220 NERVOUS SYSTEM — VISCERAL GANGLIA. 
and composed of several distinct, secondary gangdionic enlargements, 
known as the pleural, the pallial or visceral, and the abdominal 
ganglia, which are developed upon the conrse of the commissure, 
which connects together the moities of 
the pleural ganglia and would a})pear to 
correspond with the pleuro-visceral cords 
of the Isopleura, they innervate the 
circulatory, the reproductive and the 
excretory organs. 
The Pleural constituents of the visceral 
centre are the most anteriorly placed, and 
it is naturally with them that the cerebral 
and pedal connectives are joined. They 
are exceedingly unstable in position, not 
only in reference to their degree of 
approximation with the associated ganglia 
Fig. 433.— Visceral ganglion cells • 
with their nerve prolongations, from ot tlieir SpeCUll gTOUp, but tO Otliei’ CeiltreS, 
Unio piciortim (L.), X 500 (after i n • 
Rawitz). as they may fuse into one mass with 
their own centre, or become intimately fused with the cerebral or 
with the pedal ganglia, and widely separated from their true position 
and associations; they chieHy innervate the mantle, the coluniellar 
muscle, and the body wall behind the head. 
The Pallial, parietal, intestinal or visceral ganglia, as they are 
variously termed, have been supposed to represent, in a concentrated 
form, the many nerves supplying the gill leaflets in the Isopleura ; 
they innervate the respiratory organs, the osphradia and the mantle 
generally, and are developed upon the pleuro-abdominal commissures, 
and when distinct from the abdominal centre divide them each into 
an anterior Pleuro-pallial commissure, uniting the pleural and pallial 
ganglia, and a posterior Pallio-ahdominal commissure connecting the 
pallial with the Abdominal ganglion, the latter ganglion terminates 
the nerve loop and more especially innervates the genital gland and 
viscera, and although now usually apparently single, has probably 
arisen by fusion from a primitively paired condition. 
In the Streptoneura the visceral ganglia are often much scattered 
and a considerable distance apart, and owing to the rotation of the 
visceral dome, described at page 206 et seq., tlie organs originally 
occupying the right side of the mantle cavity at the rear of the animal 
have become transferred to the left siile above the neck, and the 
