1908 ] 



EXCRETORY ORGANS OF METAZOA. 599 



appear strongly modified or at least quite different from those in 

 other groups, then it is highly probable that these two organ systems 

 are not strictly homodynamous. For the pronephroi arise as solid 

 thickenings of the somatic mesoblast, that later become tubular and 

 only secondarily join with the coelom ; and their vascular supply is 

 an unsegmented glomus opposite their nephrostomes. On the other 

 hand the mesonephroi are abstricted portions of the coelom (nephro- 

 ccels), they are from the start peritoneal and in open communication 

 with the coelom; and the vascular connection of each is a ]\Ialpighian 

 corpuscle. The pronephroi are retroperitoneal, the mesonephroi, 

 peritoneal funnels in the main ; the former develop in close con- 

 nection with the segmental duct, while the latter arise much later 

 than it and join it secondarily. In view of these differences pro- 

 nephros and mesonephros are probably only incompletely homo- 

 dynamous. 



As to the metanephros, its ureter being an outgrowth of the 

 segmental duct is a new structure; while the glandular kidney arises 

 from mesoblast that may represent a late generation of mesonephric 

 tubules. Accordingly, the metanephros can be only in part homo- 

 dynamous with the mesonephros. 



Homolognes of Nephridia. — Here are to be placed the genital 

 organs that I will treat very briefly. Particular genital ducts are 

 absent in the Cyclostomes, Lccmargus and certain Teleostei ; here 

 the genital cells fall into the coelom and are discharged through 

 peritoneal canals, supposed peritoneal funnels (Weber, 1886), the 

 development of which has not been studied. 



In the males of Teleosts and certain other fishes the genital 

 ducts are simply outgrowths of the gonads, while in all other forms 

 the segmental ducts (or portions of them) are urogenital. The vasa 

 efferentia of the testis, the paradidymis and the hydatid of Morgagni 

 are modified mesonephric tubules. 



In the females of all forms except most Teleosts and Lepidosteus, 

 where the ducts are outgrowths of the gonads, the oviduct (with 

 uterus when present) is distinct from the urinary canal (segmental 

 duct or ureter) and is known as the Miillerian duct. This is paired 

 and arises in the Selachii as a longitudinal abstriction of the seg- 

 mental duct, but in other forms as a structure independent of the 



