Page:Popular Science Monthly Volume 29.djvu/695

Rh has to be subserved, and when the cuttle-fish desires to swim, it propels itself through the water by aid of a veritable hydraulic engine. The effete water from the gills is ejected with force from the funnel, and by the reaction of this jet d'eau upon the surrounding medium the animal is enabled to execute its aquatic flights. Economy of a very rigid order is illustrated clearly enough in octopod existence. The otherwise useless "breath" of the animal becomes converted into a means of locomotion.

A still closer parallel to the human chest-recoil, perchance, may be found in the case of certain poor relations of the octopus. These lower forms are the mussels, oysters, cockles, clams, and other bivalve shell-fish which frequent our own and other coasts of the world. Incased in its shell, a mussel or oyster, all headless as it is, and possessing in its way a strictly "local habitation," in that it is a fixture of the coast or sea-depth, presents us with the type of' an apparently vegetative life. But there is abundant activity illustrated within the mussel or oyster shell. There are millions of minute living threads the cilia of the naturalist—perpetually waving to and fro as they crowd the surface of the gills. These cilia, acting like so many microscopic brooms, draw in the currents of water necessary for food and breathing, while the same incessant movement which draws in the fresh water circulates it over the gills, and in turn sweeps it out as waste material from the shell. The oyster implanted in its bed, or the mussel attached by its "byssus" or "beard" to the rock, exhibits a half-open condition of the shell as its normal state. The animal lives—as may be seen on looking at a tub of oysters as they lie amid their native element—with the shell unclosed for purposes of nutrition and breathing. If, however, we tap the living oyster or mussel ever so lightly, we find the shell to close with a snap that renders the persuasion of the oyster-knife necessary for its forcible unclosure. In such a case the animal's senses, warned of possible danger by the tap on the shell, communicate to its muscular system a nervous command, resulting in a movement which, as regards the oyster, reminds one of nothing so forcibly as the cry and action of "shutters up" in a Scotch university town when snow-balling begins.

The muscular system of these shell-fish is disposed in simple fashion. Look at the inside of an oyster-shell, and note the thumb-like impression you see occupying a nearly central position. This is the mark of the "adductor" muscle of the oyster, or that which draws the shells together. The secret of successful oyster-opening is simply the knowledge, acquired by much practice, of hitting the exact position of the "adductor" muscle, and of dividing its fibers with the knife. The enormous power of this muscle to keep the valves in apposition can be appreciated most readily, perhaps, by the amateur "opener" of these bivalves. In the mussel there are two such "adductors," one at either extremity of the shell, and we note the impressions which these