Class IV.—Moss Animals, or Polyzoa, are the animals which form a coral-like shell. They are abundant on the seashore, and are called sea mosses.
Class V.—Lamp Shells (Brachiopoda). These worms are marine, and form a bivalve shell, the valves being on either side of the body. The body has long arms on one side of the mouth, which bear fringes; the motion of the fringes draws food into the mouth. They are also used in respiration. But few species of the Brachiopods are now living, though they were once very plenty.
Class VI.—Annelidæ. This last class includes the leeches, a flat worm, whose body is divided into segments; the earth or angleworm, a familiar worm of many segments, and the marine worms. Each segment of the latter bears clusters of bristles, used in swimming.
SUB-KINGDOM VI.
Mollusca (soft). General characteristics—Mollusks, a numerous branch of the animal kingdom, are so called from the softness of their bodies, which usually have no internal skeleton or framework to support them. They are covered with a tough, muscular skin, and generally protected by a shell. They have a gangliated nervous system, in some cases well developed, the medullary[6] mass not enclosed in a cranium or spinal column—of which they are mostly destitute—but distributed more or less irregularly through the body. They have hearts, and an imperfect circulative system, the blood being pale or blueish. Some breathe in air, some in fresh, more in sea water. Some—both of those on land and those in water—are naked, others have a calcareous covering or shell. The larger marine mollusks are often guarded by very strong, heavy shells. Some are viviparous, others oviparous, and multiply rapidly.
SNAIL.
Three general, and many subdivisions are recognized. The total number of living species is said to exceed twenty thousand, of which only a few can be mentioned. The classes under this division are Lamellibranchiata, Gasteropoda, and Cephalopoda. The chief representatives of the first class are all ordinary bivalves.
Ostrea (oysters) are well known bivalve mollusks. The shells are so irregular in surface and shape that it would be impossible, as it is unnecessary, to describe them. The animal itself is very simple in structure, proverbially stupid, low in the scale of animal life, but highly esteemed as a delicious article of food. They are found in almost all seas, and in water of from two to six fathoms, but never very far from some shore. They multiply so rapidly that, though the consumption increases with the increase of population and the facilities for distributing them, the supply is equal to the demand. Boston is mostly supplied from artificial beds, and the flats or shallow bays in the vicinity of our maritime cities abound in such “plants.” Baltimore and New York have each an immense local trade, and the oysters exported from the Chesapeake Bay fisheries amount to about $25,000,000 annually.
Class II.—Gasteropoda (stomach-footed). This class, including the great snail family, is a very large division of terrestrial, air-breathing mollusks. Their light shells vary much in form; when spiral and fully developed they have as many as five or six whorls, symmetrically arranged. The shell can contain the whole body, but the animal often partially crawls out and carries the shell on its back. The locomotion is slow and accomplished by the contractile muscle of the ventral foot.