240 Records of the Indian Museum. [Vol. XXII, 



anastomose in a series of loops and at certain points of the 

 network so formed slight swellings can be detected, which pro- 

 bably- correspond to local collections of ganglion cells. From the 

 posterior surface of the pedal ganglia and the pleuro-pedal commis- 

 sure several nerves pass backwards and enter the muscles of the 

 foot. Each parietal ganglion gives rise to two nerves. The most 

 anterior and smaller of the two is the mantle nerve, and the larger 

 and more posterior is the parietal nerve. The two parietal nerves 

 pass backwards and form a figure-of-eight loop in the visceral 

 hump. Each nerve crosses over to the opposite side of the body 

 from which it originated, and having done so sends off a lateral 

 branch which anastomoses with the mantle nerve of that side, thus 

 forming the ' dialyneural ' connection. 



The right parietal nerve, or supra- intestinal nerve as it is 

 called, passes obliquely across the upper aspect of the oesophagus 

 just behind the buccal mass. In this portion of its course it is 

 closely connected to, and surrounded by the branching follicles of 

 the salivary glands. Having reached the left side it gives off a 

 large branch, the anterior branchial nerve, which passes to the left 

 and, having given off a small branch to join the mantle nerve, 

 breaks up into a number of fine branches which suppl}' the 

 anterior region of the gill and the osphradium. At the point where 

 the anterior branchial nerve arises from the supra-intestinal nerve 

 a slight swelling is to be seen, this is known as the supra-intestinal 

 ganglion. From this point the nerve passes up beneath the floor 

 of the branchial chamber on the left side of the oesophagus. 

 During this part of its course it gives off a series of fine branches 

 to the gills and finally, at the extreme apex of the branchial 

 cavity, a considerably larger branch, which soon subdivides into 

 smaller twigs, is given off to the upper part of the gill. 



The left parietal or sub-intestinal nerve crosses over from 

 left to right below the oesophagus. It then diverges somewhat to 

 the right and gives off a branch which again subdivides ; one twig 

 passes forwards and outwards to join the mantle nerve of the 

 same side and the conjoined nerve so formed supplies branches to 

 the outer wall of the syphon and the terminal portions of the 

 excretor}- and genital ducts and the anus. The main nerve 

 continues backwards above the columellar muscle at some distance 

 from the oesophagus and gives off a series of branches to the mantle 

 roof and its dependent structures. At the apex of the branchial 

 chamber both nerves are continued up for a short distance in the 

 floor and outer wall of the pericardium and then unite to form a 

 loop, in front and to the inner side of the U-shaped bend of the 

 oviduct in the 9 . At the apex of the nerve loop the nerve is 

 slightly swollen and is known as the visceral ganglion. From it a 

 series of branches arise which supply the neighbouring viscera. 



