174 WILLIAM F. ALLEN 



and fifth nerves. There is a nerve cell (N.C.) on a caudal branch 

 of the sixth dorsal sensory ramus a little above the median 

 dorsal cartilaginous bar, midway between two dorsal fin rays. 

 No peripheral nerve cells were found in connection with the 

 seventh and eighth spinal nerves. The dorsal sensory ramus 

 of the ninth nerve has a cluster of three peripheral nerve cells 

 opposite the base of a dorsal fin ray, and the caudal branch of 

 the dorsal sensory ramus of the tenth nerve has two cells oppo- 

 site the median dorsal bar and one cell some distance out in 

 the dorsal fin. Numerous scattered nerve cells appear in a 

 a plexus of sensory nerves that enter the last two spinal ganglia. 

 These cells are more abundant in the dorsal, than in the caudal 

 or ventral nerves. As was noted previously, there is one more 

 sensory nerve (sensory ramus dorsalis) on the opposite or left 

 side of this specimen. This nerve is shown in figure 2 as having 

 no well-defined spinal ganglion, but it has a number of isolated 

 nerve cells (N.C.) scattered along its course; some six or nu^re 

 are outside the neural arch and one within it, suggesting some- 

 what the arrangement found in Amphioxus. 



A reconstruction (fig. 4) of two spinal nerves from the region 

 of the posterior extremity of the caudal heart from another adult 

 Polistotrema series shows a number of scattered nerve cells 

 about the dorsal sensory rami in the neighborhood of the median 

 dorsal bar. They are more numerous on the first ramus than 

 on the second. In addition to the above mentioned cells, sev- 

 eral nerve cells are clustered about a cephalic branch of the first 

 dorsal sensory nerve; which branch runs in close proximity to 

 a blood vessel in the connective tissue of the dorsal fin. No 

 nerve cells were found in the ventral sensory rami of either of 

 these nerves. Attention should be directed to the fact that the 

 cells in the dorsal pole of the spinal ganglia of these nerves 

 (fig. 4, Sp.G.) are very diffuse and scattered as compared with 

 the cells of the central and ventral portions of the ganglia. 

 This arrangement of cells in the dorsal pole of the ganglia indi- 

 cates that the above mentioned peripheral nerve cells in the 

 dorsal sensory rami must have migrated peripherally from the 

 neural crest, suggesting a tendency in certain spinal nerves to 



