55 



course of dissection. In addition to these, several smaller 

 twigs can be seen in sections, passing off from the ganglion 

 into the surrounding tissue, some of these most probably 

 innervate the labial palps. 



The parieto-splanchnic ganglion (Ga.sp., fig. 31, PI. VI.) 

 is best dissected by placing the animal (removed from the 

 shell), ventral surface uppermost, and cutting through the 

 wall of the lower siphon, the fused inner lamellae of the 

 internal branchiae, and the horizontal shelf formed by the 

 concrescence of the two ctenidia and the septum between 

 dorsal and ventral siphons. These parts are then folded 

 back and the ganglion and its nerves are fully exposed. 



This is the largest of the ganglionic centres, it is nearly 

 square in shape. Indications of its origin from paired and 

 lateral ganglia are seen in the two little anterior lobes, 

 from which the connectives take origin. It is covered only 

 by a single layered epithelium, and lies in a cavity; three 

 nerves leave it on each side. 



The cerebro-visceral connectives are the most delicate 

 of the nerves leaving the ganglion. Each connective 

 originates in one of the anterior lobes already referred to, 

 and at once plunges into the renal sinus and passes 

 through the latter, lying apparently free in the blood 

 space. The further course has been described above. 



The branchial nerves (N.br.) leave the ganglion from 

 the anterior corners, and pass along in the wall of the 

 afferent branchial vessels to the bases of the ctenidia. 

 Where the latter become free from the body-wall the 

 nerves bend round at a right angle, and pass backwards 

 to the tip. Following Duvernoy and Drost* I have 

 termed these the " branchial nerves," though I have been 



* Drost, K. Uber das Nerveusystemu.d. Siiinesepithelieu der Herznmschel 

 (Cardium edule), &c. Morphologisches Jahrbuch. Bd. XII., pp. 164 — 201 ; 

 Tafx., 1886—7. 



