G. A. DREW ON YOLDIA LIMATULA, SAY. 19 



branches sends a small nerve to the posterior adductor muscle and 

 another below the adductor muscle, to the postero-dorsal portion of the 

 mantle. The main nerve bends ventrally and is distributed to the 

 siphons. The other branch passes ventrally, beside the nerve just 

 described, sends a large nerve to the siphonal tentacle, on the side where 

 this tentacle is present, and is continued to supply the postero-ventral 

 margin of the mantle. Several branches are given to the posterior 

 expansion of the mantle. 



Each pedal ganglion gives rise to a nerve that runs directly to the 

 byssal gland, and to six or more nerves that supply the muscles of the 

 foot. Frequently one nerve, and sometimes several nerves, may run 

 up the cerebro-pedal commissure for some distance before issuing to 

 be distributed to the muscles of the foot. 



The nerve that supplies the otocyst issues from the cerebro-pedal 

 commissure but, judging from the direction of the fibres in the com- 

 missure, probably has its origin in the cerebral ganglion. 



OTOGYSTS. 

 FIGURES 4, 21 AND 22. 



The otocysts are rather large and, as Kellogg 5 has pointed out, 

 each contains a single large otolith, which plainly shows concentric 

 structure. The epithelial cells of the otocysts carry rather long and 

 slender cilia. In adult specimens there is a well-defined canal 

 connected with each otocyst. Pelseneer 11 describes this canal as 

 opening at the surface of the foot and gives a figure indicating the 

 position of its opening. 



I have found that, by the careful dissection of specimens preserved 

 in formaline, the cerebro-pedal commissures, with the otocysts, otocystic 

 nerves, and otocystic canals, all held together by connective tissue, can 

 be removed. Study of these preparations, of which I have made 

 about thirty, shows the otocystic canals to be cylindrical and of 

 uniform size for about three-fourths of the length of the otocystic 

 nerves, Fig. 21, ot., at which point there is nearly always a swelling. 

 A thin strand of fibrous tissue, probably connective tissue, is continued 

 beyond the swelling. At the distal end of the swollen portion of each 

 canal there is generally, probably always, a wall that can easily be 

 seen in dissected preparations, and verified in sections. This wall is 

 the rounded end of the otocystic canal. 



