URIXOGEXITAL ORGANS. 449 



The large elongated kidneys are placed in the excavations of 

 the sacrum between the transverse processes, and are divided 

 by indentations into a number of lobes (usually three). The 

 ureters run backwards and open into the middle chamber of the 

 cloaca internally to the genital aperture. The urinary excretion 

 is not liquid, as in Mammalia, but is a white semifluid mass 

 which contains a considerable quantity of urates. The absence 

 of water in the urine of birds is a remarkable fact. In mammals 

 the nitrogenous waste comes away in solution, which causes a 

 considerable loss of water from the blood. In birds, in which 

 this loss does not take place and in which there are no sweat 

 glands, loss of water must be mainly confined to the internal 

 surface of the air passages. No doubt the evaporation which 

 takes place on the very extensive internal air-passages and sacs 

 is an important factor in the regulation of the body temperature. 



The generative organs closely resemble those of the Reptilia. 

 The males are generally distinguished, not only by their superior 

 strength, but also by the brighter colour of their plumage and 

 the greater power of their song. There are two oval testes at 

 the anterior end of the kidneys ; they become much enlarged 

 at the breeding season, and the left is usually the larger. The 

 epididymis, which is but little developed, leads into the vas 

 deferens, which passes back along the outside of the ureter. 

 The ends of the vasa deferentia are frequently swollen so as to 

 form seminal vesicles, and open on two conical papillae placed 

 on the hinder (dorsal) wall of the cloaca. 



A copulatory organ is, as a rule, wanting ; in some of the 

 larger water birds, however (Ciconia, Platalea, etc.) a rudimen- 

 tary penis is present as a wart-like process on the front (ventral) 

 wall of the cloaca. It is larger in most of the Struthionidae, the 

 ducks, geese, swans, and in the curassows and guans (Penelope, 

 Urax, Crax). In these birds a curved tube, supported by two 

 fibrous bodies, is attached to the ventral wall of the cloaca. 

 The end of the tube can be retracted by an elastic band. A 

 superficial groove serves to conduct the sperm during copula- 

 tion. In the ostrich, the penis attains a still higher structure, 

 analogous to that of the male copulatory parts of the Chelonia 

 and Crocodilia. Below the two fibrous bodies, the broad bases 

 of which arise from the front wall of the cloaca, there is a third 

 cavernous body the extremity of which is non-retractile and 

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