Page:Popular Science Monthly Volume 31.djvu/767

Rh very small body proper. In this may be found a short bowel with a large liver, the nervous system, and, perhaps, also the heart. The bowel has a real mouth only at the junction of the two arms, but is entirely closed at the other end. The inner surface of the valve is covered with a fine, transparent skin, which is called the mantle, in the somewhat thickened border of which are planted the stiff, transparent bristles, which are moved back and forth by the contraction of the membrane in which they are fixed. The organs of generation also lie in the mantle, and are shown in the drawing as two lumps in the raised back valve. Thus, small is the body in proportion to the vigorously developed arms, from which the name of the whole class, brachiopods, is derived. But are they really arms? They are hardly movable. If we stick a needle into one of them, it does not stir; and there are species in which they are completely calcified. Only the fringes move and respond immediately to excitations.

We cut off a piece of the arm and place it under the microscope. Each fringelet is a tube made of a firm, elastic membrane, in the hollow of which are laid one or two bundles of fibers of a muscular or nervous character. On the outside the tubes are clothed with delicate cells connected into a texture bearing fine, actively vibrating ciliæ. These ciliæ generate a bubbling stream in which dance the minute bodies that are floating in the sea-water. The whole stream, which the ciliæ of the thousands and thousands of tubes produce, flows from the periphery toward the mouth. The little tubes all open into the chief pipe of the arm, and are, like that vessel, filled with fluid.

With this observation a considerable number of functions are explained at the same time. The fluid in the little tubes and the chief pipe doubtless plays a part in the movements, in that it is at times pressed into the smaller vessels and expands them, and at other times is held back by the foldings and contractions brought about by the muscles in the chief pipe of the arm. But the constant stream which the ciliæ keep up is all the time bringing new particles of water, heavily charged with oxygen, in contact with the inner surface of the mantle. An exchange of gases through their thin walls is certain to arise; the stream also produces a respiration, which is simply an exchange of the carbonic acid generated within the tissues of the body for the