412 University of California Publications in Zoology. [V L - 6 



a ring surrounding the canal. Lonnberg also differs from Spen- 

 cer and Monticelli in placing the posterior commissure "ven- 

 trally. " The apparent contradiction in these results is due to 

 incomplete rather than erroneous observation. 



The longitudinal nerve stems or lateral connectives (long. n. 

 st., pi. 39, fig. 42) lying in the dorsal half of the central region 

 of the body, run from the anterior to the posterior commissure. 

 They are separated from each other by one-third of the width 

 of the body in the anterior quarter of the body, but spread 

 further apart at the level of the birth pore, lying near the lateral 

 margins of the uterus. In the region of the ovaries they ap- 

 proach the median line again. Each stem gives off two sets of 

 branches, one in the sagittal and the other in the horizontal plane. 

 These branches are heaviest and most profuse in the regions of 

 the ovaries and the testes. 



The acetabular nervous system consists, as elsewhere in the 

 body, of a central and peripheral portion. The peripheral system 

 comprises eight longitudinal strands with many anastomosing 

 branches. These strands innervate the peripheral muscle layers 

 of the body, and the homologous muscle layers of the acetabulum. 

 They come into relation with the central nervous system by 

 means of the anterior ring in the margin of the acetabular open- 

 ing (acet., pi. 39, fig. 42.) 



The central system includes an anterior bridge commissure, 

 lying just in front of the posterior margin of the acetabulum on 

 its dorsal surface ; a pair of anterior lateral stems with branches ; 

 and an anterior nerve ring (pi. 39, fig. 42). The commissure 

 forms a bridge between the two longitudinal nerve stems. There 

 is a ganglionic enlargement of the lateral stem at a point where 

 it is joined by the bridge commissure. The commissure is in the 

 shape of an arch, enlarged at the ends and smaller in the middle 

 of its course. Its mass with respect to the rest of the body varies 

 widely, perhaps with the state of contraction of the animal ; it is 

 fairly constant with respect to the nervous system as a whole. 

 No branches are given off from the commissure itself. The mar- 

 ginal ganglionic knots are about double the diameter of the 

 lateral stem (ant. br. comm., pi. 39, fig. 42). They are enlarge- 

 ments of the lateral stems, beginning at the point where the 



