72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



the body wall. Above the diaphragm on each side of the blood vessel 

 are masses of small individual cells, probably "nephrocytes." The 

 circulatory system of the Onychophora thus resembles that of the 

 Chilopoda and the Hexapoda in the simplicity of its structure. Since 

 many of the arthropods, in common with the annelids, have a highly 

 developed blood vascular system, it would seem probable that the 

 simpler forms represent reductions from a more elaborate primitive 

 system such as that of the Annelida. 



THE NEPHRIDIA 



The nephridialike excretory organs of the Onychophora are paired 

 segmental structures usually present in all the somites between the 

 somite of the oral papillae and that of the genital ducts, though they 

 may differ much in size and in the relative development of their parts. 

 They lie in the lateral compartments of the definitive body cavity at 

 the bases of the legs (fig. 29, Nph), and open externally in grooves 

 on the ventral surfaces of the leg bases (figs. 29, 31 F, 33 A, Npr), 

 except those of the fourth and fifth pairs, which in most species open 

 at the bases of the distal rings of the legs (fig. 33 C). 



A well-develoi>ed onychophoran nephridium consists of five distinct 

 parts (fig. 33 A) : First, beginning externally, is a short outlet duct 

 {Nd) ; second, a bladderlike enlargement, or reservoir {Bl) ; third, 

 a tubular canal {Cnl) varying in length and usually coiled; fourth, 

 a funnel-shaped enlargement of the inner end of the canal {Fun) ; 

 and fifth, a thin-walled end-sac (ESc). The walls of the funnel (B) 

 are relatively thick and are histologically different from the rest of 

 the canal ; they are clothed with long vibratile cilia directed toward 

 the nephridial exit (see Dakin, 1920, Cuenot, 1926, Zilch, 1936). 



The funnel and the canal of an adult onychophoran nephridium 

 are comparable with an entire metanephridium of the annelids ; the 

 end-sac is a remnant of the coelomic sac of the embryonic somite. 

 The opening of the nephridial funnel into the end-sac, therefore, is 

 the nephrostome (fig. 33 B, Nst). The canal is developed in the 

 embryo as an exit duct of the coelomic sac, formed by the union of 

 a ventral diverticulum of the sac (fig. 32 C, c) with a tubular in- 

 growth (d) from the ectoderm of the same segment mesad of the 

 leg rudiment (D). The primitive function of the coelomoducts un- 

 doubtedly was the discharge of excretory products and, in the 

 genital segments, of the gametes. Embryonic coelomoducts occur, 

 according to Evans (1902), in connection with the coelomic sacs of 

 the antennae (fig. 27 D, rf), and in all the postoral somites except 



