308 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



the system. They are paired organs which consist of glandular por- 

 tions, the nephridia (kidneys), and their ducts. The reproductive 

 organs include the gonads or sexual 'glands,' which (ovaries) pro- 

 duce the eggs or (testes) the spermatozoa, and the passages by which 

 these products are carried to the external world. To these are fre- 

 quently to be added accessory reproductive structures by which, in 

 certain cases, the sperm is transferred to the female. 



FIG. 313. Urogenital organs of Emys europea, after Bojanus. &, urinary bladder; 

 g, opening of vas deferens into the urogenital sinus; k, kidney; /, testis; u, ureter; vd, vas 

 deferens. 



THE EXCRETORY ORGANS. 



The nephridia consist of a series of excretory tubules, specialized 

 in different ways, and of the ducts into which the tubules empty. As 

 the function of the nephridia is the elimination of the nitrogenous 

 waste (uric acid, urea, etc.) which accumulates in the blood, they have 

 an abundant blood supply, entirely derived, in the younger stages of 

 all vertebrates and in the adults of the higher groups from the dorsal 

 aorta, while in the later developmental stages and in the adults of most 

 anamniotes the aortic blood is supplemented by blood coming from 

 the tail and hind limbs by way of the caudal and iliac veins (fig. 303). 



In its extreme development one of the excretory tubules may con- 

 sist of the following parts (fig. 314): At the proximal end the tubule 

 opens into the ccelom (metaccele) by a ciliated funnel, the nephro- 



