KIDNEYS 27 



Much more radical has been the change in the excretory system. 

 No trace whatever of true nephridia, such as occur in Amphioxus 

 (Weiss, Boveri, Goodrich [174]), have yet been discovered in any 

 Craniate. 



The kidney tubes of the Craniates are generally somewhat 

 loosely compared to the nephridia or segmental organs of the 

 Annelids (Gegenbaur, Semper [404], Hatschek, etc.). Now this 

 comparison was first made at a time when the development of the 

 renal organs of the vertebrates was incompletely known, and when 

 both the. structure and the ontogeny of the nephridia of Annelids 

 were very imperfectly understood. Moreover, the theory was to 

 some extent founded on observations which have since been shown 

 to be erroneous. The question now wears a very different aspect 

 (Goodrich [172]). Since then it has been ascertained that nephridia 

 are found in almost all the invertebrate Coelomata, but that they 

 may, or may not, be connected with the coelom ; it has been shown 

 that there occurs in these animals a second series of organs open- 

 ing to the exterior the genital funnels or coelomostomes, which 

 develop from the wall of the genital or coelomic sacs ; further, the 

 excretory organs of Amphioms are now known to be certainly 

 homologous with the nephridia of Annelids [174]. It is therefore 

 quite clear that if the kidney tubes of the Craniata are to be 

 compared to any organ in the Invertebrata, it is with the coelomo- 

 stomes, and not with the nephridia, that they must be homologised. 

 Should Boveri's suggestion, that the genital pouches of Amphioxus 

 represent the kidney tubes of the Craniates, prove to be true, it 

 will be a striking confirmation of this conclusion. 



The excretory system of the Craniates is founded on a series of 

 paired segmental funnels and tubules derived, directly or indirectly, 

 from the coelomic epithelium, and leading into a longitudinal 

 connecting duct which opens to the exterior behind the anus. No 

 Craniate is known in which these tubules open independently to 

 the exterior, but it is reasonably conjectured that such must have 

 been the original state of things. The renal tubes develop from 

 before backwards. The earlier and more anterior become first 

 functional, and are succeeded by the more posterior. Thus, the 

 earliest set of tubes to appear occupy the pericardial region 

 immediately behind the gill-slits, and form the pronephros. These 

 subsequently become functionally replaced by the more posterior 

 mesonephros of the abdominal region. Finally, in the Amniota, 

 a separate and more posterior set of tubes, the metanephros, alone 

 persist as the adult kidney. A more detailed account of the 

 structure and development of these organs will be given later (p. 83). 



There can be little doubt that originally some or all of these 

 tubes carried the genital products to the exterior, and this function 

 is still retained by the mesonephric tubes in the male sex of all 



