78 



indicate by their structure the double origin of the so- 

 called cerebral ganglia. 



V/liile these concentrations of the visceral ganglia 

 with each other and of the pleural with the cerebral have 

 been taking place, the cerebral commissure is constantly 

 elongating, along v;ith the grov/th of the oesophagus, so 

 that in t'ne adult, the cerebral ganglia are separated from 

 each ot;.;--' y a long commissure. 



The Nervous System of the Adult. - Along with the great 

 change in the general relations of the various systems 

 that have taken place from the "Ship-worm" larva to the 

 adult, the nervous system has changed, principally in the 

 changed position of the visceral ganglia, which lose their 

 position in front of the posterior adductor, and come to 

 lie much posterior to it. However, the same three pairs 

 of ganglia are present in the "Ghip-worms" as in other 

 types of Lamellibranchs. The general arrangement is shown 

 in fig. 60. Lying almost at the sides of the mouth are 

 the two cerebral ganglia (c. figs. 10,60), well developed 



