THE PHYLUM MOLLUSCA 



rougfhened areas termed muscle scars. Contraction of the powerful adductor 

 muscles brins^s about closure of the valves. When the muscles relax, the 

 elasticity of the hinge ligament causes the valves to gape ventrally. Other 

 muscle masses, retractors and protractors, are variously developed in different 

 species. GeneralK' speaking, they occur in pairs both anteriorly and pos- 

 teriorly, originating in the body wall of the clam and inserting near the 

 adductor muscles on the internal surfaces of the valves. In effect, the re- 

 tractor muscles suspend the body of the organism from the valves, and varia- 

 tion of their states of contraction and relaxation adjusts the position of the 

 animal within the shell. Enclosed between the two flaps of the mantle is a 

 space, the mantle cavity, filled with circulating water. The ventral edges of 

 the mantle flaps, held closely together at the margins of the valves, effectively 

 seal off the mantle cavity from the external environment. The posterior 

 margins of the mantle, however, are modified into two extensible tubes, the 

 siphons, through which the mantle cavity communicates with the surrounding 

 water. The siphons may be protruded between the valves, and in 1'enu.s 

 mercenana they are provided with special triangular siphon retractor muscles 

 attached to each valve. Through the ventral or incurrent siphon water is 

 drawn into the mantle cavity; through the dorsal or excurrent siphon it is 

 conducted outward. 



Ventricle 



Pericardial cavity 

 Right atrium 



Eflferent 

 branchial vessel 



Afferent 

 branchial vessel 



Fig. 13.2. Diae;rammatic cross section of a typical fresli-water pelecvpod, Anodonta, siiowine; 

 the heart and principal blood vessels. (From W. Stempell, 1926, ^oologie im Gnmdriss.) 



371 



