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.

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.

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.

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.