150 A. B. DAWSON 



to the layer of longitudinal muscle fibers, and in this position 

 each gives rise to dorsal and ventral rami. The dorsal rami run 

 between the two muscle layers of the body wall as far as the 

 middorsal line, forming the dorsolateral portions of the so-called 

 nerve ring. From each dorsal portion there is given ofT first of all 

 branches to the intersetal tract and along the rest of its course 

 dorsally many small peripheral branches pass vertically or 

 obliquely toward the epidermis. 



The ventral rami run ventrally in the same relative position 

 as the dorsal ones, extend to the midventral line, and give rise 

 along their course to numerous peripheral branches. In this 

 way three main nerve rings, an anterior, middle, and posterior, 

 are established for each body segment. 



In referring to the nerve cells, their locations will be designated 

 in the terms of the preceding description and the direction of their 

 processes will be indicated by the phrases proximal, distal dor- 

 sally, distal ventrally, and peripheral, according as the processes 

 extend toward the ventral chain, toward the middorsal line in the 

 dorsal portion of a nerve ring, toward the midventral line in the 

 ventral portion of a nerve ring, and toward the epidermis in a 

 peripheral nerve. 



REVIEW OF LITERATURE 



Hesse ('94) dsecribed groups of ganglion cells in the course of 

 the nerves of the prostomium of the earthworm, but in the nerve 

 rings of the body segments he observed only one nerve cell. It 

 was bipolar. The brief statement by Hesse regarding ganglion 

 cells caused Langdon ('95) to reexamine the material she had 

 studied. As a result of the reexamination she discovered that a 

 series of alum-carmine preparations showed a number of ganglion- 

 cells in the nerve rings and gave in a foot-note the following 

 record for the first thirteen metameres: first metamere, none; 

 second metamere, one on its median nerve ring; third metamere, 

 one on its anterior ring and four on its posterior ring. In meta- 

 meres three to thirteen, two to eight ganglion cells were found 

 on every nerve ring. All the cells noted were of the bipolar type. 



