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, c.p.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 Dentalium (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 



to 



cm 



FIG. 214. 



Nervous system of Nucula nucleus, viewed from the left side, and a little ventrally. ce.g, cere- 

 bral ganglion ; c.n, cerebral nerve ; c.p.c, cerebro-pedal connective ; o.n, otocystic nerve ; o.o, 

 otocystic orifice ; os, ospliradium ; ot, otocyst ; pa.n, pallial nerve ; p.g, pedal ganglion ; pl.g, 

 pleural ganglion ; pl.p.c, pleuro-pedal connective ; vl.c, visceral commissure ; vl.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, r.o), 

 and bears on the posterior part of its course a large pair of ganglia 



