2o8 Journal of Comparative Neurology, 



branching fibers the ganglion cells, forming a loose fibrillar cap- 

 sule around each of the latter ; continuations of these fibers 

 along the axis cylinder produce a sheath of Schwann. Another 

 layer of neuroglia cells lie just on the periphery of the fibrous 

 core, and send their branches into the latter. The only fibrils 

 within the nervous system are those of the neuroglia and hence 

 Burger, who described the axis cylinder as a dense fibril, prob- 

 ably mistook the neuroglia fibrils for axis cylinders. 



Ganglion cell i occurs only in the dorsal lobe of the brain ; 

 it is the smallest and probably sensory. 



Ganglion cell 2 occurs on the ventral surface of the ventral 

 brain lobe, as well as along the lateral nerve chords. These 

 cells are arranged in radial clusters ; and in the lateral chords 

 these clusters have a regular alternating arrangement. 



Ganglion cell 3 occurs on the median surface of the brain 

 lobes, and, more sparingly, in the lateral chords. These are 

 large, pyriform cells, and it is on them that the structure of the 

 axis cylinder may be best determined. 



Ganglion cells 4, or the neurochord cells, have been found 

 by me only in Cerebratidus, but by Biirger also in Langia, Pjvs- 

 adenoporus and Drepanophorus. (Biirger found in the two latter 

 genera only a single pair of these cells, situated in the brain ; 

 and in Langia and Cerebratidus one pair in the brain, and a large 

 number along the lateral nerve chords). In Cerebratidus lacteus 

 I found the following distribution of these giant cells. There 

 are 3 pairs in the ventral lobe of the brain, situated one behind 

 the other. On the nerve chords there are none in the oeso- 

 phageal region ; behind the latter region they are found again, 

 and become more numerous towards the distal end of the 

 chords ; near the proximal end of the chord they are more nu- 

 merous on the dorsal surface of the chord, but distally this po- 

 sition is reversed. They are arranged on the chords without 

 regularity, and there is no symmetry in the arrangement on the 

 two surfaces' of the same chord, nor on the two sides of the 

 body. Zones where they are relatively numerous alternate 

 with those where they are relatively scarce. About four-fifths 

 of a worm six inches in legth was serially sectioned, giving the 



