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.