URINOGENITAL ORGANS 363 



with lymphoid and adipose tissue, and are closely attached to the 

 outer border of the kidneys ; when ripe, they become greatly 

 enlarged, so as to embrace the gut ventrally. The oviducts, which 

 doubtless correspond to Mullerian ducts, are long and coiled, re- 

 sembling those of the Amphibia: posteriorly they unite before 

 opening into the cloaca, and each communicates anteriorly with the 

 body-cavity by a funnel-shaped aperture. The wall of the ovi- 

 duct secretes albumen round the eggs as they pass along it. 



The manner in which the sperm is conducted to the exterior in 

 Ceratodus is not understood. In Protopterus (Fig. 288, B) the 

 seminal tubules of the testis open into a duct, embedded between 

 the lobes of the testis ventrally. Behind the testis the duct ex- 

 tends freely for a short distance, and then unites with the base of 

 the Mullerian duct, which, as in the female, joins with its fellow 

 to open into the cloaca on the summit of a papilla. The rest of 

 the Mullerian ducts, with their ostia, can still be recognised in 

 immature individuals. The sperm-duct of Protopterus is ap- 

 parently a structure sui generis ; like that of Teleosts, it is probably 

 developed quite independently of the nephridial system. 



In the greater number of Elasmobranchs the ovaries are 

 paired, and the oviducts, like those of Dipnoans, are separate 

 from the ovaries, and correspond to Miillerian ducts. Their 

 anterior portion has a common opening into the body-cavity, 

 and further back each is provided with an oviducal or shell-gland. 

 The anterior part of the oviduct is always narrower and more 

 delicate than the posterior, which dilates to form a kind of uterus, 

 in which (when the Shark is viviparous) the embryo undergoes de- 

 velopment. Posteriorly, the oviducts open into the cloaca some- 

 what behind the aperture of the ureters either separately, or by 

 a common canal (Figs. 278, B, 289). 



The oviducal gland secretes the horny material for the egg-case or 

 "purse," which is usually elongated and produced at its four angles into 

 longer or shorter tendril-like threads ; in Cestracion it has a spiral ridge. 

 The majority of Sharks are viviparous, and in these the egg-shell becomes 

 more or less reduced. In Mustelus antarcticus the uterus becomes divided 

 into several compartments, each containing an embryo surrounded by a 

 membrane apparently representing the horny egg-shell of other forms. 



The testis of Elasmobranchs is paired and symmetrical, but the 

 two organs may become partially fused with one another. Vasa 

 efferentia connect each testis with the anterior end of the corre- 

 sponding mesonephros (parorchis), the Wolffian duct serving as a 

 vas deferens (p. 346), and giving rise to a dilated portion or 

 vesicula seminalis, as well as to a caacal sperm-sac where it com- 

 municates with the urinogenital sinus : the latter opens into the 

 cloaca on the apex of a papilla (Figs. 206, 278, c, 290). Vestiges of 

 the anterior end of the Miillerian ducts can be recognised even in 

 the adult as a short transverse tube with a central aperture,, 

 situated below the oesophagus anteriorly to the liver. 



