9. Vermes, Worms. With lengthened body without divisions; for instance, worms found in the intestines.
10. Radiariæ, (radiated animals,) with the different parts of which they are formed arranged like rays round one common centre; as, for instance, the Star-fish.
11. Polypi, (many feet.) The animal that forms the Coral is a Polypus.
12. Infusoria, (Infusory animals.) These are found in infusions of vegetable substances in water, and are, in general, too small to be visible to the naked eye.
In the present little book we shall treat of the first five of these Classes.
The Molluscous animals are, on account of their organization, placed first among the invertebral animals, a few of the species resembling, in some respects, the more simply-formed fishes.
The systematic arrangement of the Molluscous animals, considered not only as regards their shells, but having reference also to the anatomical distinctions of the creatures themselves, is a modern study. In ancient authors we merely find a few scattered facts, the beauty of the shells attracting their notice more than the value or nature of the animals.
Although, at the first glance, the inhabitants of shells appear to be beings of a very uninteresting nature, a due consideration of the valuable properties of many, and the usefulness of all, will enable us to perceive, that, regarding them merely in an interested point of view, they are worthy of the strictest attention of the naturalist. In the first place, the whole of them afford food for the different species of fish, and other inhabitants of the deep. The Tyrian dye, the royal purple of the ancients, was produced by the inhabitant of a small univalve shell, of the genus Purpura. That beautiful ornament in dress, the oriental pearl, is the consequence of disease in a species of mussel, and the inner portion of the shell of the same animal, is the well-known substance, mother-of-pearl. A kind of silk is obtained from the beard of the pinna, which, in some places, is made into gloves. As an article of food we may mention the well-known oyster, the mussel, scallop, &c., and some of the larger kinds form no small portion of the subsistence of the natives of the South-Sea Islands, and the Negro population in the West Indies.
The [Teredo navalis], or ship-worm, has, by its destructive powers, ruined the noblest vessels, and rendered useless the timbers, on which many of the constructions in harbours mainly depend for security; on this account great attention has been bestowed on its natural history and habits. The barnacle, which attaches itself to the bottoms of ships, renders the planks so foul, as to interfere materially with the rate of sailing of the vessel itself. These are only a few of the useful and noxious qualities of these inhabitants of the deep.
The shell with which a Molluscous animal is covered, is absolutely necessary to protect its delicate body from injury; this shell is, in general, composed of much the same substances as bone; but the bone of a bird, or quadruped, is formed by the agency of the blood, and the particles of which it is composed are deposited by that fluid, and again taken up and restored to the circulation, a circumstance which does not take place in the substance of a shell. The shell is formed by the deposition of layer upon layer, in the course of the growth of the animal, and the ridges we perceive on many shells, point out their periodical increase.