326 



THE OCEAN WORLD. 



tube are subjected to constant maceration. In this state a mechanical 

 action, even very inconsiderable, would suffice to clear away the bed 

 of fibre thus softened, and, if this action is in any degree continuous, 

 it suffices to explain the excavation of the galleries, however extensive 

 their ramifications. Again, the upper cutaneous folds, especially the 

 cephalic hood already mentioned, having the power of expanding at 

 will by an afflux of blood, covered with a thick coriaceous epidermis, 

 and moved by four strong muscles, seems to me very capable of per- 

 forming the operation. It appears very probable that it is this hood 

 which is charged with the removal of the woody fibre, rendering it 

 incapable of resistance by previous maceration, which may also be 

 assisted by some secretion from the animal." That the fleshy parts 

 of the mollusc, acting upon the surface, softened by long maceration 



Fig. 130. Pholas dactylus having hollowed out a shelter in a block of gneiss. 



in water, is the true boring implement employed by the Teredo, is, 

 probably, the only explanation the case admits of; at all events, in 



