472 The Animal Kingdom 



Fortunately the anatomy of the majority of these forms is so 

 similar that a fresh-water clam of the genus Anodonta can be used as 

 an example. 



The Anatomy of Anodonta sp. — Dorsally the shells or valves 

 are connected by the liingc teeth and an elastic hinge ligament. The 

 hinge teeth not only hold the valves together, but also aid in their 

 opening and closing. Some species of clam also have other teeth at 

 the anterodorsal portion of the shell which act in coordination with the 

 hinge teeth. These are known as the pseudocardinal teeth. On the 

 outer surface of the shell, lines of growth niay be seen with the heavier 

 lines representing the annual growth rings. The oldest part of the valve 

 is the swollen anterior dorsal portion known as the umbo. In section. 



PERIOSTRACUM 



HINGE TOOTH 

 POSTERIOR RETRACTOR 

 POSTERIOR 



UMBO 



ANTERIOR 

 RETRACTOR 

 -ANTERIOR 

 ADDUCTOR 



PROTRACTOR 



Fig. 152. — The fresh-water clam, Anodonta. A, Left valve of shell; B, cross 



section of shell and mantle. 



the shell is seen to be made up of three distinct layers, an outer perios- 

 tracum which is thin and darkly colored, the middle prismatic layer 

 which is thickest and is formed chiefly of calcium carbonate, and the 

 inner layer, the nacre or mother-of-pearl, which is usually iridescent. 

 The outer periostracum is insoluble in the carbonic acid which is so 

 often present in fresh water and thus offers protection to the inner 

 layers. 



Fastened to the dorsal inner portion of the shell are the large 

 muscles which open and close the valves. These may be seen as scars 

 when the shell is opened. There are two large muscles, the anterior 

 and posterior adductors, which draw the valves together. Other mus- 

 cles are the anterior and posterior retractors which withdraw the foot, 

 and the anterior protractor which extends the foot. 



Within the valves are located the various portions of the clam's 

 body. There is a dorsal median visceral mass which contains most of 



