FRESH-WATER MUSSEL 



375 



due to carbonic acid in the water. Extending out from the umbo on 

 each valve in a concentric manner are the growth lines of the shell, 

 evidenced as slight, medium, or heavy ridges. 



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. 



Ligamentous hinqe Umbo 



.^jQrowihlims 



Ventral siphon 

 I Dorsal siphon 



Expanded 



^°^^^fee'^'^"'^l_----^^^^C l^r^^nor cdduztor 



■^^-a^0K:k J:' r'^ir/ protractor 

 ''^'^^■^.:^:0:.i^:^^py& retractor 

 muscle 



Pallial Unz Q 



Fig. 143. — Exteinal (A) and internal {B) shell features of Lainpsilis anodontoides. 



Internal Anatomy 

 (Detailed description based on Lampsilis) 



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- 

 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. 



