244 MARTIN F. WOODWARD. 



the dorsal aud ventral roots of a vertebrate spinal nerve. In 

 addition to these large latero-ventral nerves there are present 

 certain small nerves, which apparently arise from the pleural 

 portion of the cord and pass to the dorsal pedal muscles. 

 When, however, we remember that tbe distinction between 

 these two portions of the cord is practically only an external 

 one we shall probably be right in concluding that all the 

 nerves derived from these cords are connected with both 

 subdivisions. 



In connection with the apparent separation of the pleuro- 

 pedal cords into two distinct portions by means of a longi- 

 tudinal groove it is interesting to note that Haller' had 

 already come to the conclusion that this groove has no mor- 

 phological significance; thus he found in other Rhipidoglossa, 

 as I have found in Pleurotomaria, that transverse sections of 

 this cord failed to reveal any line of separation between the 

 pleural and pedal portions of nerve tracts running from one 

 into the other. 



Visceral Commissure. — As suggested by Bouvier and 

 Fischer from the study of an imperfect specimen, PI euro - 

 tomaria exhibits a typical streptoneurous condition in its 

 visceral loop (figs. 6, 27, and 28) ; but at the same time this 

 mollusc is most peculiar among the Diotocardia in the point 

 of origin of its visceral nerves. 



If the cerebro-pleural connective on either side of the 

 body be examined, it will be seen that between its origin 

 from the cerebral ganglion and its fusion with the pedal 

 system it gives rise to a very large nerve, whose relations at 

 once identify it with the visceral nerve, that on the right side 

 being the supra- and that on the left the sub-intestinal nerve. 



As already mentioned, the points of origin of these nerves 

 appear, after the removal of the thick nerve-sheath, slightly 

 orange-coloured, owing to the presence of a considerable 

 number of nerve-cells which are continued, though in smaller 

 numbers, from this point up to the branchial ganglion. The 



I " Uiitersuchuiigea liber marine Rliipidoglossen. II. Textus des Central- 

 uerveusj&tem uiid seiner Hiillen," * Morpli. Jalirb.,' Bd. xi, 1886. 



