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OBJECT AND NATURE OF ZOOLOGICAL CLASSIFICATION.
-CLASSES OF THE MOLLUSCOUS ANIMALS.
| NIMALS differ very much from one another A not only in their form, size, and habits, but also in their internal structure; and we intuitively group them according to their resemblances and differences, and give to each group a certain distinctive or characteristic name. From childhood our minds have been engaged, consciously or unconsciously, in the observation of natural objects, noting their shapes and qualities, and rudely comparing and classifying them. The differences of internal structure have led naturalists to divide the animal kingdom into five divisions, each division being distinguished by some striking peculiarity of
structure, the animals comprised in each being constructed upon a plan differing from that of any of the other divisions. These primary groups, which are called sub-kingdoms, are as follows :
1. Back-boned animals, termed Vertebrata, exemplified in beasts, birds, reptiles, and fishes.
2. Jointed animals, termed Annulosa; as insects, crabs, worms, &c.
3. Soft-bodied animals, termed Mollusca ; as the common garden snail, the oyster, and cuttlefish.
4. Hollow-intestined animals, termed Coelenterata ; as the sea-anemone, the coral polype.
5. Jelly animals, termed Protozoa; as infusory animalcules and the sponge.
The creatures living in our land and freshwater shells, which form the subject of the present volume, belong to the group of softbodied animals, to which the term Mollusca is applied. They have, as the name implies, soft and fleshy bodies, not divided into segments, without bones or jointed limbs, enveloped in a muscular coat called the mantle, and the shell with which they are commonly protected is composed of either one or two, rarely more, pieces: the bodies of some are naked. The cuttle-fish and slug are examples which will
give a general notion of the naked forms. The shelled species are familiar to us. The nervous system of the molluscous animals presents very marked peculiarities : in the back-boned and jointed animals, the principal mass of the nervous system, comprising the brain and spinal column, forms a continuous trunk throughout the length of the body, from which the nerves branch off at determinate points; whereas in the soft-bodied animals, the nervous system is composed of two or more pairs of brain-like masses scattered throughout the body, and united by cords of nerve-substance, which also send off nerves to the several parts of the body.
The sub-kingdom Mollusca admits of a very ready division into minor groups or classes. Thus we have cuttle-fishes, sea snails, land snails, bivalves, &c. In the language of the naturalist, the cuttle-fishes are the Cephalopoda, or head-footed mollusks; the sea snails are the Gasteropoda, or belly-footed mollusks; the land snails somewhat resemble the sea snails, but breathe air instead of water, and are hence termed Pulmonifera ; the bivalves, as the mussel and oyster, are the Conchifera. There are four other classes of the Mollusca, but which, together with the cuttle-fishes, do not come into the scope of the present subject, because they are all marine ; so that all our land and fresh