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.