MOLLUSKS 75 



These are also united by two great adductor muscles, located 

 at opposite ends of the animal (Fig. 45, A, a.a., J9.a.), which 

 in times of disturbance contract and firmly close the shell. 

 Upon their relaxation the shell opens, the clam extends its 

 foot, and plows its way leisurely through the mud, or re- 

 mains buried, leaving only the hinder portion of its gaping 

 shell exposed. Through this opening a current of water 

 is continually passing in and out, owing to the action of 

 the cilia covering the gills, and by placing a little car- 

 mine or coloring matter in the ingoing stream we may 

 trace its course through the body. Passing in between the 

 mantle and the foot it travels on toward the head, giving 

 off small side streams which are continually made to enter 

 minute openings in the gills, whence they are conducted 

 through tubes in each gill up to a large canal at its base, 

 where it is carried backward to the exterior. In this pro- 

 cess oxygen gas is supplied to the number of blood-ves- 

 sels traversing the gills, and at the same time considerable 

 quantities of minute organisms and organic debris are 

 hurried forward toward the head, where they encounter the 

 whirlpools made by the cilia on the lips and are rapidly 

 whisked down into the mouth and swallowed. 



75. Rock- and wood-boring clams. Other similar forms 

 are rendered even more secure through their ability to 

 bore in solid rock. In the common Piddock, for example 

 (Fig. 46), the shell is beset with teeth like a rasp, which 

 gradually enlarge the cavity as the animal grows, until it 

 becomes a prisoner with no means of communication with 

 the exterior save the small opening through which the 

 siphons project. This is also the case with the Teredo, 

 improperly called the shipworm, which swims about for 

 some time during early life and then, about the size of a 

 small pinhead, settles down upon the timbers of wharves 

 or unsheathed ships, into which it rapidly tunnels. 

 Throughout life its excavation is extended sometimes to a 

 distance of two to three feet, and imprisoned yet safe at 



