476 The Animal Kingdom 



epithelium and live as parasites. For several weeks, they remain as 

 parasites. Finally the cysts rupture, and the young drop off and de- 

 velop into free-living adults. 



This knowledge of the life history of the fresh-water clams has 

 been used to increase their numbers and to prevent extinction of these 

 forms in some of our inland waters. 



Among marine forms, the life history differs from this. Many 

 species possess a typical ciliated veliger larva; others have direct de- 

 velopment. Sessile forms such as the oyster have a free-swimming 

 stage. 



Fig. 156. — Some representatives of the phylum MoUusca. a, Busycon, the whelk, 

 with eggs; b, Pecten, the scallop; c, Clione, a pteropod; d, Dendronotus, a nudibranch; 

 e, Tridacna, the giant clam; /, Fissurella, a limpet; g, Mytilus, the sea mussel with 

 byssus; h, Buccinum, a whelk; i, Ischnochiton, a chiton; j, Cypraea, a cowrie; k, 

 Littorina, the periwinkle; I, Teredo, the shipworm; m, Pteroccra, the finger shell; 

 n, Dentalium, the tooth shell; o. Verms, the quahog; p, Bittium. 



Economic Importance. — Since ancient times man has relied upon 

 the bivalves as a source of food. Along the coastal areas tremendous 

 shell mounds attest to their popularity among the early Indians. At 

 the present time, oysters, clams, mussels, and scallops are of great 

 commercial value, perhaps being worth more than $1,000,000 an- 

 nually. So valuable are these natural resources that the fish and wild- 

 life service devotes much time to the study of the breeding habits and 

 the propagation of the various species. Oysters, particularly, are cul- 

 tivated by supplying young oysters in suitable places and providing 

 proper substrate for them to settle upon. 



If an irritating grain of sand or other material is introduced be- 

 tween the mantle and the shell, the mantle may secrete nacre about the 



