234 THE LAMELLIBRANCHIA 
Lamellibranchia. In the Protobranchia alone are there four dis- 
tinct pairs of ganglia, cerebral, pleural, pedal, and visceral, in the 
adult (Fig. 214). In this group there is a pleural ganglion attached 
to the cerebral ganglion on either side, as is the case in the 
Scaphopoda and the “ epiathroid” Gastropoda (Figs. 182; 123, B). 
There are therefore two pedal connectives on each side (Solenomya 
and Nucula), namely, the cerebro-pedal and the pleuro-pedal con- 
nectives, which are free in the initial part of their course (Fig. 
214, cp.c, pl.p.c), but are fused together half-way towards the 
pedal centres in Nucula, and for nearly the whole of their length 
in Solenomya, as is the case in Dentaliwm (Fig. 182) and the 
Atlantidae. In all other Lamellibranchia the pleural centres are 
intimately fused with the cerebral, and there is only a single pedal 
peg pe pue vi. 4 
Fie. 214. 
Nervous system of Nucula nucleus, viewed from the left side, anda little ventrally. e.g, cere- 
bral ganglion ; ¢.n, cerebral nerve; c.p.c, cerebro-pedal connective ; 0.n, otocystic nerve 3 0.0, 
otocystic orifice; os, osphradium ; of, otocyst; pa.n, pallial nerve; p.g, pedal ganglion ; pl.g, 
pleural ganglion; pl.p.c, pleuro-pedal connective; vi.c, visceral commissure; vi.g, visceral 
ganglion. 
connective on each side (Fig. 215, B). Nevertheless, in all the 
Lamellibranchs belonging to different groups that have been studied 
from this point of view, a pleural ganglion distinct from the cerebral 
has been recognised in the course of development (Modiolarca, 
Dreissensia, Lasaea, Teredo). 
In all the Lamellibranchia the cerebral, or rather the 
cerebro-pleural, centres are connected with the pedal and visceral 
centres: thus there are two distinct commissures, the pedal 
and the visceral. In Mytilus, however, the visceral commis- 
sure is fused for a certain distance with the pedal connectives. 
The visceral commissure is always long, and issues from the pleural 
ganglia in Nucula and Solenomya, from the cerebro-pleural ganglia 
in other Lamellibranchia. It runs somewhat superficially right 
round the viscero-pedal mass, inside the renal orifices (Fig. 231, 7.0), 
and bears on the posterior part of its course a large pair of ganglia 
