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TEXTBOOK OF ZOOLOGY 



are covered with thin shells which prevent undue water loss. In 

 some marine snails, such as Busycon, eggs are deposited in disc- 

 shaped capsules which are spaced equally apart and held together 

 by a tough band. Some snails, such as the fresh-water Campeloma, 

 have a brood pouch in which eggs are deposited and the young are 

 born alive. The latter is ovoviviparous reproduction in contrast to 

 oviparous reproduction, as described above. 



FRESH- WATER CLAMS 



(Detailed description based on Lampsilis) 



Habitat and Behavior 



Mussels or clams are usually found partly buried in the mud, 

 sand or gravel of ponds, lakes, or streams. By means of the mus- 

 cular foot which is protruded from between the two valves at the 



Anodonto 

 ^tevjortiana 



Qaadrula 

 forsheyi 



Proptey-Q 

 purpurata 



I 



Amblema 

 costota 



Leptodea Carunculina .Musculium 



fraqilis texasen5is rejrnsst 



Fig. 140. — Some common fresh-water bivalves. 



anterior end of the shell they plow their way slowly through the 

 stream or pond bed, feeding on the microscopic organisms in the 

 water. At the posterior end of the shell are two openings : the 

 ventral siphon which pulls in food and water, and the dorsal siphon 

 through which wastes and deoxygenated water are eliminated. 



