74 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



nephriclia of the higher Annehda and the coelomic exits of the Ony- 

 chophora have been separately acquired and developed in each group. 

 On the other hand, there can be little doubt that the nephridial organs 

 of Arthropoda (antennal, maxillary, and coxal glands) are entirely 

 comparable with the onychophoran nephridia. 



THE ORGANS OF REPRODUCTION 



In the evolution of specific reproductive organs the Onychophora 

 are far in advance of any of the polychaete or oligochaete annelids ; 

 but the development and the definitive structure of the genital organs 

 are so closely parallel in the Onychophora and the Arthropoda that 

 we can scarcely question the probability of the genital systems in 

 these two groups having had a common origin. In fact, it is the 

 fundamental similarity in the genital system that would appear to 

 constitute the closest bond of union between the Onychophora and 

 the Arthropoda, and which most strongly suggests that the two groups 

 have been derived from a common progenitor. The germinal centers 

 of the Onychophora, as in the arthropods, are entirely enclosed in 

 gonadial sacs of coelomic derivation, and the gametes are discharged 

 through ducts whose lumina are continuous with those of the gonads. 

 An approach to a closed genital system is seen in the Oligochaeta in 

 the development of coelomic seminal vesicles containing the genital 

 outlet funnels, and a system as completely closed as that of the Ony- 

 chophora and Arthropoda is perfected in the Hirudinea ; but the 

 ontogeny of the organs in these several groups shows that there is 

 no possibility of the onychophoran-arthropod reproductive system 

 having been evolved from that of the higher annelids. 



The primary germ cells of the Onychophora become localized at 

 an early stage of embryonic development in the median dorsal parts 

 of the splanchnic walls of one or several posterior pairs of coelomic 

 sacs (fig. 34 A, Gnn). According to Evans (1902) there are four 

 embryonic genital somites in Eoperipatus weldoni, while Kennel 

 (1888) says the germ cells of Peripatus edzvardsi occur in but 

 one somite. Whatever the number of genital segments may be in 

 modern forms, we must suppose that the germ cells once occupied 

 most of the somites, for the early embryonic relation of the germinal 

 centers to the coelomic sacs is identical with the adult condition in 

 the Polychaeta, and undoubtedly means that in the primitive Ony- 

 chophora the gametes were discharged into the coelomic sacs (A, 

 Sps), and were liberated from the latter through the coelomoducts 

 (d). As we have seen, the upper parts of all the coelomic sacs between 



