THE ELASMOBRANCH FISHES 



295 



wliicli the part of the kidney lying posterior to tlie urinary sinus is drained by 

 collecting tubules (fig. 254b) some of which unite anteriorly into one or two 

 groups (ureters), while others of them enter enlarged horns of the urinary 

 sinus (female of R. clavata). In the female of the sting ray, Trygon, all the 



w.d. 



A 



B 



Fig. 255. Renal corpuscles, Acanthias. (From Borcea.) A. Active. B. Atrophied and in the 

 service of the male sex system. 



cil., ciliated cells; cj)., Bowman's capsule; c.t., collecting tube; gJ., glomerulus; r.t., renal 

 tubule; tv.d., Wolffian duct. 



collecting tubules enter the large horns of the urinary sinus independently, 

 excepting the single anterior one which is a direct continuation of the small 

 Wolffian duct. 



The lower part of the Wolffian duct in a type in which this does not receive 

 tubules may be enlarged as the so-called urinary vesicle. In Sqimtina the duct 

 is swollen and in Scyllium it is of large size (p. 189, fig. 177a, u.v.). 



From this it is seen that the Wolffian duct decreases in importance in the 

 female as we approach the rays. In a type like Squalns sucklii, however, a plan 

 is given in which the Wolffian duct assumes a more important role. Here in 

 the female (fig. 253a) the ducts receive the collecting tubules from practically 

 the whole of the kidney, so that a ureter may be said to be absent, or if present 

 to receive only a few tubules. 



The urinary sinus (u.s., fig. 253) into which the Wolffian ducts empty varies 

 greatly in size and shape. In the sharks it is simpler in its external form than 

 in the rays and may be described as a delta-shaped sac which empties posteri- 



