UROGENITAL SYSTEM. 327 



der opens to the exterior at the tip of a genital papilla, but in the male it connects 

 with a urogenital sinus, into which a pair of reservoirs of sperm empty. The duct 

 from the urogenital sinus opens into the cloaca at the tip of a urogenital papilla. 

 In Chinuzra the anterior end of the mesonephros lacks Malphigian bodies and forms 

 a large (Leydig's) gland, the secretion of which may possibly be used in dissolving 

 the spermatophores (fig. 331). 



GANOIDS. In Polypterus the pronephric tubules are two in number, belonging 

 to the second and fifth post-otic somites; mLepidosteus there are five or six; sturgeon 

 six; and Amia eight to eleven. The large size of the pronephros in Polypterus is 

 due to the extensive coiling of the anterior end of the duct. In the sturgeon a part 

 of the excretory organ is separated from the rest but it is not certain that this is 

 really a pronephros. 



The mesonephros is markedly segmental, the glands of the two sides being en- 

 larged and united behind in the sturgeon. Nephrostomes are late in appearance, not 

 being formed until after the tubules have joined the duct. The urinary bladder 

 differs from that of teleosts in that the Mullerian ducts enter it. 



TELEOSTS (fig. 327) have a pronephros which extends over from one to five 

 somites. It is usually transitory in character, but it persists through life in several 

 species and functions during the larval stages in many more. The mesonephros 

 varies considerably in shape. Where there is an air bladder this covers some or all 

 of the ventral surface of the mesonephroi. Frequently the organs of the two sides 

 are united behind, while lobes may extend forward from the main mass, or back 

 into the tail. The duct is sometimes visible from below, sometimes it is immersed 

 in the mass of the organ. There is no sexual part to the mesonephros and there 

 are no nephrostomes in the adult. The urinary ducts of the two sides unite behind 

 and from the united portion and from the ventral wall of the cloaca the urinary 

 bladder is formed. Later the opening of the bladder separates from the cloaca and 

 usually comes to lie behind the anus, sometimes united with the sexual openings. 



DIPNOI. In Ceratodus there are two pronephric tubules, that of the third 

 somite being complete, that of the fourth rudimentary. The glomerulus lies beside 

 the open nephrostome. The mesonephros is at first strongly metameric. There 

 are no nephrostomes in the adult and none appear at any time mLepidosiren. The 

 adult mesonephros is widest behind, but the relations of the efferent ductules of the 

 male are differently arranged in the separate genera, as mentioned above. 



AMPHIBIA. The pronephros (developing from two somites in the urodeles, 

 three in anura and twelve or more in gymnophiones) retains its functions in uro- 

 deles and anura until the metamorphosis, when its tubules degenerate. At first 

 the mesonephros consists of a tubule with nephrostome and renal corpuscle for each 

 somite, but in the adult this metamerism is lost, except at the anterior end, by the 

 development of secondary tubules, each complete like the original ones, the nephro- 

 stomes sometimes amounting to over a thousand on the ventral surface of each 

 Wolffian body. In the adult anura the nephrostomes lose their connexion with 

 the excretory system and join branches of the renal arteries, thus placing the ccelom 

 in connexion with the circulatory system. 



In the urodeles the mesonephroi form a pair of ridges on the dorsal wall of the 

 ccelom, but they occasionally project as folds. Their length is somewhat propor- 



