The Triton has been wriggling grotesquely in our grasp while we have made him our text, and, now he is restored to his vase, plunges to the bottom with great satisfaction at his escape. This water-snail, crawling slowly up the side of the vase, and cleaning it of the green growth of microscopic plants, which he devours, shall be our representative of the second great division—the Mollusca. I cannot suggest any obvious character so distinctive as a backbone, by which the word Mollusc may at once call up an idea of the type which prevails in the group. It won’t do to say “shell-fish,” because many molluscs have no shells, and many animals which have shells are not molluscs. The name was originally bestowed on account of the softness of the animals. But they are not softer than worms, and much less so than jelly-fish. You may know that snails and slugs, oysters and cuttlefish, are molluscs; but if you want some one character by which the type may be remembered, you must fix on the imperfect symmetry of the mollusc’s organs. I say imperfect symmetry, because it is an error, though a common one, to speak of the mollusc’s body not being bilateral—that is to say, of its not being composed of two symmetrical halves. A vertebrate animal may be divided lengthwise, and each half will closely resemble the other; the backbone forms, as it were, an axis, on either side of which the organs are disposed; but the mollusc is said to have no such axis, no such symmetry. I admit the absence of an axis, but I deny the total absence of symmetry. Many of its organs are as symmetrical as those of a vertebrate animal—i.e. the eyes, the feelers, the jaws—and the gills in Cuttlefish, Eolids, and Pteropods; while, on the other hand, several organs in the vertebrate animal are as unsymmetrical as any of those in the mollusc, i.e. the liver, spleen, pancreas, stomach, and intestines.[15] As regards bilateral structure, therefore, it is only a question of degree. The vertebrate animal is not entirely symmetrical, nor is the mollusc entirely unsymmetrical. But there is a characteristic disposition of the nervous system peculiar to molluscs: it neither forms an axis for the body—as it does in the Vertebrata and Articulata—nor a centre—as it does in the Radiata—but is altogether irregular and unsymmetrical. This will be intelligible from the following diagram of the nervous systems of a Mollusc and an insect, with which that of a Star-fish may be compared (Fig. 18). Here you perceive how the nervous centres, and the nerves which issue from them, are irregularly disposed in the molluscs, and symmetrically in the insect.
But the recognition of a mollusc will be easier when you have learned to distinguish it from one of the Articulata, forming the third great division,—the third animal Type. Of these, our vases present numerous representatives: prawns, beetles, water-spiders, insect-larvæ, entomostraca, and worms. There is a very obvious character by which these may be recognized: they have all bodies composed of numerous segments, and their limbs are jointed, and they have mostly an external skeleton from which their limbs are developed. Sometimes the segments of their bodies are numerous, as in the centipede, lobster, &c.; sometimes several segments are fused together, as in the crab; and sometimes, as in worms, they are indicated by slight markings or depressions of the skin, which give the appearance of little rings, and hence the worms have been named Annelida, or Annulata, or Annulosa. In these last-named cases the segmental nature of the type is detected in the fact that the worms grow, segment by segment; and also in the fact that in most of them each segment has its own nerves, heart, stomach, &c.—each segment is, in fact, a zöoid.[16]
Fig. 18.
Nervous System of Sea-Hare (A) and Centipede (B).
Just as we recognize a vertebrate by the presence of a backbone and internal skeleton, we recognize an articulate by its jointed body and external skeleton. In both, the nervous system forms the axis of the body. The Mollusc, on the contrary, has no skeleton, internal or external;[17] and its nervous system does not form an axis. As a rule, both vertebrates and articulates have limbs—although there are exceptions in serpents, fishes, and worms. The Molluscs have no limbs. Backboned,—jointed,—and non-jointed,—therefore, are the three leading characteristics of the three types.
Let us now glance at the fourth division—the Radiata,—so called because of the disposition of the organs round a centre, which is the mouth. Our fresh-water vases afford us only one representative of this type—the Hydra, or fresh-water Polype, whose capture was recorded in the last chapter. Is it not strange that while all the Radiata are aquatic, not a single terrestrial representative having been discovered, only one should be found in fresh water? Think of the richness of the seas, with their hosts of Polypes, Actiniæ, Jelly-fish, Star-fishes, Sea-urchins, Sea-pens (Pennatulæ), Lily-stars (Comatulæ), and Sea-cucumbers (Holothuriæ), and then compare the poverty of rivers, lakes, and ponds, reduced to their single representative, the Hydra. The radiate structure may best be exhibited by this diagram of the nervous system of the Star-fish.[18]
Fig. 19.
