274 



MOLLUSC A. 



Motor. 



We have already referred to the adductor muscles for 

 closing the shells and the protractors and retractors of the 

 foot. The main substance of the foot is mus- 

 cular and it is thrust out ventrally at the will of 

 the animal, acting as a burrowing organ. 



The heart is situated dorsally and is three-chambered. 



The median ventricle envelops the intestine and passes for- 



Bi od- wards and backwards into main arteries. It is 



fed by paired lateral auricles which open into it 



by valves. They receive blood from the ctenidia. 



The heart and this part of the intestine lie in a spacious 



cavity, the pericardium^ which is coelomic in origin. The 



Fig- 193- — Dorsal View of Heart and Pericardium 

 OF Anodonta. (Adnat.) 



Aperture of Kidney. 



Anterior Artery. 



Intestine. 



Ventricle. 



Auricle. 



Posterior Artery under Intestine. 



venous system, as in the snail, is lacunar, and formed of 

 sinuses and cavities in the body. A large median sinus 

 below the pericardium feeds the ctenidia. Hence the blood- 

 vascular system closely resembles that of the snail ; the chief 

 difference is the paired condition of the auricles (like that of 

 the shells). 



The brain, situated laterally to the mouth, consists of a pair 

 ^. of cerebral ganglia joined forwards by a connec- 



tive. From the brain there run paired connec- 

 tives to \Si^ pedal ganglia in the anterior part of the foot, and 



