232 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
course of dissection. In addition to these, several sinaller 
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, Pl. 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 lamelle of the 
internal branchie, 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 Nervensystem u.d. Sinnesepithelien der Herzmuschel 
(Cardium edule), &c. Morphologisches Jahrbuch. Bd. XII., pp. 164—201 ; 
Taf x., 1886—7. 
