Page:Popular Science Monthly Volume 13.djvu/425

This page has been validated.
409
THE TEREDO AND ITS DEPREDATIONS.

of the denticles on the two parts of the same valve. Hence it is clear, after the description which we have given above, that if the large adductor muscle contracts, the denticles of the anterior or spiral part of the valve cut the wood; if, on the contrary, the small adductor muscle is shortened, it is the middle part of the valve which undergoes a movement of rotation, and the teeth which it bears are set at work. Thus, then, whether the two muscles contract simultaneously or one by one, the woody cells are cut by successive incisions, which would divide them into small quadrangular pieces, if there be no rending of the fibres. It is evident that the hardest task is demanded from the spiral part of the valve, for it is that part which is first brought to bear upon the wood. This part also has a more solid structure and the denticles are much finer; it is moved by a muscle of considerable size; the power of this muscle is, moreover, sensibly increased by the fact of its being implanted upon the two middle parts of the valves, each of which can be considered as a long arm of a lever whose extremity passes over a space at least four times as large as the portion of the valve which, strictly speaking, does the work.

The foot remains fixed in the same spot a very short time only. The form of the end of the cavity, that of a regularly rounded basin, suffices to prove that the valves of the shell are placed every instant in contact with a different spot. The foot displaces itself, little by little, so as to give a rotating movement to the shell, and at the same time to all that part of the body beyond the shell, even as far as the palettes. When the torsion thus produced becomes excessive, the foot loosens its hold, and the body returns to its former position. Thus, then, the rotary movements remarked by some observers, far from being the cause, should be considered rather the effect; they are only the shifting of position of the animal, and nothing more.

The teredo does not bore out his galleries, but he hollows them out with an action analogous to that of a file, by means of the thousands of cutting teeth with which its valves are armed. If the teeth do not break away rapidly, it is due to their wedge-like form and to the oblique direction of the planes which bound each of these wedges. Moreover, as the animal grows, new rows of teeth are formed, so that the rows which have served in youth are no longer of any use in more advanced age; they are principally the outer rows of teeth, the last formed, which do the work.

The sense of touch exists in the teredo in the suction-disk. This is not only a muscular organ, but one rich in nerves. Quatrefages has already pointed out the two small ganglia, situated on the intestines, which furnish the nerves for that part of the body. The foot, when extended, commences by feeling the place before attaching itself to it and drawing the shell after it. Naturally, it avoids the places which seem to offer too much resistance; but he avoids with equal care the parts where there only remains a wall of wood too thin for sufficient re-