498 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



embryonic life, an organ on either side of the heart ; 

 and, lastly, it would be as easy to derive the single 

 from the double, as the double from the single 

 arrangement, when we bear in mind that so primitive 

 a form as Peripatus has the two testes completely 

 separated (Fig. 207). 



jir 



Fig. 207. Male Organs of Peripatus. 



te, Testes ; vd, vasa deferentia ; pr, prostates ; p, common duct of vd. 



(After Balfour.) 



We find, then, that the generative glands are 

 either distinctly double, united by an obvious bridge, 

 or converted into a more compact single mass, which 

 retains more or less obscurely indications of a primi- 

 tively double arrangement. 



The male glands are not always rounded off as in 

 the crayfish ; in Squilla they are tubular, and from the 

 sides of the walls short caeca, in which the generative 



