NERVOUS SYSTEM. 



The three pairs of ganglia that are usually found in lamelli- 

 branchs are present in this form, but they differ greatly in size 

 and they are not all placed in the usual positions. 



The cerebral ganglia (fig. 15, eg.) are placed some distance 

 ventral to the mouth, just beneath the outer covering of the 

 body. They, like the other ganglia, are yellowish in color, and 

 may frequently be faintly seen through the covering of the body. 

 Each cerebral ganglion is somewhat elliptical in outline with 

 the long axis directed dorso-ventrally and has a rather distinct 

 swelling on the ventral (actually anterior) and outer side (the 

 side away from the median plane of the body) (fig. 24, eg). 

 The anterior end of each cerebral ganglion presents a forked 

 appearance, due to the origin of two large nerve cords. The 

 inner and ventral one of these two cords (figs. 23 and 24, cc.) 

 is the commissure that joins the two cerebral ganglia. As the 

 ganglia lie some distance ventral to the oesophagus, this com- 

 missure forms a long loop that passes dorsally around the 

 oesophagus just posterior to the mouth. The outer and pos- 

 terior of the two large cords that leave the anterior end of each 

 ganglion is the anteror pallial nerve (figs. 15, 23 and 24, apn). 

 This runs parallel with the commissure as far as the oesophagus 

 and is then continued along the side of the liver and in the 

 mantle, to the margin of the mantle in the region of the anterior 

 ear of the shell, where it joins by several branches the circum- 

 pallial nerve (cpn.) that follows along the margin of the mantle 

 near the bases of the tentacles and eyes. The circum-pallial 

 nerve will receive attention later. 



Between the points of origin of the cerebral commissure and 

 the pallial nerve, a small nerve (figs. 23 and 24, pn.) leaves the 

 ganglion to be continued dorsally, and to supply the labial palp. 



From the inner, ventral surface of each cerebral ganglion, a 

 little in front of the middle, the cerebro-pedal connective leaves 

 to join the pedal ganglion of the same side. The cerebro-pedal 

 connective is smaller near the cerebral than the pedal ganglion 

 (fig. 24, cpc.) and bears a ganglionic swelling on its outer side 

 very near the pedal ganglion. 



In the acute angle formed by the surface of the cerebral 

 ganglion and the cerebro-pedal connective, a small nerve (otn.), 



43 



