374 



OILMAN A. DREW. 



type as is found in Chiton, and that each class has developed 

 ganglia according to its needs^ than that the ancestors of 

 Lamellibranchs possessed the comparatively complex system 

 of ganglia found in Gastropods. If this is true, it is easy to 

 understand why Gastropods with their complicated head 

 apparatus should develop ganglia for which Lamellibranchs 

 have no need. Accordingly the necessity to homologise all 

 of the ganglia in the two classes disappears. 



In most Lamellibranchs the otocystic nerves spring from 

 the cerebro-pedal commissures, and they are supposed to 

 originate in the cerebral ganglia. In Solemya to gat a, 

 Stempell finds that the otocystic nerves leave the cerebral 

 ganglia direct, and are not included in the cerebro-pedal 

 commissures in any part of their length. Is it not possible 

 that the posterior root of the cerebro-pedal commissure, in 

 forms where there are two roots, is the central end of the 

 otocystic nerve ? 



The pedal ganglia (fig. 48, pg.) are rounded and nearly 

 equal to the cerebral ganglia in size. They lie close together, 

 and they are connected by a moderately large commissure. 

 The nerves from the pedal ganglia supply the muscles of 

 the foot. They need no special mention. 



The visceral ganglia (fig. 48, vg.) are the smallest of the 

 three pairs of ganglia. In shape they resemble the cerebral 

 ganglia, but they are turned in the opposite direction. Each 

 visceral ganglion is elongated, and gradually tapers ante- 

 riorly into the cerebro-visceral commissure. The two ganglia 

 lie far apart, and are connected near their posterior ends by 

 a long and rather thick commissure. A rather large nerve 

 leaves the posterior end of each ganglion, runs posteriorly 

 ventral to the posterior adductor muscle, and, besides giving 

 branches to this muscle, supplies the posterior and ventral 

 portions of the corresponding lobe of the mantle. Anterior 

 and ventral to the posterior pallial nerves another rather 

 large nerve leaves each ganglion. This nerve runs along the 

 inner side of the suspensory membrane of the corresponding 

 gill nearly to its posterior end, 



