Page:Popular Science Monthly Volume 13.djvu/424

This page has been validated.

as Deshayes has pretended, an animal having very few if any muscles, is, on the contrary, richly provided with those organs. There are the longitudinal and transverse muscles through the whole length of the mantle, a true sphincter (Fig. 10, c) at the base of the siphons, a muscular organ which receives and covers a part of the valves, two adductor muscles for the movement of bringing the valves together, and a foot provided with a suction-disk and susceptible of extension and retraction—truly a profusion of motive organs which one would not expect to find in an animal which passes its entire life in a narrow canal which it can never quit. Moreover, all these motive powers have only one essential end, namely, to endow the teredo with the power of boring his gallery—his home.

But all the muscles which we have enumerated do not coöperate to that end in an equally direct manner. When water has entered by the incurrent siphon, the animal can, by contracting the transverse muscles of the mantle, force the water through the whole length of its body up to the end of the gallery, and then drive it out by the excurrent siphon. The teredo undoubtedly makes use of this as a means of getting rid of the fine filings of wood which the valves of the shell have detached. He can then draw back a little the anterior part of his body by the action of the longitudinal muscular fibres, supporting himself by the two palettes pressed against the inner walls of the calcareous tube at a distance of two or three millimetres from the exterior opening, by which the siphons project outside the wood. It is probable that the teredo takes that position during his periods of repose, which come from time to time, and which he uses for repairing his tools.

The teredo possesses, on the other hand, the means of preventing or hindering the outflow of water at will, so that its body, distended by the liquid, occupies at that time the whole extent of the gallery, and his anterior portion touches with the valves of its shell the end of the gallery. In this position he can carry on his work of miner. He commences then by extending his foot (Fig. 1, d), which he fixes by suction against the side of the cavity. At the same time the valves separate a little; then, while the foot draws the shell to itself and thus presses its exterior surface against the wood, the valves close up again, and the denticles with which they are furnished cut into the wood.

In this labor there are still two peculiarities worthy of notice: First, the limited extent of movement with which the valves are endowed, their anterior extremities moving only a very short distance from each other. But this circumstance, in view of the narrow space in which the teredo works, gives him this advantage, that, by the rapid succession of movements of opening and closing the shell, he attains his end—namely, to reduce the wood to an impalpable powder—better than if each blow of the instrument had a wider range. In the second place, we should recall the fact that the directions of movement of the two adductor muscles are at right angles, as are also the directions of the cutting surfaces