
lines, shorthand, abbreviations, and contractions; five are devoted to
Greek palæography, seven to Latin.”—_The Critic._

“Covering as this volume does such a vast period of time, from the
beginning of the alphabet and the ways of writing down to the
seventeenth century, the wonder is how, within three hundred and
thirty-three pages, so much that is of practical usefulness has been
brought together.”—_New York Times._

  _MAN AND THE GLACIAL PERIOD._ By G. FREDERICK WRIGHT, D.D., LL.D.,
        author of “The Ice Age in North America,” “Logic of Christian
        Evidences,” etc. With numerous Illustrations. 12mo. Cloth,
        $1.75.

“The author is himself an independent student and thinker whose
competence and authority are undisputed.”—_New York Sun._

“It may be described in a word as the best summary of scientific
conclusions concerning the question of man’s antiquity as affected by
his known relations to geological time.”—_Philadelphia Press._

“The earlier chapters describing glacial action, and the traces of it in
North America—especially the defining of its limits, such as the
terminal moraine of the great movement itself—are of great interest and
value. The maps and diagrams are of much assistance in enabling the
reader to grasp the vast extent of the movement.”—_London Spectator._


          New York: D. APPLETON & CO., 1, 3, & 5 Bond Street.

------------------------------------------------------------------------




                          TRANSCRIBER’S NOTES


 1. P. 70, changed “japonicus” to “M. japonicus”.
 2. P. 150, changed “fibres” to “fishes”.

    .Moved advertising between half-title & Frontispiece to between the
    index and the other advertising.

 3. Silently corrected typographical errors and variations in spelling.
 4. Archaic, non-standard, and uncertain spellings retained as printed.
 5. Enclosed italics font in _underscores_.
 6. Enclosed bold font in =equals=.