THE TEREDO AND ITS DEPREDATIONS. 



405 



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 gallerj r , which is always per- 

 fectly 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 svstem 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, 0) ; by means of this ring, when 



