•JIG NERVOUS SYSTEM — PELECYPODA. 
cerobro-pleural, the pedal and the visceral or parieto-splaiichuic centres, 
which are joined together by connectives and connnissnres forming two 
nerve loops encircling the alimentary 
canal, the long and important cerebro- 
plenro-visceral commissures, which ex- 
tend almost the whole length of the 
body, containing many ganglion cells. 
The efi'erent nerves, which give rise 
to the contraction of the adductor 
muscles, always arise from the ganglia 
in their immediate proximity, but the 
inhibitory nerve tibres, which produce 
their relaxation and alloAV the shell to 
gape owing to the elasticity of the liga- 
ment, are stated to arise exclusively 
from the cerebral ganglia. 
The buccal or stomato-gastric and 
the j)leural ganglia and the pleuro- 
pedal connective, wddch were formerly 
considered as peculiarly characteri.stic 
of the (xastropoda, are also really 
ju'esent in Pelecypods, but are usually 
so intimately fused with the neigh- 
bouring parts of the nervous system 
that their combination can only be 
satisfactorily demonstrated by the 
microscoi)ical examinations of suitable sections. 
The Cerebral or Sensory ganglia, sometimes also termed the 
liuccal, sui)ra-cesophageal or post-oesophageal ganglia, originate as a 
paired epihlastic thickening within the velar area and are sometimes 
fused together, but when separate the constituent parts are joined by 
commissures above the oesophagus, by the siile of which these ganglia 
are placed. In adolition to innervating the head and its sensory 
organs, they give off branches to the anterior body wall ami to the 
otocysts, and are the chief point of convergence of the afferent nerves, 
lint vary in development in correlation with the functional importance 
of the organs innervated, and are joined by connectives with the 
sub-oesophageal centres forming nerve loops which surround the 
alimentary canal. 
of a Pelecypod, Anodonta cygnea (L.). 
a. ad. anterior adductor nerves; a.itt. 
anterior mantle nerves; h.n. branchial 
nerves ; c.c. cerel)ral commissure ; c.pl.g. 
cerebro-pleural ganglia ; c.pl.p.c. cere- 
bro-pleuro-pednl connectives ; c.p/.7>.c. 
cercbro-pleuro-visceral commissure ; /.«. 
rectal and cardiac nerves ; m.n. median 
mantle nerves ; of. otocyst ; p, posterior 
mantle nerves ; p.n. pedal nerves ; r.ti. 
renal nerves ; s.n. nerves to siphonal 
tentacles ; x>.g. visceral ganglion ; v.p.n. 
nerves along kidney to hinder part of foot. 
