16 THE MOLLUSCA 
6. Nervous System and Organs of Sense.—In the nervous system of 
Mollusca a perioesophageal collar is normally present, of which the 
dorsal moiety is the cerebral and the ventral moiety the labial com- 
missure. From either side, at the junction of the two moieties of 
the collar, nerve cords distributed to the integuments take their 
origin. These latter are differentiated into a dorsal pair, innervat- 
ing the mantle, and a ventral pair, innervating the foot: they may 
be ganglionated throughout the whole of their extent, as in the 
pallial cords of Amphineura and the pedal cords of Amphineura 
and Aspidobranchs, or they may bear localised ganglia, known as 
the pleural and pedal ganglia respectively, near their origins. 
These two paired cords are connected by anastomoses, the most 
anterior anastomosis being always preserved and known as the 
pleuro-pedal connective. The pedal cords or centres are united by 
anastomoses ventrad of the digestive tract, the most anterior and 
at the same time the largest of the anastomoses being always 
retained as the pedal commissure. ‘The pallial cords are often 
united by an anastomosis dorsad of the rectum as in Amphineura, 
Cephalopods, Lamellibranchs, and various Gastropods. The 
. nervous system of Molluses is thus characterised by its oesophageal 
ring, from which issue four, originally parallel, tegumentary nervous 
cords (Fig. 19). 
The visceral organs are innervated firstly by trunks given off 
from the labial commissure. These trunks, uniting under the 
oesophagus, form an anterior or stomato-gastric visceral commissure, 
bearing on its course two ganglia which are situated near and 
partially innervate the buccal bulb and also the whole of the 
oesophagus and stomach. In some cases, e.g. the Cephalopods and 
certain Tectibranchia, the stomato-gastric commissure _ bears 
stomachal ganglia. The viscera are innervated, in the second 
place, by trunks issuing from the pallial cords and distributed to 
the circulatory, excretory, and genital viscera. In all Molluses 
except the Amphineura the two most important of these trunks are 
united below the digestive tube, thus forming an infra-intestinal 
loop or “ visceral commissure,” provided with one or more ganglionic 
centres (Fig. 19, v.g). These two visceral loops, the stomato-gastric 
and the visceral properly so called, are generally united together 
by anastomoses (Cephalopods and Gastropods). 
There are, therefore, three kinds of nerve-centres in Molluses : 
(1) sensory centres, represented by the ganglionated cerebral 
commissure or differentiated cerebral ganglia; (2) tegumentary 
centres, represented by the pleural and pedal cords or ganglia ; 
(3) the visceral centres, represented by the stomato-gastric and 
the visceral loop properly so called. 
The nerve-centres consist of a superficial portion, made up of 
ganglion cells, and a central fibrillar portion which is almost 
