218 



GENERAL ZOOLOGY 



mouth. The whole alimentary canal is ciliated and the 

 muscles accordingly take little part in moving the food 

 through it, which is very unusual. 



Most lamellibranchs move very slowly by protruding the 

 foot (Fig. 87 A',/) and pressing against the bottom, but some 





D' 



FIG. 87. Life history of fresh water mussel. A, adult clam giving off glochidia 

 through excurrent siphon; A', anatomy of mature clam; a, adductor muscle; 

 /, foot; p, marsupium, or swollen portion of gill, in which glochidia mature from 

 eggs. B, B', young glochidia on bottom; C, C', glochidia encysted in fishes' 

 fins; D, D', young clam. 



clams are able to leap a short distance; the scallops swim 

 actively by squirting water from the mantle cavity, and the 

 fresh-water Sphceridce climb about on aquatic plants. A 

 number of clams are able to burrow with great facility. 

 The oyster moves about only while young, for it grows 

 fast to the bottom and is then unable to change its position. 

 On this account oysters are often smothered by mud 



