62 Journal of Comparative Neurology. 



oesophageal commissure and some are found in this branch even 

 after it has entered the brain itself. Each of these cells is plainly a 

 ganglion cell and from each a single process can be seen pass- 

 ing into the main dorsal branch of the commissure just mentioned 

 and then turning ventrad in this branch. I have not been able 

 to trace the further course of these axis cylinders, but Hamaker 

 ('98) states that they pass to the commissural ganglion and he 

 apparently considers it possible that they are the ganglion cells 

 which give rise to the " three giant fibers which traverse the 

 ventral cord throughout its entire length." Each cell is itself 

 enclosed in a fibrous capsule and its process or axis cylinder is 

 surrounded by a large sheath which is a prolongation of this 

 capsule. Retzius ('95) noted these ganglion cells in N. diversi- 

 color and considered it possible that they were concerned with 

 the innervation of the anterior eyes. Although Hamaker ('98) 

 considered that this could not possibly be true, yet he has desig- 

 nated each of these groups of ganglion cells as an "optic gan- 

 glion." Such a designation is not only incorrect in itself since 

 given to a ganglion having no connection with the eye except, 

 in some species, the accidental one of position, but also leads 

 to confusion since Racovitza ('96) has already designated as 

 optic ganglia the regions in the posterior brain which receive 

 the central fibers of the retinal cells. 



The anterior paired cephalic organs are situated in the pro- 

 stomium, one on either side, just at the angle which this part of 

 the head makes with the palps (Plate I, Fig. 28). In a surface 

 view of the prostomium taken from a worm injected with meth- 

 ylene blue, this organ appears as a deeply stained, ovoid body 

 apparently formed of very coarse fibers whose peripheral ends 

 branch in a twig-like manner just beneath the cuticula. Sections 

 reveal the fact that this ovoid is really formed of very large 

 bipolar cells which are mostly spindle-shaped (Plate I, Fig. 29). 

 The central processes from each group of cells form a small 

 nerve which passes directly to the anterior margin of the brain 

 slightly mesad and ventrad of the palp nerve. This nerve now 

 joins other nerves from the anterior part of the prostomium and 

 the common nerve thus formed can be traced along the lateral 



