THE TEREDO AND ITS DEPREDATIONS. . 407 



backward within the anterior, spiral parts of the valves (Fig. 11, p) 

 below the line which passes by the two tuberculous extremities, 

 coupled together, i. e., by the centre of rotation of the valves. From 

 this principal mass some slender muscular fibres extend over the two 

 thorny protuberances (Fig. 11, e e), which may be compared to two 

 arms of levers, whose common fulcrum is 

 found in the centre of rotation of the 

 valves. It is clear that, by this arrange- 

 ment, the action of the muscle is consider- 

 ably strengthened. 



The two adductor muscles are com- 

 posed of the same microscopic elements, 

 that is, of fibres and fibro-cellular tissue, 

 easily separated in form of a ribbon, six 

 mmm. wide and one mmm. thick ; their 

 length is relatively considerable, and prob- 

 ably equal to that of the muscles them- 

 selves, inasmuch as one can nowhere dis- 

 cover any free extremities. These fibres 



are distinguished from the fibro-cellular tissue of the mantle not only 

 by their greater length, but also by their darker outlines, which indi- 

 cate thicker walls and, consequently, greater solidity and strength. 



The effect produced by these muscles in contracting is very evident. 

 The large adductor muscle, situated on a plane a little above the gen- 

 eral centre of rotation of the valves, serves to draw together the rounded 

 sides of the valves as well as all the other parts of the valves situated 

 posteriorly on the same side of the centre of rotation. The small ad- 

 ductor muscle, placed in front of the centre of rotation, exercises a more 

 limited action. When it contracts, the anterior extremities of the spiral 

 part of the valves approach each other ; simultaneously, all the parts 

 of the shell situated behind experience a slight displacement forward, 

 as if they tried to turn about an axis passing by the centre of rotation ; 

 but the one which they thus describe is necessarily very small, on ac- 

 count of the shortness of the muscle. It is evident that the direction 

 of the movement made by one of these adductor muscles is nearly a 

 right angle with that of the other ; the anterior and middle parts of the 

 valves, which are first acted upon by the action of the muscles, meet at 

 an angle of 90. 



Finally, the teredo has also a muscular organ, without which it 

 would be impossible for him to pierce his galleries. It is the part known 

 as the foot, which has the power of projecting outside between the an- 

 terior opening of the valves (Fig. 5, b ; Fig. 1, d the dotted line indi- 

 cating the outline of the foot in the state of extension). This foot has 

 the power of extension and retraction, and terminates at the end with a 

 suction-disk, by the aid of which the animal can attach itself to the wood. 



From what precedes, it is evident that the teredo, far from being, 



