MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 



109 



kidney proper, and the urinary tube. The whole organ has 

 two openings, one of which opens into the cavity of the 

 I.)ericardium or sac around the heart while the other opens 

 on the side of the abdomen near the base of the inner gill. 

 It is from the latter that the excretions of the kidneys are 

 carried away by the current of water through the exhalent 

 siphon. 



The heart and circulation. — The heart is situated in the 

 doi'sal part of the Ijcjdy just below the hinge ligament and 

 is surrounded by a membranous sac, tlie pericardiutn. It 

 consists of one ventricle and a right and left auricle. Two 

 arteries arise from the ^'entriele, one from the anterior end 

 which passes above the intestine and conveys the blood to the 

 forward parts of the jjody and another from the posterior- 

 end which rims below the intestine and carries blood to the 

 posterior part of the body. By the contractions of the 

 heart the blood is sent out through the arteries into irreg- 

 ular channels that 

 reach all parts of the 

 animal. On its re- 

 turn the lilood passes 

 first through the kid- 

 neys, where it gives 

 up nitrogenous waste 

 matters. Then it 

 flows to the gills, 

 where it is aerated, 



and finallv from the Fig. 59. — Part of nervous system of a dam. 



gills, it passes directly into the thin-walled auricles. These 

 open into the ventricle. Thus the blood reaches its start- 

 ing point, having comideted the circuit of the body. 



The nervous system. — This system consists of three pairs 



