6(5 CEYLON PEAEL OYSTER REPORT. 



The cerebral ganglia are pre-oral or supra-cesophageal in position, and a nerve cord 

 or commissure, passing over the oesophagus, connects the two cerebral ganglia ; while 

 a single stout transverse cord the visceral commissure joins the two parieto- 

 splanchnic ganglia (fig. 18). 



The cerebro- visceral connectives surpass all the other commissural nerves in length. 

 Taking their rise at the posterior end of the cerebral ganglion, each passes backwards 

 and downwards, buried within the visceral mass, till it emerges opposite the upper 

 angle of the base of the font. Then it passes ventrally, overlaid by the renal sinus, 

 whose course it follows till, entering the tissue at the base ot the gills, it turns 

 slightly forwards still passing ventrally and ends in its respective parieto- 

 splanchnic ganglion (fig. 15). 



In addition to the supra-cesophageal cerebral commissures and to the connectives 

 passing to the pedal and parieto-splanchnic ganglia respectively, the- cerebral ganglion 

 of each side gives off anteriorly a stout nerve the anterior common pallial. This 

 passes forwards, bifurcating almost immediately. The outer branch (the external 

 pallial nerve) courses along the pallial edge, meeting and anastomosing with the 

 corresponding external pallial branch of the posterior common pallial trunk. The 

 labial palps and the otocysts are also innervated from the cerebral ganglia. 



The cerebro-pedal connectives arise from the posterior and outer sides of the 

 cerebral ganglia, and run downwards within the visceral mass and just behind the 

 levator muscles of the foot to the pedal ganglion. They lie close together in their 

 course, and about midway each gives off a nerve, passing posteriorly into the visceral 

 mass. 



The double nature of the pedal mass is distinctly seen in sections (Plate IX., figs. 1G 

 and 16a). Three principal nerves arise from the pedal ganglion to innervate the foot 

 and byssal gland (Plate VI., fig. 16). One, the dorsal (or superior) pedal nerve, given 

 off from the upper anterior part, passes along the dorsal region of the foot to the tip 

 of this organ, throwing off twigs as it goes. Its terminal portion innervates the 

 locomotor or crawling portion of the foot. The second, the ventral (or inferior) pedal 

 nerve, arises immediately below the last described, passes forwards and downwards 

 and supplies the byssal groove and disc-pit. The byssal nerve is the third offshoot 

 from the pedal ganglion ; it comes away from the ventral end and passes direct to the 

 byssal gland, dividing into numerous branches. 



Each of the visceral or parieto-splanchnic ganglia receives from above the stout 

 cerebro-visceral connective, the two ganglia being themselves united by a single 

 transverse visceral commissure. In addition to these connecting nerves, each 

 ganglion gives off two stout distributory nerves (Plate VI., fig. 18) an anterior 

 lateral (the branchial, n.br.) and a posterior (the pallial, n.pal.). Each branchial nerve 

 leaves the ganglion at the anterior lateral corner, turns down at once into the base of 

 the gills, and then passes backwards to the posterior tips following the -course of the 

 afferent vessel. The posterior common pallial nerves emerge from the posterior end 



