SHELLS. 



47 



truding and looking like a row of little frills. This edge secretes 

 carbonate of lime from the water and adds it to the shell all along 

 the outer edge, forming a new line of growth. Thus, as the clam 

 increases in size his house grows proportionately, so that it always 

 exactly fits him. The two halves of the shell are joined by a cu- 

 rious hinge. In some kinds of shells the hinge is external and in 

 some internal. It consists of teeth (two or more) with spaces be- 

 tween on either half. These lock together, and are held by a 

 strong, elastic muscle. On the inside of the clam shell are two 

 slight depressions, where the powerful adductor muscles of the 

 body were fastened. 



The clam's body is completely enshrouded in the mantle, ex- 

 cept for two openings, through one of which the foot can be 



Clam Shell. Exterior showing lines of growth ; interior showing muscle scars. 



pushed out. The other is for the siphon, or what is commonly 

 known as the "neck" of the clam. In some respects the clam 

 may be better off than we are, for he has a little brain in his foot 

 and also a gland for secreting strong fibers. With this he spins 

 a byssus by which he can attach himself to whatever he likes. 

 He does not even have to search for his food, but waits for it to 

 come to him. He makes a burrow in the mud or sand, attaching 

 himself to the bottom by the byssus. Then he thrusts his siphon 

 up through the mud and water until it reaches the surface. The 

 siphon is made up of two tubes, the water flowing in through one 

 and out through the other. 



When the inflowing current, laden with minute plants and 

 animals, reaches the gill chamber, some of these are sifted out and 

 retained for food, while the water and waste matter flow out 

 through the other tube. 



The clam's eggs are carried by the mother on her gills. When 

 there are fish in the water with them the mother clams discharge 

 the eggs, which soon hatch, but if there are no fish they carry the 

 eggs until they decay. The reason of this strange behavior is 



