THE CLAM AND OTHER BIVALYED SHELL-FISH. 59 



palpi. It is quite hard to find, as the lips are very soft 

 and movable. Just below the mouth, and attached to the 

 body, is a tongue-shaped fleshy mass, called the "foot." 

 This is used in burrowing, as it projects through a round 

 or oval hole in the mantle in front. 



How the clam moves and burrows into the mud, and the 

 use of the siphon, can be observed by placing a living clam 

 in a dish of fresh sea-water. If allowed to remain in per- 

 fect quiet for a while, the clam will open its shell a little 

 way, and gradually thrust out its siphon ; then one can. 



Fio. 60.— One valve of tbe clam removed to show the siphon (si), the giBs (g), 

 feelers (p), foot (/), and position of the mouth (mo); om, anterior, po, posterior, 

 adductor muscle; o, orince in the mantle (ni) for the protrusion oi the foot; the 

 arrows pointing inwards mark the course of the currents of water from the si- 

 phon to the mouth; the single outward pointing arrow marks the exit of waste 

 matter out of the upper division of the siphon. 



with a lens, observe the two openings at the end, and the 

 tentacles' guarding them; also, perhaps, the current of 

 water passing into the lower opening in the siphon, and a 

 current passing out of the upper opening, as through this 

 passage are expelled the hard and other indigestible parts 

 of the animals it has swallowed. 



The clam burrows deep in the mud by means of its 

 foot. This is very strong and muscular, and swells so that 

 it can be thrust out from between the valves of the shell 

 into the mud ; then, by movements somewhat like those of 



