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TEXTBOOK OF ZOOLOGY 



The shell is covered by a horny, pigmented periostracum. Under- 

 lying this is the prismatic layer composed of carbonate of lime. 

 The inner mother-of-pearl or nacreous layer consists of many thin, 

 usually smooth plates, that in reflected light produce an iridescence 

 in many species. 



Internal Anatomy 



The valves are held together by two powerful transverse muscles, 

 the anterior and posterior adductors. Upon cutting these muscles 

 the shells gape open, exposing the underlying organs. The valves 

 are lined with a mantle which secretes the shell. On the inner sur- 



Manble cut free 



Perkaniial cavity 

 ' AuHcle 



Rectum 



Ant retractor 



Anbsyior 

 adductor 1 



I 



Poit retractor 

 Post, adductor 

 I Ex siphon 



Protractor Ext . labial palp 



Left qill plate 



Fig. 142. 



-Lampsilis anodontoides with the left mantle partially removed and 

 turned back to expose the underlying organs. 



face of each shell may be seen the curved pallial line which extends 

 between the two adductor muscles and indicates the partial attach- 

 ment of the mantle. Teeth which strengthen the closure of the 

 shell may be present where the two valves come together. Between 

 the two walls of the mantle is the mantle cavity which contains the 

 leaflike gills, the foot, and visceral mass. 



Digestion 



During the activity of the clam a constant current of water is 

 maintained in the mantle cavity. Food material is circulated for- 

 ward to the mouth which lies between ciliated labial palps. Upon 



I 



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