236 GILMAN A. DREW. 



Posteriorly the cerebral ganglia taper rather gradually into the 

 cerebro-visceral connectives, which run along the sides of the 

 visceral mass very near the adductor muscle, until the visceral 

 ganglia are reached. 



The pedal ganglia lie very near each other (Fig. 9, pg), so the 

 commissure that connects them is short and broad and presents 

 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 fre- 

 quently be distinguished through the body wall beneath the 

 mouth. Two large nerves (fit) 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. 6, 8 and 10, 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 

 antero-ventral surface of the adductor muscle, nearly opposite the 

 external openings of the kidneys. They are imbedded in a mass 

 of connective tissue and are fused to each other, so the commis- 

 sure that connects them is nearly as broad as the ganglia them- 

 selves and shows ganglionic structure. The chief indication of 

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

 that leave them, and of the cerebro-visceral connectives 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 ganglia of lamellibranchs. 

 The dorsal surfaces of the ganglia are quite smooth, but when 

 seen from the ventral surface (Fig. 10) the regions that are indi- 

 cated in the figure are always visible. On each cerebro-visceral 



