UROGENITAL SYSTEM. 



343 



other groups fertilization is internal. In some cases the transfer of 

 the sperm from the male to the female is effected by the apposition 

 of the cloacae of the two sexes, but in others copulatory organs of an 

 intromittent character occur. These are formed on several plans and 

 are not homologous throughout. 



FIG. 342. Hemipenes of Crotalus horridus, after J. Miiller. One hemipenis is 

 exserted, the other retracted but laid open, cl, cloaca; g, seminal groove; p, hemipenis; 

 r, rectum, rp, retractor muscle of hemipenis; u, ureter; vd, vas deferens (Wolffian duct). 



In the male elasmobranchs the posterior or inner side of the 

 pelvic fins are specialized for this purpose. The metapterygium 

 (p. 1 1 6) and the basalia connected with it are more or less completely 

 separated from the rest and form the so-called clasper ('mixip- 

 terygium.'). Each of these is grooved along its medial surface and 



