THE LAMELLIBRANCHIA 235 
(Fig. 215, ¢), and in exceptional cases a little ganglion at about the 
middle of its length on either side, at the point of origin of 
the branchial, pallial, or visceral nerves. These additional ganglia 
occur in Dreissensia and in some other genera of Eulamellibranchia, 
viz. the Unionidae, Cardium, Lutraria, Mya, and Solen. Each of 
the three chief pairs of ganglia presents the same general 
characters in the entire class, and special characters in certain 
forms. 
The cerebro-pleural ganglia are supraoesophageal and are 
situated above the buccal orifice ; generally on the posterior face 
of the anterior adductor muscle when this exists (Figs. 230, 251, ¢.9). 
In the Solenomyidae alone do they occupy a more posterior position 
(Fig. 231, c.g). In the Protobranchia, and in MJactra corallina and 
Venus, the cerebro-pleural ganglia are attached to one another, but 
in all other forms they stand 
apart. They innervate the labial 
palps, the anterior adductor 
muscle, and the anterior part of 
the mantle, and send fibres to the 
osphradia and otocysts. 
The pedal ganglia are situated 
in the mass of the foot at a 
greater or less distance from the 
cerebral centres ; they are always 
attached to one another (Figs. 
245.95; 215, 6), When—the 
foot is atrophied they become 
more and more reduced—e.g. in 
Teredo (Fig. 195, p.g), Ostraea, 
Pecten (Fig. 215, C)—and the 
pedal connectives become very 
short. 
The visceral ganglia are situ- 
ated some distance behind the 
posterior adductor muscle in all Central nervous system. <A, of Teredo; 
B, of Anodonta; C, of Pecten. a, cerebral 
the Protobranchia (Fig. 231, v.g), ganglion-pair; , pedal ganglion-pair; ¢, 
: see : visceral ganglion-pair. (From Lankester 
but in other Lamellibranchia they after Gegenbaur.) : 
are to be found on the ventral 
face of this muscle (Figs. 188, 218, 219, ete., v.g), except in Thracia, 
in which they are in front of it, and in some highly specialised 
forms in which they are again behind it, as, for example, in Pholas, 
and particularly in Zeredo, in which the posterior adductor is 
shifted forward (Fig. 195, v.g). The visceral ganglia are generally 
superficial, and barely covered by the tegumentary epithelium 
(Figs. 188, 236), but in Lima they are somewhat deeply embedded 
in the visceral mass. The two ganglia are primitively distant 
ees 
— 
Fic. 215. 
