326 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



canal, closed in front. In the other type a groove of the covering 

 epithelium forms on the surface of the ovary. This closes over and 

 sinks inward, forming what is termed as an entovarial canal. Either 

 canal may extend backward to the hinder end of the body cavity, thus 

 forming an oviduct, or the oviduct may be formed from both kinds of 

 canals, one in front, the other behind. From this it would appear 

 that the ovary originally extended back to the hinder end of the ccelom 

 (as it does in Cydopterus) or that the par- or entovarial canal had 

 united with a Mullerian duct which has otherwise been entirely lost. 

 The oviducts thus formed usually unite before opening to the exterior, 

 either directly r via a urogenital sinus. The oviducts in the dipnoi 

 (fig. 326, A) are much like those of the selachians, emptying inde- 

 pendently into the cloaca. They persist, though of small size, in the 

 males (fig. 325, D). 



EXCRETORY ORGANS IN THE SEPARATE GROUPS. 



CYCLOSTOMES. In the lampreys the pronephros extends over thirteen 

 somites, but only the anterior five form complete tubules, the remainder, however, 

 join the pronephric duct. The pronephros is best developed in the Ammocoete, 

 10 mm. long, and in this stage the mesonephros is also developed and both are 

 functional. With increase in size there is a degeneration of the mesonephric tubules 

 in front and a formation of new ones behind, the definitive organ extending over 

 about two-fifths of the body length. Each pronephros projects into the coelom as 

 a band supported by a fold of the peritoneal membrane. The two pronephric ducts 

 unite a little in front of the hinder end, forming a urogenital sinus into which the 

 abdominal pores empty, and which, in turn, opens at the tip of a urogenital papilla 

 just behind the anus. 



In the myxinoids the nephridial tubules develop as a continuous series, the 

 organ in the earliest stage known extending over somites 11-80. Later the organ 

 becomes divided into two parts by the degeneration of the intermediate tubules. 

 The anterior part projects into the body cavity and is provided with nephrostomes, 

 while the posterior part, reaching through some twenty or thirty somites, has its 

 tubules strictly segmental, each with a Malphigian body. This is the functional 

 excretory organ. 



ELASMOBRANCHS. The pronephros is never functional as an excretory 

 organ. The Wolffian bodies of the two sides are somewhat influenced in form by the 

 other viscera, and are sometimes asymmetrical. Usually the nephrostomes are closed 

 in the adult, but they persist in several genera, among them Acanthias, while they are 

 lacking in Scyllium and Raia. The anterior end of each mesonephros is narrowed 

 and serves as the connexion with the testes in the male, while the anterior end of 

 the Wolffian duct forms a much-coiled epidymis in the same sex. A urinary blad- 

 der is formed by the union of the ducts of the two sides. In the female the blad- 



