136 TEREDINID^. 



end of the tunnel, and that it occupies the whole of the 

 front or hemispherical cavity. That part of each valve 

 which may be supposed to have a rasping power is 

 placed at the side, and not at the bottom. I believe 

 that the valves, instead of the foot, serve as a fulcrum, 

 and that they are pressed equally against both sides, 

 while the tissue of the foot is employed in absorbing 

 and detaching, slowly but gradually, minute par- 

 ticles of the moistened wood. If the shell were 

 the instrument of perforation, it would be applied to 

 the bottom, and not to the sides of the tunnel ; and no 

 muscle has yet been detected which could effect such a 

 change in the relative positions of the valves and foot. 

 Mr. Osier strongly advocated the theory that the wood 

 is rasped away by the shell; yet he admitted that, 

 owing to the shortness of the lateral muscles in Teredo, 

 it was not probable that this mollusk could bore, like 

 the Pholas, by the action of these muscles alone. 

 Quatrefages agrees with Deshayes in considering the 

 muscular apparatus by no means adapted for putting 

 the valves in action as perforating-instruments, by either 

 a rotatory or a twisting movement. He attributes this 

 agency to the anterior fold of the mantle, especially 

 that part which lines the back or beaks of the shell 

 (called by him the ' ' capiichon cephalique ") aided by 

 continual soaking of the water, and perhaps also by 

 some secretion of the animal, as well as possibly by the 

 siliceous particles observed by Hancock in the mantle of 

 certain other perforating mollusks, and by Deshayes in 

 the integuments of the Teredo. But no part of the 

 mantle is placed in contact with the excavated end of 

 the tunnel or canal, which is entirely occupied by the 

 foot. In a memorandum which I received from the 

 late Dr. Lukis on this subject, he says (after summarily 



