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 cloace 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. 
Fic. 342.—Hemipenes of Crotalus horridus, after J. Miller. One hemipenis is 
exserted, the other retracted but laid open. cl, cloaca; g, seminal groove; #, 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. 116) 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 
