NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS y^i 



the somite of the jaws. During embryonic development the coelomic 

 sacs of those segments that eventually contain nephridia become each 

 constricted into a dorsal section (fig. 32 C, a) and a ventral section 

 {b), which soon become entirely separate compartments (D). Except 

 in the genital region the dorsal compartments disappear; in the 

 nephridial somites the ventral compartments become much reduced, 

 but they retain their open connections with the coelomoducts, and 

 persist as the delicate end-sacs of the nephridia (fig. 33, ESc). 



Fun Cil 



Fun 



Fig. 22>- — Structure of onychophoran nephridia. 



A, diagrammatic transverse section of leg and nephridium of mature embryo 

 of Peripatopsis capensis Grube (from Sedgwick, 1888). B, inner part of nephrid- 

 ium of Peripatoidcs sp., showing ciliated funnel (Fun) with nephrostome open- 

 ing into coelomic end-sac (from Dakin, 1920). C, diagrammatic transverse sec- 

 tion of leg and nephridium of adult Peripatus tholloni Bouvier (from Fedorow, 

 1926). 



Bl, nephridial bladder ; Cil, cilia ; Cnl, nephridial canal ; ESc, coelomic end- 

 sac of nephridium ; Fmi, nephridial funnel ; NC, nerve cord ; Nd, nephridal duct ; 

 Npr, nephropore ; Nst, nephrostome. 



It is commonly held that the excretory organs of the Onychophora 

 are homologous with the annelid metanephridia (see Glen, 1919). 

 The simple development' of the canals as open ventral diverticula of 

 the coelomic walls (not of the septa), the direct opening of the canals 

 to the exterior on the same segment, and the occurrence of embryonic 

 coelomic ducts in the head, however, are all features distinctive of 

 the Onychophora. Considering, therefore, that there is little proba- 

 bility on other grounds that the Onychophora have been derived from 

 annelids having metanephridia, we may conclude that the open 



