lOG PROCEEDINGS OF THE ACADEMY OF [Feb., 



opening or fenestra. Just beyond this point, in the posterior part of 

 the first trunk segment, each commissure gives off a large nerve laterad 

 to the ventro-lateral longitudinal muscle, which it accompanies for an 

 indeterminate distance. It was successfully traced, however, only 

 as far as the middle of the second trunk segment. This muscle nerve 

 is flat and embedded in the hypodermis throughout its course, lying 

 just beneath the muscle. It rapidly diminishes in thickness as it 

 passes caudad, and for this reason could not be traced farther with 

 certainty. At the point where the muscle nerve is given off ganglion 

 cells reappear, and the circumoesophageal commissures may be con- 

 sidered to have passed into the lateral nerve cords. The latter, com- 

 posed of nerve fibres longitudinally disposed, remain buried in the 

 hypodermis while traversing the trunk to the posterior portion of the 

 last segment. Here they end, after giving off a branch laterad to 

 the body wall. They are situated apart, throughout their length, at 

 an average distance of about one-half of the diameter of the body, or, 

 as seen in cross sections of the trunk, about 60 degrees of the circle. 

 They are thus slightly mesad of the ventro-lateral longitudinal muscles. 

 In the second trunk segment and the anterior half of the third the 

 ventral cords broaden out greatly, having a rather irregular contour 

 on their mesial side, and are united by three transverse commissures., 

 These commissures, broad where they join the longitudinal nerve 

 cord, become so narrow at the midline that their continuity can only 

 be demonstrated in good transverse sections (fig. 9). Above, below, 

 and laterad of the nerve cords in this region of the body, but within 

 the hypodermis, are numbers of ganglion cells (fig. 9, g.c). The cell 

 bodies of these latter cannot be distinguished from those of the sur- 

 rounding hypodermis cells; their nuclei, however, are readily distin- 

 guishable, being precisely like those of the ganglion cells of the brain, 

 except as to size, ranging from 1.12 /z x 1.87 /« to 1.80// x 2.25 ,m. Their 

 average size is therefore nearly one-third smaller. These cells together 

 form a long and diffuse ganglion, extending from the anterior of the 

 second trunk segment to the middle of the third. At the latter point 

 the ventral nerve cords rather suddenly contract, almost immediately 

 widening again, and in the posterior half of the third trunk segment 

 are again united by two commissures, situated apart at a distance of 

 ca. 15 fi. Ganglion cells are also present here, forming a second pair 

 of ganglia; the number of ganglion cells is, however, much less than in 

 the preceding, and the second pair of ganglia are conseciuently of much 

 less extent (fig. 9). The third pair of ganglia are similar to the second 

 pair, but are composed of still fewer cells, and situated between the 



