1 6 THE MOLLUSC A 



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 Molluscs 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 Molluscs 

 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 Molluscs : 

 (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 



