ON THE HEAD KIDNEY OF BDELLOSTOMA. 125 



of the cardinal vein on each side, projecting into its lumen. They 

 are best developed in the region of the mesonephros. In the 

 region of the hind kidney, these bodies are replaced by a 

 median, impaired structure, the lobuli composing it being 

 closely connected on each side with the adjacent parts of the 

 tidneys. 



In Teleostei suprarenals are at all events frequently 

 absent ; or, as I would rather suggest, they are represented by 

 the greatly metamorphosed head kidney described by Balfour.-^ 

 In other cases, where suprarenals have been detected, they 

 have always been attached to the surface of the kidney.^ 



In Amphibia, they are embedded in the substance of the 

 kidney, either on its ventral surface (frog), or on its internal 

 border (Triton) ; and they, like the kidneys, receive blood 

 from the renal portal vein.^ 



In Reptiles* the adult structure of the suprarenals strongly 

 supports the view that they are modified portions of the meso- 

 nephros. In the male lizard, for example, they lie suspended 

 in the mesorchium, between the testis and the seminiferous 

 Wolffian tubules, with which latter they are closely connected. 



In snakes the relations are very similar, while there is a 

 remarkably well developed ''adrenal portal" circulation. 



In Birds and Mammals the highly specialised suprarenals 

 retain, as might be expected, fewer traces of their mode of 

 origin than is the case in lower forms. 



It is evident, from the above sketch of their relations, that 

 the only case among lower Vertebrates, in which any well- 

 marked separation between kidneys and suprarenals occurs, is 

 among Elasmobranchs, where the anterior paired portion of 

 the suprarenal system is very distinctly separated from the 

 mesonephros. This fact, however, is probably due simply to 

 the extreme degree of specialisation undergone by the meso- 



1 Loc. cit. 



^ Ecker, ' Der feinere Bau der Nebennieren,' 1846. 

 ' Owen, 'Anatomy of Vertebrates,' vol. i, p. 543. 



* Braun, "Bau u. Entwick. d. Nebennieren d. Reptilieu," 'Arb. Zool. 

 Inst.,' Wurzb., 1872. 



