the otocystic nerve, leaves the gangHon to be continued around 

 the dorsal surface of the cerebro-pedal connective to the otocyst 

 of the same side. This nerve will receive attention later. 



Posteriorly the cerebral-ganglia taper rather gradually into 

 the cerebral-visceral connectives which run along the sides of 

 the visceral mass very near the adductor muscle until the vis- 

 ceral ganglia are reached. 



The pedal ganglia lie very near each other (fig. 24, pg.), so 

 the commissure that connects them is short and iDroad and pre- 

 sents ordinary ganglionic structure. They are separated from 

 the cerebral ganglia only by a short interval, and lie anterior 

 and slightly ventral to them, some distance dorsal to the base 

 of the foot. They lie so near the surface that their color may 

 frequently be distinguished through the body wall beneath the 

 mouth. Two large nerves (fn.) leave each pedal ganglion to 

 be continued into the foot where they supply the muscles of the 

 foot and probably the byssal gland. The swellings on the cere- 

 bro-pedal connectives near the pedal ganglia have already been 

 described. The otocystic nerves which usually leave the cerebro- 

 pedal connectives near the pedal ganglia, in this form originate 

 directly from the cerebral ganglia near the point where the con- 

 nectives leave the ganglia. 



The visceral ganglia (figs. 15, 23, and 25. vg.) are by far the 

 largest and most complicated of the ganglia, and from them 

 nerves are sent to most parts of the body. They are situated 

 on the antro-ventral surface of the adductor muscle, nearly oppo- 

 site the external opening of the kidneys. They are imbedded in a 

 mass of connective tissue and are fused to each other so the 

 commissure that connects them is nearly as broad as the ganglia 

 themselves and shows ganglionic structure. The chief indica- 

 tion of the presence of a pair of ganglia is the arrangement of 

 the nerves that leave them, and of the celebro-visceral connec- 

 tives that join them. The ganglia are divided into very definite 

 regions, each of which is connected with definite bundles of 

 nerve fibers and, no doubt, has a particular function to perform. 

 I have not had time to make a detailed study of the structure 

 and nerve tracts of the ganglia, but I am satisfied that there is 

 much more complexity than is ordinarily attributed to the gan- 

 glia of lamellibranchs. The dorsal surfaces of the ganglia are 



44 



