48 WALTEE K. FISHER, 



places, and by two nerves and also with the pleural ganglion direct, 

 by a third, the pleuro-osphradial connective. 



The osphradial, ctenidial, and parietal nerves. The 

 logical method perhaps would be the description of the visceral com- 

 missure and its ganglia before mentioning the osphradial nerves, which 

 arise from ganglia in this commissure — namely from the supra- and 

 sub-intestinal. It seems desirable, however, to finish all those nerves 

 which have any connection with the mantle, or other portions of the 

 body wall, and defer the description of the visceral commissure and 

 its ganglia until the visceral nerves are considered. 



The right osphradial nerve arises from the subintestinal ganglion, 

 and is considerably smaller than that of the left side. It passes out- 

 ward and usually slightly forward (depending largely on whether the 

 gonad is very large and encroaches on the head cavity). A tiny nerve 

 accompanies it, in front, and apparently joins the complex of nerves 

 under the osphradium. For a short distance the right osphradial 

 nerve is free, but sooner or later enters the dorsal wall of the head 

 cavity, and just before reaching the osphradium sends a prominent 

 branch backward, the posterior right parietal (R.Par). This nerve 

 lies in the muscular floor of the rectal portion of the right kidney, 

 and as already described joins the posterior branch of the right an- 

 terior pallial, within the spindle muscle. Just after leaving the os- 

 phradial nerve it gives off' one or two fine branches ventrally into the 

 muscular wall of the body. I have been unable to trace either of 

 these to the pleural ganglion, so that, on the right side, the pleuro- 

 osphradial connective does not seem to exist. 



Before entering the spindle muscle to join the pallial nerve, the 

 right posterior parietal gives off a delicate branch (Nph.N) which 

 lies in the wall of the nephridial blood sinus, and follows this sinus 

 back, close beside the inner edge of the attachment surface of the 

 spindle muscle, nearly to the hind end of the animal. Here it is 

 seen to leave the sinus and pass into the vein-net of the right kidney, 

 nearly to the median line. It may or may not join that (Nph.N 1 ) of 

 the left side. The nerve is so fine that it has been impossible to tell 

 whether branches are given off, along its length, to the vein-net, or 

 into the spindle muscle. 



The main osphradial nerve of the right side then passes behind 

 the osphradium and is here strongly ganglionic. On reaching the 

 spindle muscle it turns abruptly forward (R.Par'), follows the edge 

 around, being imbedded in the pallial hood, and joins the anterior- 



