Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/15

Rh appear when fastened to a stone or shell. They are bag-like in form, and have two very distinct apertures, called "siphons," through the upper of which the water continually passes into the interior, and through the lower of which the same water is ejected, having been deprived of its nutritious particles in its passage through the animal.

Ordinarily, the flow of water is steady and moderate, but when the animal is irritated, it contracts itself so violently that the water is spirted out to a considerable distance, as out of a syringe.

At fig. 1 is seen the mechanism by which this act is performed. Surrounding the entire animal is a soft and membranous skin, called the "test," which is in many cases covered with the smaller algæ and marine animals. If this be cut open and turned back, a second skin, or inner tunic, is seen. This is formed chiefly of muscular fibres, laid in different directions, but being more closely gathered, and therefore more powerful, near the orifice. The best plan of separating these two skins is to place the animal for some little time in spirits, when the inner tunic contracts so violently that it separates itself from the outer envelope, and saves the trouble of careful dissection. Within is a third and very delicate tunic, which belongs to the respiratory system.

We now come to the digestive organs. Though the water enters the animal at the aperture which has already been mentioned, it does not reach the true mouth until it flows as far as the spot marked m, when it passes into the stomach, thence into the intestines, and thence into the surrounding water through the second orifice. If one of these animals be held to the light, the shape of the digestive apparatus can be easily traced by its dark opacity, contrasting with the partial translucence of the rest of the animal; and as the two upper tunics can be stripped off just like a grape-skin, it is easy to see the general outlines of the structure.