UNSEGMENTED SCHIZOCOELS 



22) 



Figure 13.8 Class Pelecypoda, bivalve types: A, A^y//7us, a mussel, marine; B, Pecien, o scollop, 

 marine; C, Sponc/y/us, o thorny oyster, marine; D, Osfreo, on oyster, marine; E, Chomo, jewel box or 

 rock oyster, marine; F, Ensis, a jocknife clam, marine; G, Anodonta, a fresh-water clam. 



shipworms. These bivalves, vi'hen burrowing in 

 wood, swallow and digest the fine wood particles. 

 Most pelecypods live in sandy mud, but others are 

 found in sand, clay, and burrowing in rocks or wood. 

 They are found from intertidal to deep water. 



In almost all pelecypods the sexes are separate and 

 the egg hatches into a free-swimming larva which 

 eventually transforms into a miniature adult. Some 

 marine bivalves have brood pouches in which the egg 

 is retained until hatching, but most shed their eggs 

 into the water for fertilization. Fresh-water animals' 



larvae differ from the marine larvae by being fish 

 parasites prior to becoming adults. 



Man has relied upon clams, oysters, and the like 

 for thousands of years. Coastal midden heaps of 

 ancient men contain many bivalve shells, indicating 

 that primitive man used them for food. The shells 

 were also used for ornaments, as they are today to 

 form jewelry. Pearls are formed by secretion of a 

 calcareous substance around irritating sand grains. 

 Circular portions of many shells (mother of pearl) 

 were strung by American Indians for wampum; today 



