334 



INVEBrEBRATE MORPHOLOGY. 



(a) whicli opens into tlie suprabranchial chamber (sir) in the 

 vicinity of the exhalent siphon. The relations of this rectum 

 to the heart have already been noted (p. 332). 



The nervous system of the Pelecypoda differs somewhat 

 apparently from that of the Gasteropods, a smaller number 



P y ne dp' 



of ganglia being discernible. Above the oesophagus a short 

 distance behind the mouth is on either side a -flell-marked 

 ganglion (Fig. 149, eg) connected with its fellow of the oppo- 

 site side by a transverse commissure. In the more primitive 

 forms (Nucula) two ganglia are found on either side, of which 

 one evidently corresponds to the cerebral and the other to the 

 pleural ganglion of the Gasteropods. Where, therefore, as in 

 the majority of the Pelecypods, but a single ganglion occurs on 



