Ill] THE PEARL OYSTER 31 



attached or inserted on to the shell valves. Two of 

 them are known as Retractors. They originate in 

 the walls of the byssus gland and pass backwards one 

 to each shell valve. The other four are Levators, 

 two of which are anterior and two posterior. By the 

 contraction of these muscles the foot is moved from 

 side to side or retracted. 



The protrusion of the foot to the outside of the 

 shell is produced by a rush of blood into the cavities 

 of this organ. 



There are numerous other smaller and diffuse 

 muscles in the body which need not be detailed. 



TJie Viscera. 



There is no large cavity in the molluscs which, 

 like the human body-cavity or coelom, apparently 

 contains the viscera. The cavity which corresponds 

 to this is small ; it is termed the pericardium, and 

 contains the heart. As in all bivalves there is a 

 definite heart with blood vessels, but the vessels often 

 expand into mere cavities in the tissues and it is 

 difficult to speak of arteries and veins, except when 

 referring to definite large vessels with cellular walls. 



The heart, in the pericardium, is situated dorsal 

 to the adductor muscle and between it and the so- 

 called liver (see fig. 4). 



The mouth of the animal is situated high up near the 

 hinge-line, above the foot and consequently anterior 



