In fact, each valve presents in a certain way a combined auger-bit, gouge, and file. The ordinary steel file is made with two rows of notches, in order that the tool may cut simultaneously in two different directions; in this shell the same end is attained by the two rows of denticles, the action of which is equally in two directions perpendicular to each other; and our shell has another advantage, that it does not foul so readily with the filings as does an ordinary file.
Nevertheless, the winding direction of the galleries, in which it is not unusual to find right angles, or even somewhat acute angles; the defective cylindricity in the form of the galleries, which often appear as if composed of rings piled up one upon another, some larger and some smaller; the form of the end of each gallery, which is always perfectly smooth and hemispherical without any projection in the centre—all these facts show, according to Harting, that the action brought to bear upon wood by the teredo is not that of an auger boring a hole by rotary motion, but rather that of a file; this is rendered more apparent from the results of the careful anatomical study given by Harting to the muscular system of the teredo.
Although confined during its entire life to the dwelling which it has itself constructed, the teredo still has a strongly-developed muscular system. It is evident, moreover, that he uses all his muscles, excepting only those which serve to move the siphons, more or less directly, in the perforation of his galleries.
The first system of muscles is that which one finds in the mantle. That organ is provided through its whole length with longitudinal and
|Fig. 9.||Fig. 10.|
transverse muscular fibres. These fibres give the teredo the power of elongating or shortening its body; and also, by the partial action of some bundles of fibres, to make a slight movement of torsion.
At the base of the palettes, at the posterior portion of the mantle, is a powerful muscular ring (Fig. 10, c); by means of this ring, when