Fig. 233.—Fresh-water Mussel. A, from left side; B, from behind. ft, foot; in. sph., inhalant siphon; ex. sph., exhalant siphon; lg, ligament; m, mantle; um, umbo. (× ⅔.) (After Howes.)
Molluscs.—Molluscs are soft-bodied animals, in most cases protected by a hard, external shell, but they differ essentially from crustaceans and all other arthropods in not being segmented, and in not possessing jointed limbs. Familiar and instructive examples of two classes of the group are found in the fresh-water mussel and the garden snail.
The fresh-water mussel ([Fig. 233]) is to be found in streams, along the bed of which it ploughs its way by means of a muscular foot (ft.). The body is enclosed in a brown shell, which consists of halves called valves, hinged together along the straight, dorsal edge by an elastic ligament (lg.). The action of the ligament is to separate the valves slightly unless they are forcibly held together by the contraction of closing muscles which run from one valve to the other. Hence the shell of a dead mussel always gapes open. The rounded end of the shell is anterior, the more pointed end posterior. The oldest part of each valve is the umbo (um., [Fig. 233]), a knob just in front of the ligament; and concentric lines surrounding the umbo mark successive positions of the margin of the valve as the animal increased in size. The valves are formed by the activity of the mantle lobes (m.)—a pair of delicate membranes which hang down from the sides of the body. The foot is a median prolongation of the body itself, and on each side a pair of plate-like gills lies between the foot and the mantle lobe of that side. “Thus the whole animal has been compared to a book, the back being represented by the hinge, the covers by the valves, the fly-leaves by the mantle lobes, the first two and the last two pages by the gills, and the remainder of the leaves by the foot.”[38]
When the living mussel is undisturbed, the mantle folds project slightly at the hinder end of the shell, their edges being so placed in contact that they form two short tubes. A current of water flows in at the lower of these (in. sph., [Fig. 233]), carrying to the mouth a supply of food-particles, and to the gills and mantle a store of dissolved oxygen; while an outgoing current leaves by the upper tube (ex. sph.).
The garden snail ([Fig. 234], B) seems at first sight to have but little resemblance to a mussel; but it also is a mollusc—consisting of a soft, unsegmented body, which is produced ventrally into a foot, and is protected by a shell formed by the activity of a mantle fold of the body. In this case, however, the shell is one piece, and is spirally coiled. The snail has also a distinct head, which bears two pairs of tentacles; at the tip of each of the longer and upper tentacles (t) is an eye (e). The animal crawls about by wave-like contractions of the muscular foot; it feeds upon vegetation, which it rasps into small particles by means of a toothed tongue, and then swallows. The snail is entirely adapted to a terrestrial life, and breathes air—the mantle fold under the shell enclosing a lung chamber with blood-vessels in its walls, which opens to the exterior by a respiratory pore (p.o., [Fig. 234]) on the right side. The snail spends the winter, in a state of torpor, under logs or stones, the body being entirely retracted into the shell, the mouth of which is closed by a plate of hardened slime.
Fig. 234.—A, A Fresh-water Snail (Limnaea); B, Garden Snail. e, e, eyes; p.o., respiratory pore; t, t, tentacles, (× 1.)