110 Illinois State Laboratory of Natural History. 



ventral portion of the Crt4om, in front of the ventral setae. 

 They terminate finally in the ventral vessel, on the same verti- 

 cal plane as that of their origin. The anterior arches are less 

 contorted, — the first, indeed, pursuing a nearly direct course 

 from above downward. This vessel is no larger than the others, 

 and is doubtless non-contractile. It is given off at the posterior 

 end of the first segment (suboesophageal), and on the same ver- 

 tical plane the ventral vessel takes its origin, — probably formed 

 by the union of these arches. This vessel is supported by a 

 vertical mesentery except in the anterior segments, where it is 

 borne at the middle of a delicate transverse membranous par- 

 tition, which disappears with the formation of the first dissepi- 

 ment. It is also valved, but imperfectly, at a considerable 

 distance behind the dissepiment. 



The cerebral ganglion is transverse, slightly convex in 

 front, and slightly three-lobed, the large anterior nerves going 

 off from the anterior lateral angles by bulbous processes. Gan- 

 glion cells are most abundant on the anterior and dorsal sur- 

 faces, the inferior posterior surface being nearly free of them. 

 Three pairs of nerves arise from the cephalic ganglion, the first 

 and second large and the third small. The first go outward 

 and downward from their origin to the cephalic wall ; the 

 second, arising just behind the first, pass directly downward : 

 and the third, springing from the lateral part of the dorsal sur- 

 face just before the origin of the commissure, pass directly up- 

 ward. The commissures send each five nerves to the wall of 

 the head, the four anterior arising in pairs, and the posterior 

 and largest, given off just before the commissures meet in the 

 sub(rsophageal ganglion, going singly outward. No branches 

 to the pharynx were detected. 



The subasophageal ganglion is transversely oval in front, 

 nearly cylindricfd behind, very richly cellular on the lower sur- 

 face, especially at the middle, and also ))osteriorly on the sides. 

 The four anterior ventral ganglia are closely approximated and, 

 including the suboesophageal, have but a single pair of branches 

 each. 



The ventral cord (PL VL, Fig. 5, & PI. VII., Figs. 6 & 7) 

 generally presents two elongate ganglionic swellings to each 

 somite, corres])onding to the two sets of lateral nerves arising. 



