292 THE ELASMOBRANCH FISHES 



UROGENITAL SYSTEM OF ELASMOBRANCHS IN GENERAL 



The two systems, urinary and genital, included nnder this head, although 

 differing in function, are so closely associated anatomically that they are usu- 

 ally considered together. First, however, we shall consider them separately 

 and then discuss their secondary relationship. 



Urixary System 



KIDNEY (MESONEPHROS) 



Upon opening the body cavity by a ventral incision and removing the viscera, 

 the urinary organs of an Elasmobranch appear as dorsally placed structures 

 on each side of the spinal column. In the sharks they may extend as ribbon- 

 like bands, narrow at the base of the liver anteriorly, and only slightly wider 

 at the cloacal region posteriorly. In the rays they are characteristically en- 

 larged posteriorly where most of the tissue is confined. These statements are 

 true only in general, for great variation is present in different species of 

 sharks and rays; differences are further to be noted in the different sexes of 

 the different species. 



The long ribbon-like type of mesonephros or kidney characteristic of Squa- 

 lus (fig. 253a) and Galeus loses something of this shape in ScylUum where its 

 anterior part is narrower and, in the female, ends short of the base of the liver. 

 Again, the type of kidney characteristic of the shark although extending far 

 anteriorly may be at the same time like the kidneys of the rays, broader poste- 

 riorly (Sqiiatina, fig. 254a) . In the rays, excepting Torpedo, the kidney rarely 

 extends far forward, but sometimes a large part of it is located back of the 

 cloaca {Raja clavata, fig. 254b, Trygon). 



Sexual differences which are marked in the kidney of the Elasmobranchs 

 are produced by two factors : one is the reduction of the anterior part of the 

 kidney in the female; the other is the hypertrophy of this part in the male. 

 This hypertrophy results from the fact that the anterior part in the male 

 comes into the service of the genital system and takes on a secondary function. 

 In Torpedo sexual differences are slight, for a ray. In the female a band of 

 tissue is continuous forward, and this is only slightly less developed than 

 in the male. Sexual differences are evident in a type like Squatina in which 

 the kidney of the adult female does not extend anteriorly to the liver. In 

 Scyllium, where the difference in the two sexes is marked, the anterior part of 

 the kidney of the female falls short of the base of the liver; and in R. clavata 

 the kidney is limited to the posterior segment, while the anterior part in the 

 male (fig. 254b) represents a great mass of tissue. 



Furthermore, the kidney varies greatly in its degree of complexity in the 

 different Elasmobranchs. In some of the sharks it retains in part a simple 

 metameric arrangement by which in dorsal view it agrees with the segmenta- 

 tion of the body, characteristic of the embryo {Squatina, fig. 254a). In most 

 other sharks, however, this simple arrangement is lost at least in the posterior 



