Mollusca which live exclusively on algae and other forms of plant life are almost entirely wanting in the great depths, where vegetation is probably unknown. The struggle for existence must be much less keen than in the thickly populated shallows, where vicissitudes of every kind occur. The absence of rapid motion of water must obliterate many of those mechanical effects which tend to produce modifying influences upon the animals affected. In the absence of circumstances tending to cause variation, in the unbroken monotony of their surroundings, species must, one would think, preserve a marked uniformity over an exceedingly wide area of range.
Vegetable food being wanting, those genera which in shallower waters never taste flesh, are compelled to become carnivorous. Characteristic of the great depths are very remarkable forms of Trochidae, in whose stomachs have been found the remains of Corallines and Foraminifera. According to Dr. Dall, the results of this diet show themselves in the greatly increased space occupied by the intestine, in the diminution, as regards size, of the masticatory organs, the teeth and jaws, and also in the prolongation of the anal end of the intestine into a free tube, which carries away the excreta in such a way that they do not foul the water taken into the gills. The amount of nutriment contained in the bodies of dead Foraminifera is so small that a comparatively large quantity must be swallowed to keep the vital energies active, and therefore the amount evacuated must be proportionately larger also. The abyssal Trochidae, then, and many other genera, sustain themselves by feeding on the ‘rain’ of dead animal matter which falls upon the ocean floor, not so much hunting their prey as opening their mouths and eating whatever happens to fall into them. Genera which are normally carnivorous would appear to do the same. The Pleurotomidae, for instance, are a family markedly characteristic of very deep water. Representatives of the genus which occur in shallower water are known to secure their prey while in the living state. But, according to Dr. Dall, a singularly small proportion of deep-sea Mollusca, as compared with those from the littoral region, show signs of having been drilled or attacked by other Mollusca. This could hardly be the case if the Pleurotomidae retained their predatory habits, since they are more numerous in the great depths than any six other families taken together. It has already been mentioned (p. [186]) that a large proportion of deep-sea Mollusca are perfectly blind.
Amongst other remarkable forms from the great depths may be mentioned Pleurotomaria, with its singular anal slit (Fig. [269], p. 407) extending in some cases half way round the last whorl. Three or four species of this genus, so characteristic of almost all fossiliferous strata down to the Cambrian, have been obtained in very limited numbers off the West Indies and Japan. Dentaliidae, especially the sub-genus Cadulus, find a congenial home in the slimy ocean mud. One of the greatest molluscan treasures procured by the Challenger was Guivillea alabastrina Wats., a magnificent Volute as white as alabaster, 6½ inches long, which was dredged from 1600 fath. in the South Atlantic, between Marion Island and the Crozets. Another very curious form, belonging to the same family, is Provocator pulcher Wats., a shell about half the size of Guivillea, of stouter proportions, and with an angulated and patulous mouth. This shell was dredged by the Challenger in comparatively shallow water (105–150 fath.) off Kerguelen Island. Among the Trochidae are the fine new genera Basilissa, Bembix, and Gaza. The exploring voyages of the American surveying steamer Blake, in the Gulf of Mexico and the Caribbean Sea, have given us the remarkable new forms Benthobia (possibly akin to Admete), Mesorhytis (a sub-genus of Fasciolaria hitherto only known from the Cretaceous of North America), and Benthodolium (possibly = Oocorys), a genus akin to Cassis.
In his report on the Pelecypoda obtained by the Challenger, Mr. E. A. Smith remarks that as a rule “very deep-water ‘benthal’ species certainly have a tendency to be without colour and of thin structure, facts no doubt resulting from the absence of light, the difficulty of secreting lime, the scarcity of food, and other unfavourable conditions of existence.” At the same time, he notices that most of the species obtained belong to genera which, even when occurring in shallow water, are thin and colourless, e.g. Neaera, Lima, Cryptodon, Abra, Verticordia, etc. Deep-water species of such genera as have a decided periostracum (Malletia, Limopsis, Leda, Nucula, Arca) retain it with little if any modification. The deep-water Pelecypoda of the Atlantic and Pacific Oceans present no special features of interest. The species are few in number, and the genera are not remarkable either for novelty or peculiarity of form.
The greatest depth at which Pelecypoda have been obtained is 2900 fath. mid North Pacific (Callocardia pacifica Sm., Abra profundorum Sm.); the greatest depth at which Gasteropoda have been obtained is 2650 fath. South Atlantic (Stylifer brychius Wats.), both by the Challenger. The deepest Challenger Nudibranch came from 2425 fath., and the deepest Chiton from 2300 fath. The greatest depth ever dredged is 4575 fath. off the east coast of Japan.
CHAPTER XIII
CLASS CEPHALOPODA
The Cephalopoda present a complete contrast to the majority of the Mollusca in habits and in many points of organisation. In their power of rapid movement and their means of progression, their extreme ferocity and carnivorous habits, their loss, in so many cases, of a shell, and in its constitution when present, in the general symmetry of their parts, in their reproductive and nervous system, they stand in a position of extreme isolation with nothing to connect them with the rest of the phylum.
Professor A. E. Verrill has collected many interesting details with regard to gigantic Cephalopoda occurring on the north-eastern coasts of America. From these it appears that the tentacular arms of some species of Architeuthis measure as much as 32, 33, 35, and 42 feet in length, while the total length, including the body, sometimes exceeds 50 feet. Even off the Irish coast a specimen was once captured whose tentacular arms were 30 feet long, the mandibles 4 inches across, and the eyes about 15 inches in diameter.[393] The strength of these giant Cephalopods, aided as they are by formidable rows of suckers and other means of securing a grip, is almost incredible. Cases are not uncommon, in which persons diving or bathing have been attacked, and have with difficulty made their escape.
Great damage is frequently inflicted by Cephalopoda upon shoals of fish on British coasts. Off Lybster (Caithness) Loligo and Ommastrephes devour the herring, large numbers of which are cut up and bitten on the back of the neck by these creatures. On the American coasts the mackerel fisheries are sometimes entirely spoiled by the immense schools of squid which infest the Bay of St. Lawrence.[394] When excited in the pursuit of fish Cephalopoda leap high out of the sea. Dr. W. H. Rush[395] relates that when about 300 miles off the coast of Brazil, a swarm of hundreds of decapods flew from the water and landed on the deck of the ship, which was 12 feet above the surface level, and they had to go over the hammock nettings to reach it.
