250 FUNCTIONS AND INSTINCTS. 



by the water, and slowly increasing its crypt 

 as its dimensions increase in this case the most 

 powerful action seems to be at the anterior end, 

 though assisted, it may be, by some motion at 

 the posterior. This kind of action appears best 

 suited to its slender body. 



Let us next examine the pholads, all the 

 genuine ones are rough like a rasp, strengthened 

 near the base with accessory valves and a thick 

 interior margin, indicating that here is the great 

 action, and here it is that the foot revolves, thus 

 maintaining a rotatory motion, causing the valves 

 to act as files upon the walls of its crypt and thus 

 to enlarge it when necessary ; perhaps this 

 action may also be connected with its respiration 

 and nutriment ; it is probably very slow and 

 gradual, so as not to injure the frail apex of 

 its shells. 



In another rock-borer, of a form not suited to 

 effect an excavation by a rotatory motion, the 

 deficiency, we see, is compensated for, and it 

 effects its purpose by employing chemical agency 

 when its crypt becomes too small for it. 



The sand-boring razor-shell above described, 

 would be impeded by a rough shell, in excavating 

 its deep burrow, its valves therefore are smooth 

 and polished, and its body very narrow, and 

 consequently meets with less resistance in its 

 motion either upwards or downwards while the 

 cockles which do not bore to a great depth are 



