MOLLUSKS 15 
These are also united by two great adductor muscles, located 
at opposite ends of the animal (Fig. 45, A, a.a., p.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 débris 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. 
%5. 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 
