10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



ducts. (In the male of Scolopendra only the right duct becomes the 

 functional genital exit.) By comparison with Onychophora, it seems 

 very probable that the ventral ampullae of the genital coelomic sacs 

 of Scolopendra represent rudiments of coelomoducts that formerly 

 opened on the bases of the segmental appendages. 



The coelomic origin of the gonads and gonoducts is shown also in 

 the embryonic development of various other arthropods, particularly 

 in the Chelicerata. Hence, it is to be inferred that in cases where the 

 gonads and their ducts appear first as solid cell masses, as with the 

 insects, that this condition is a secondary modification, and that the 

 lumen subsequently formed in the organs represents a part of the 

 coelome. In certain insects it has been observed that the solid strands 

 of cells that become the lateral genital ducts end with hollow ampullae. 

 These ampullae lie in the appendage rudiments of the seventh ab- 

 dominal segment in the female, or in those of the tenth segment in 

 the male; in some cases the duct branches to each of these segments 

 (see Heymons, 1892, 1895; Wheeler, 1893). When no appendage 

 rudiments are present the ducts usually end in the same respective 

 segments. 



Inasmuch as the coelomic sacs of insects are seldom perfect and 

 their cavities soon become confluent with the haemocoele, the evidence 

 from embryology in this case does not show that the genital ducts are 

 formed from entire coelomic sacs. The gonads and gonoducts appear 

 first as solid strands of cells in the splanchnic walls of the mesoderm. 

 It seems quite possible, therefore, that gonoducts may be formed by 

 the closure of channels in the coelomic walls leading from the germi- 

 nal areas to the coelomic outlets, just as the gonads are derived from 

 a series of coelomic pockets containing the germ cells. 



It is often said that the gonoducts are " modified nephridia ". The 

 assertion may be true with respect to some of the Annelida, but evi- 

 dently when applied to Onychophora and Arthropoda it is not a correct 

 statement of the facts. An onychophoran nephridium is a segmental 

 coelomoduct connecting a remnant of the lateral coelomic compart- 

 ment of the same segment with the exterior (fig. 2D, Nph). The 

 excretory head glands of Crustacea are organs of the same type of 

 structure. An onychophoran or chilopod gonoduct, as shown by its 

 development in Pcripatus and Scolopendra (fig. 3 B, D), represents 

 an entire coelomic sac together with its exit tube. Hence, if a 

 nephridium of the onychophoran type is once formed in a segment, 

 it cannot be converted into a genital duct. The onychophoran nephridia, 

 on the other hand, might be regarded as remnants of segmental gono- 

 ducts, if it be assumed that each coelomic sac primitively contained a 



