CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 235 



The cerebral ganglia (Fig. 6, 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. 9, eg). The 

 anterior end of each cerebral ganglion presents a forked appear- 

 ance, due to the origin of two large nerve cords. The inner and 

 ventral one of these two cords (Figs. 8 and 9, cc) is the commis- 

 sure that joins the two cerebral ganglia. As the ganglia lie 

 some distance ventral to the oesophagus, this commissure forms a 

 long loop that passes dorsally around the oesophagus just pos- 

 terior to the mouth. The outer and posterior of the two large 

 cords that leave the anterior end of each ganglion is the anterior 

 pallial nerve (Figs. 6, 8 and 9, apii). 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 circumpallial nerve (cpii) that fol- 

 lows along the margin of the mantle near the bases of the ten- 

 tacles and eyes. The circumpallial nerve will receive attention 

 later. 



Between the points of origin of the cerebral commissure and 

 the pallial nerve, a small nerve (Figs. 8 and 9, /;/) 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. 9, cpc] and bears a ganglionic swelling on its outer side 

 very near the pedal ganglion. 



In the acute angle formed by the surface ot the cerebral 

 ganglion with the cerebro-pedal connective, a small nerve (otii), 

 the otocystic nerve, leaves the ganglion to be continued around 

 the dorsal surface of the cerebro-pedal connective to the otocyst 

 of the same side. 



