520 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the female. Union of the sexes is common among 

 the Amphibia, but it is in the Csecilise only that 

 an external copulatory organ is developed from the 

 cloaca ; this may be as much as five centimetres 

 long (Wiedersheim). In Reptiles the penis is nearly 

 always distinctly double ; in the lizard the two halves 

 ordinarily lie beneath the skin just behind the 

 cloacal slit, but they are here, as they are also in the 

 snakes, capable of protrusion and erection ; a spiral 

 groove runs along the inner face of each half, and 

 serves as the duct by which the semen is conveyed 

 from the vas deferens to the female. The penis of 

 the Crocodilia and of the Cheloiiia is feebly, if at all, 

 protractile, but an advance is to be noted in the greater 

 union, of the two halves, and the development of blood 

 spaces or corpora cavernosa in its substance. Where 

 a penis is developed in birds, it has (except in the 

 ostrich) the form of a coiled protrusible tube, the 

 groove on whose upper surface leads into a canal. 



Among the Eutheria the penis always presents a 

 certain number of common characters, in so far as the 

 outgrowth of the cloaca from which it is formed 

 always becomes separated by the perinaeum from 

 the rectal orifice, that the primitive groove always 

 becomes a canal, and the basal or proximal end is 

 continuous with the urogenital sinus, or portion 

 common to the urethra and the vasa deferentia. 

 This penis is erected by a median and two lateral 

 corpora cavernosa, and at the free end, which is 

 covered by a fold of the skin (pirepiice), there is 

 developed a glans. In addition to the corpora, an 

 os penis may be found, and this is sometimes, as 

 in the walrus, of great size. The portion of the 

 urethra found in the penis is knoAvn as the penial 

 urethra, and it sometimes, though by no means 

 always, traverses the body of the glans before opening 

 to the exterior. 



