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).

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.

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.

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.