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 coelom 
(as it does in Cyclopterus) or that the par- or entovarial canal had 
united with a Miillerian duct which has otherwise been entirely lost. 
The oviducts thus formed usually unite before opening to the exterior, 
either directly or 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 Ammoccete, 
zo 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 ccelom 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 furictional 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- 
