12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



Other hand, true nephridia are never developed; but coelomoducts 

 (perhaps primitive genital outlets) may be formed in each segment of 

 the Onychophora, and it is these structures, together with remnants 

 of the coelomic sacs, that give rise to the segmental excretory organs 

 of Onychophora and Crustacea commonly termed " nephridia ", and 

 which form the definitive genital ducts of the Onychophora and 

 Arthropoda in particular somites. 



When the germ cells multiply and are extruded from the germaria 

 in a genital organ of the arthropod type, they are received at once 

 into the lumen of the mesodermal gonadial sac. The gonadial sac, or 

 gonad, furnishes the germ cells a protected space within the body in 

 which they may complete their development. With the multiplication 

 and growth of the germ cells the gonad increases in size by an exten- 

 sion of its cellular walls, and it also assumes the role of nutritive 

 organ for the developing ova or spermatozoa within it. Any specific 

 part of the sac that contains and nourishes the growth stages of the 

 gametes, therefore, constitutes a trophocyst, or vitellarmin, of the 

 ovary or testis. 



The mature gonad may retain the form of a simple sac, in the walls 

 of which the germinal cells may be dififusely scattered or localized 

 at some particular place, but with many of the arthropods the capacity 

 and productivity of the gonad are augmented by a secondary lobula- 

 tion of the walls or by the outgrowth of saclike or tubular diverticula 

 (sperm tubes or egg tubes), each containing a germinal area, usually 

 at its blind end. In some of the apterygote insects the gonadial 

 diverticula have a segmental arrangement, but generally there is no 

 relationship between the germinal pockets of the gonads and the body 

 segments. Both the ovary and the testis are primarily single organs 

 in the embryo ; though primitively they may have been formed from 

 the germarial pockets of several consecutive coelomic sacs, the coe- 

 lomic components have nothing to do with the subsequent outgrowth 

 of follicular or tubular diverticula. Any theory of metamerism in the 

 reproductive organs of the arthropods, therefore, must go back to a 

 very early phylogenetic stage when the germinal centers were seg- 

 mentally arranged in the coelomic walls, and the gametes were dis- 

 charged through open coelomoducts connecting the coelomic sacs with 

 the exterior. The only case among the arthropods of multiple genital 

 openings and genital ducts associated with gonadial sacs occurs in the 

 Pycnogonida, but the great reduction of the body and the branching 

 of the gonad into the legs gives reason for suspecting that the genital 

 apertures on the second leg segments of these animals (fig. 7, Gpr) 

 are secondary formations. The pycnogonid gonad is said to be a 



