332 ZOOLOGY. 



end, and at their anterior end they cross, the ventral 

 one turning upwards to open into the pericardium, and 

 the dorsal one turning down to open to the exterior 

 in the supra-branchial chamber. This opening is called 

 the nephridiopore. The ventral limb is brown, and its 

 walls are glandular, hence it is often called the " kidney " ; 

 its nephrostome (or reno-pericardial pore) is ciliated and 

 carries off the pericardial fluid ; as the wall of the pericar- 

 dium is glandular in places (forming the so-called " organ 

 of Keber "), it doubtless has an excretory function also. 

 The dorsal limb of the nephridium is thin-walled and non- 

 glandular, and often called the " ureter." Close to the 



KlCARD/(SAf^S>< >< ' 



Fig. 172. DIAGRAM OF SIDE DISSECTION OF NEPHRIDIUM or ANODONTA. 



point where it opens to the exterior, it has a communication 

 with its fellow of the opposite side ("inter-renal opening"). 



8. The Nervous System of Anodonta is very different 

 from any we have yet seen. The central nervous system 

 consists of three pairs of small ganglia, connected with one 

 another by commissures consisting of nerve-fibres only 

 (fig. 170), 



One pair, commonly called cerebral, lies right and left of 

 the mouth, the two being connected by a commissure in 

 front of the mouth. From these ganglia motor fibres are 

 given off to the anterior adductor and other muscles in the 

 neighbourhood. They also send off commissures to the 

 other ganglia of the same side. 



