NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 55 



taining nuclei." Among the early developmental phases of the Ony- 

 chophora, however, we cannot fail to note two important likenesses 

 to annelid development. The first is the elongation of the blastopore 

 on the ventral surface of the embryo as it occurs in Peripatopsis 

 capensis (fig. 22 A, B), followed by the closure of its median part 

 (C), finally leaving only the persistent oral and anal apertures at the 

 two extremities (D). We have here evidently a condition even more 

 generalized than in the annelids, in which the anus is usually a secon- 

 dary perforation. The second suggestion of annelid development, 

 shown in several onychophoran species, is the forward growth of the 

 mesoderm as bands of cells generated from a proliferating area of 

 the blastoderm situated behind the blastopore (fig. 22 A, B, C, 

 E, MsT). The mesoderm is, therefore, a teloblastic product, though 

 it is not possible to identify in the generative area a primary pair of 

 teloblastomeres. It would appear, however, that the onychophoran 

 mesoderm may not be entirely of teloblastic origin, for Sedgwick 

 (1887) finds that the forwardly growing bands in Peripatopsis 

 capensis are augmented by cells proliferated from the lips of the 

 blastopore along the lines where ectoderm and endoderm meet. The 

 later development of the mesoderm is unquestionably a strictly 

 homologous process in both the Annelida and the Onychophora, for 

 in the latter, as in the annelids, the primarily solid mesoderm bands 

 are first segmented corresponding with the body somites (fig. 22 B), 

 and then excavated by coelomic cavities (C, G). 



Beyond the early stages of cleavage and germ-layer formation the 

 course of onychophoran ontogeny is well standardized and gives a 

 good basis for comparison of the Onychophora with the Annelida on 

 the one hand, and with the Arthropoda on the other. It will be found, 

 however, that many of the irregular earlier processes of onychophoran 

 development are duplicated among the Arthropoda. 



THE NERVOUS SYSTEM 



The onychophoran nervous system includes a brain situated in the 

 head above and before the decurved anterior end of the stomodaeum 

 (fig. 32 A, Br), and two long, widely separated nerve cords (NC) 

 extending from the brain to the posterior end of the body, where they 

 appear to be continuous in an arc above the rectum. The cords are 

 connected by numerous ventral commissures (Com), and they give 

 ofif in each segment a series of dorsal nerves against the body wall 

 (fig. 24 B) and ventral nerves that go downward to the legs and 

 other ventral parts. Opposite the legs the nerve cords are slightly 



