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water mollusks belong to the three classes, Conchifera, Gasteropoda, and Pulmonifera.

The ordinary bivalves (Conchifera)-as the oyster, mussel, and cockle-are all aquatic animals, and by far the greater number of them live in the sea. They breathe by means of two pairs of gills in the form of plates or laminæ, and hence are also called Lamellibranchiata; they are, moreover, destitute of any distinct head, and are inclosed in a shell, composed of two pieces or valves.

The snail-like animals have a distinct head, provided with eyes, horns or tentacles, and a mouth armed with cutting jaws; the under side of the body forms a single muscular foot, on which the animal creeps. The body is usually protected by a shell composed of one piece. Among these belly-footed animals some live in the water, and breathe by means of gills,-these are the true Gasteropoda; others live on the land, as the snail and slug, and have the respiratory organ in the form of a lung,-these are comprised in the class Pulmonifera. Some, indeed, inhabit our fresh waters; but, unlike the true fresh-water Gasteropods, they are compelled to come to the surface to breathe the air, and are not capable of suffering a long immersion in the

water.

To pursue the scheme of classification. Each

class contains orders, and the orders are subdivided into families; each family is subdivided into genera, and each genus contains one or more species. The animal kingdom is composed of individuals; but among these there are a certain number which have a close resemblance to each other, and are recognizable by a character which is constant and definite. Such groups of individuals constitute what naturalists call species. In familiar language, we speak of these in such general terms as the horse, the oyster, the garden snail, &c.,—meaning no horse in particular, no oyster, no garden snail in particular. Let us examine this matter more closely. Let us collect a number of snails from a garden; we see that they form a natural group, for they are all characterized by a yellowish-coloured shell, beautifully banded with brown. This assemblage of individuals with like characters we call "the garden snail." Let us generalize still further and higher, and extend our sphere of collection to the neighbouring wood. We now find that there are other undoubted snails, but which evidently form a group distinct from "the garden snail," for all their shells are of much smaller size, and are clothed with hairs; from which latter character we may appropriately call them "the hairy snail." Observe that we have naturally spoken of these two groups under the

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common term snail"-in Latin, helix. Now the name employed to designate this assemblage of species is the generic one. We distinguish "the garden snail" by a further name, which is the specific one, because it points out the species or particular snail among the general assemblage of snails. Thus the garden snail is known among naturalists as Helix hortensis; that is, Helix, a snail, and hortensis, of the garden. The second species is Helix hispida, the hairy or hispid snail.

A difficulty that meets us early in our examination of these animals, is the variation of form, colour, and size among them. Now in the case of the oyster, we are all familiar with the numerous variety of shapes that the shell of this mollusk presents, yet we do not fail to recognize them as belonging to the mollusk in question. In the case of less familiar species, the difficulty increases; and we must here try to form a series, with the normal or usual form as a centre, so as to include the extremes of variation. In most cases this is practicable. In a few instances varieties have been produced, which have become fixed or permanent; that is to say, the peculiarities which distinguish the variety have been perpetuated or handed down from generation to generation.

The principal causes of variation in shells are

abundance or scarcity of food, differences of habitat, &c. Thus, the shell of the large pond snail, Limnaea stagnalis, becomes more lengthened, tapering, and thinner, when the animal lives in running water, with only vegetable food as a diet, than the shells of more favoured individuals inhabiting stagnant ponds, which fare more sumptuously upon dead dogs and other animals.

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Classification is, then, an arrangement of all beings according to a certain order, by means of which objects are reunited into groups, recognizable by determinate characters, which, in their turn, are reunited into other groups still more comprehensive character. We have thus the ANIMAL KINGDOM subdivided into SUBKINGDOMS; these sub-kingdoms are further subdivided into CLASSES; and these classes, again, into ORDERS, FAMILIES, GENERA, and SPECIES.

"The practical utility of such a classification is easily seen by comparing it with the address of a letter. So it is with the naturalist, who by his classifications arrives speedily to the groups to which the animal belongs."*

If, for example, he wished to define a garden snail, without resorting to such means, he would be forced to compare his description with that of

* M. Milne-Edwards.

350,000 different animals. But if he says that the garden snail is a molluscous animal of the class Pulmonifera, of the family Helicidæ, of the genus Helix,-by the first he excludes all the vertebrated, annulated, coral-like, and jelly-like animals from his comparison; by the second, he excludes the bivalves, the water-breathing snails, and cuttle-fishes; by the third, he distinguishes the garden snail from the slugs, pond snails, and the like; and having arrived at the genus to which it belongs, a few distinguishing characters in addition will enable him to determine the species. Further, if the classification be based upon natural and not upon artificial characters, then it expresses the relationship of the species; for from the foregoing scheme of classification, it will be observed that differences in structure in the animals become less and less as we ascend in

the scale of subdivision. Thus, for example, animals belonging to the same family, but of different genera, differ less from each other than animals belonging to different families.

The subject will be treated according to the following plan:

The characters of each class will be studied by a full examination of the internal structures, as well as of the external or shelly parts, of a typical species of each. Technical terms, which we shall be compelled to use in describing each species,

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