But the number of animal kinds which belong to other sub-kingdoms vastly exceeds the total number of vertebrate animals, and the structural contrasts found between different non-vertebrate species is very much greater than any such contrasts as can be found to exist between any two members of the highest, or vertebrate sub-kingdom. This is only what we might expect; for non-vertebrate animals—often spoken of collectively as "Invertebrata"—form several distinct sub-kingdoms, each of which has a rank approximatively co-ordinate with that sub-kingdom to which we ourselves belong. Nevertheless, since the members of the invertebrata sub-kingdoms are, speaking generally, much less known and familiar than are vertebrate animals, and as the structural differences between them cannot be pointed out till an initial acquaintance has been made with comparative anatomy, for these reasons we may treat the various animal sub-kingdoms which have yet to be noticed at much less length than we have treated the vertebrata. The details of their peculiarities and the various degrees of significance and interest which they present will begin to appear when we proceed to treat of "The Forms of Animals."
The last class of vertebrates is, as we have seen, constituted by the fishes, which are fishes properly so called. But there are many animals which are familiarly and improperly spoken of as "Fishes," but which are even more below true fishes than whales and porpoises are above them. Thus, we hear of cuttle-fishes, and a variety of creatures are spoken of as "shell-fish," which are not in the least related to true fishes. Indeed, the many so-called "shell-fish" are not even nearly related one to another. Thus, the oyster and the lobster are both commonly thus named, but they belong respectively to two altogether distinct sub-kingdoms of the world of animals.
The oyster is an animal which belongs to a vast assemblage of species, with much variety of form and structure, which, on account of their soft bodies (whether or not enclosed in shells), are called MOLLUSCA or "Mollusks." This assemblage ranks as a sub-kingdom and contains within it at least four subordinate great groups, or "classes." All snails and whelks, with their allies, and also all cuttle-fishes, belong to the sub-kingdom of "soft animals."
Amongst the most familiar of mollusks is the common snail, which may serve as a type of the "class" of mollusks to which it belongs—the class Gasteropoda. The snail, with the slug, are representatives of land-forms of mollusca, but the bulk of the class and of the whole sub-kingdom are aquatic animals, such as the whelk (Buccinum), periwinkle (Littorina), limpet (Patella), &c. The Gasteropods generally possess spirally coiled shells (like the cowry or whelk), but some kinds have their shells in the form of simple cones—like a Chinaman's cap—as, e.g., the limpet. There are a few Gasteropods in which the shell consists of a series of similar segments as is the case with Chiton, while many are altogether naked. In some kinds the soft body is drawn out into a number of tufted processes, as in Doris and Eolis, and sometimes the body is almost worm-like, as in Phylliroë, or provided with a pair of ring-like lateral processes and a rudimentary shell, as in the sea-hare Aplysia.
Next above the Gasteropods comes a group of animals forming the class Pteropoda. These pteropods are small, active, oceanic, surface-swimming creatures, many of which live in delicate glass-like shells, and some of which form a large part of the food of the whalebone whale. They flit through the water by the aid of lateral processes which much resemble those before-mentioned as existing in the sea-hare. Allied to these pteropods is a curious little animal, the shell of which resembles a miniature elephant's tooth and which is named Dentalium.
Highest of all the mollusca stand the cuttle-fishes, forming (with the Nautilus and many extinct animals, such as ammonites and their allies) the great class Cephalopoda. The Cephalopoda, such as the cuttle-fish (Sepia) and the Poulp (Octopus), have now become familiar objects through our aquaria, where their very eccentric forms and remarkable movements naturally attract attention. To this group also belongs Spirula, the coiled and chambered shell of which is found so abundantly, but its soft tenant so very rarely. To it also belongs the extinct Belemnite, which was provided with a dense, conical internal shell, specimens of which found in rocks were at one time taken for thunderbolts. Of a lower grade of organization is the Nautilus, sole existing representative of a great group of Cephalopoda (including the ammonites and other forms) which has, with the above exception, long become entirely extinct.
The oyster is an animal which belongs to a much lower class of mollusca—namely, to the class called Lamellibranchiata, from the plate-like (or lamellar) structure of the gill. To that class also belongs the scallop (Pecten), the mussel (Magilus), the fresh-water mussel (Anodon), the razor-shell (Solen), the cockle (Cardium), species with a long fleshy tube such as Mya, stone-perforating shells such as Pholas, and the well-known wood-boring "ship-worm" (Teredo)—which is no "worm" at all—with a multitude of other forms.
Certain other animals (which, like the Lamellibranchs, all have a shell divided into two valves) form another still lower class called Brachiopoda, a class which we may, at least provisionally, consider as belonging to the mollusca. These Brachiopods are also called "Lamp-shells," from a certain resemblance which many of them show to the form of a classical lamp. They are interesting, because in very ancient times they seem to have held that place in the world's animal population which is now held by the Lamellibranchs, by which, as they died out, they have been gradually replaced till but comparatively few forms survive. Some of these, however, are of great antiquity, and one of them, Lingula, is, though still living, one of the most ancient of all known animals.
We may next pass to a small sub-kingdom which includes the curious and inert animals before referred to[16] as "Sea-squirts," Tunicaries or Ascidians, and which constitute the sub-kingdom TUNICATA. These are marine organisms of very simple but very peculiar structure which sometimes grow up in compound aggregations. Certain forms (e.g., Pyrosoma) are luminous at night and may be seen swimming about in the ocean like so many red-hot urn-heaters. As we shall hereafter see, the reproductive processes and the earlier stages of existence of these creatures possess much interest, and have afforded strong grounds for regarding them, in spite of their lowly organization, as very close allies of the highest animals or Vertebrata.
Returning now to the "lobster" (lately mentioned as one of those animals commonly called "shell-fish") we may regard it as an example of what is by far the most numerous of all the sub-kingdoms of animals. This sub-kingdom is made up of animals with jointed feet or "Arthropods," and the ARTHROPODA are subdivided into four classes—1, Crustacea; 2, Myriapoda; 3, Arachnida; and 4, Insecta; and it is to the first of these four classes that the lobster belongs.