84 OKIIAMANDAL MARINE ZOOLOGY REPORT 



The connectives follow the normal arrangement and number; a pair of long and 

 stout oerebro-viscera] connectives (C.v.con.) join the cerebral and parieto-splanchnic 

 ganglia, while a very short pair of similar cords (C.p.con.) link the cerebral and pedal 

 centres. 



In the living condition the ganglia and the stouter portions of the principal nerves 

 are bright orange yellow in colour, and since the ganglia are situated superficially this 

 enables them to be found with comparative ease. 



Tlie cerebral c/ancjlia. If the labial palps be folded back from the middle line, the 

 right ganglion (fig. 23, Cer.g.r.) may readily be seen lying superficially as a large and 

 conspicuous pale orange- coloured mass close to the united bases of the right pair of 

 palps, at a point about midway between the mouth and the base of the foot. The left 

 ganglion lies at about the same horizon, at the bases of the left palps (Cer.f/.l.) ; it is 

 distinctly smaller than the right, and as it does not lie so close to the surface it is 

 somewhat difficult to locate. A long U-shaped nerve cord, the cerebral commissure 

 (figs. 10, 11 and 13, Ccr.com.), passing dorsally over the anterior end of the oesophagus, 

 unites the right and left ganglia; in consequence of the considerable distance from tin- 

 oesophagus at which the latter are situated, this cord is unusually long. 



Ventrally, each ganglion gives off two other connectives, one short and delicate, 

 the cerebro-pedal connective (C.p.con.), to the pedal ganglion, the other very long and 

 stout, the cerebro-visceral (C.v.con.), to the parieto-splanchnic ganglionic mass. 



Each cerebro-visceral connective runs in a ventral direction along the base of the 

 palps of its own side to the anterior extremity of the gills, where it leaves the visceral 

 mass and passes, still pursuing a ventral course, to between the anterior surface of the 

 adductor and the posterior of one of the renal tubes. Its further course follows that 

 of the renal tube of its respective side along which it runs on the postero-lateral 

 aspect to junction with the dorsal margin of the parieto-splanchnic ganglion. 



In addition to the supra-cesophageal commissure and the connectives to the pedal 

 and parieto-splanchnic ganglia, each cerebral ganglion gives off anteriorly a common 

 anterior pallia! nerve (N.a.p.), which passes into the mantle at its antero-dorsal corner. 



Other small nerves also arise from the cerebral ganglia to innervate the pallia! 

 palps and adjacent tissues. 



Pedal Ganglion. The two primitive components of this nerve centre are so 

 closely approximated that they cannot be clearly resolved except by microscopic 

 examination of serial sections. The dual nature is then readily made out and it is 

 seen that the- lateral mass which represents the original right ganglion is much the 

 larger of the two ; no trace of commissure persists. The ganglion is situated medianly, 

 on the dorsal side of the base of the foot. In addition to the cerebro-pedal 

 connectives given off at its upper and external corners, a stout pedal nerve arises from 

 this ganglion and ] passes directly into the foot, where it can be traced to the tip, giving 

 off small twigs along its course. 



