146 THE URINOGENITAL ORGANS OF VERTEBRATES. 



requires notice is the division of the kidney into two portions, 

 an anterior and posterior. The anatomical similarity between 

 this arrangement and that of higher vertebrates (birds, &c.) is very 

 striking. The anterior one precisely corresponds, anatomically, 

 to the Wolffian body, and the posterior one to the true per- 

 manent kidney of higher vertebrates : and when we find that 

 in the Selachians the duct for the anterior serves also for the 

 semen as does the Wolffian duct of higher vertebrates, this 

 similarity seems almost to amount to identity. A discussion of 

 the differences in development in the two cases will come con- 

 veniently with the account of the bird ; but there appear to me 

 the strongest grounds for looking upon the kidneys of Selachians 

 as equivalent to both the Wolffian bodies and the true kidneys 

 of the higher vertebrates. 



The condition of the urinogenital organs in Selachians is by 

 no means the most primitive found amongst vertebrates. 



The organs of both Cyclostomous and Osseous fishes, as well 

 as those of Ganoids, are all more primitive ; and in the majority 

 of points the Amphibians exhibit a decidedly less differentiated 

 condition of these organs than do the Selachians. 



In Cyclostomous fishes the condition of the urinary system 

 is very simple. In Myxine (vide Joh. Muller My xinoid fishes, 

 and Wilhelm Muller, Jenaische Zeitsckrift, 1875, Das Urogenital- 

 system des A mphioxus u. d. Cyclostomeri) there is a pair of ducts 

 which communicate posteriorly by a common opening with 

 the abdominal pore. From these ducts spring a series of trans- 

 verse tubules, each terminating in a Malpighian corpuscle. These 

 together constitute the mass of the kidneys. About opposite 

 the gall-bladder the duct of the kidney (the segmental duct) 

 narrows very much, and after a short course ends in a largish 

 glandular mass (the head-kidney), which communicates with the 

 pericardial cavity by a number of openings. 



In Petromyzon the anatomy of the kidneys is fundamentally 

 the same as in Myxine. They consist of the two segmental 

 ducts, and a number of fine branches passing off from these, 

 which become convoluted but do not form Malpighian tufts. 

 The head-kidney is absent in the adult. 



W. Muller (loc. cit.} has given a short but interesting account 

 of the development of the urinary system of Petromyzon. He 



