EXCRETORY ORGANS. 70 1 



Further changes take place in connection with the excretory 

 system on the conversion of the Ammocoete into the adult. 



The segmental ducts in the adult fall into a common urino- 

 genital cloaca, which opens on a papilla behind the anus. This 

 cloaca also communicates by two apertures (abdominal pores) 

 with the body cavity. The generative products are carried into 

 the cloaca by these pores ; so that their transportation outwards 

 is not performed by any part of the primitive urinary system. 

 The urinogenital cloaca is formed by the separation of the portion 

 of the primitive cloaca containing the openings of the segmental 

 ducts from that connected with the alimentary tract. 



The mesonephros of the Ammocoete undergoes at the meta- 

 morphosis complete atrophy, and is physiologically replaced by 

 a posterior series of segmental tubes, opening into the hinder- 

 most portion of the segmental duct (Schneider). 



In Myxine the excretory system consists (i) of a highly developed pro- 

 nephros with a bunch of ciliated peritoneal funnels opening into the peri- 

 cardial section of the body cavity. The coiled and branched tubes of which 

 the pronephros is composed open on the ventral side of the anterior portion 

 of the segmental duct, which in old individuals is cut off from the posterior 

 section of the duct. On the dorsal side of the portion of the segmental duct 

 belonging to the pronephros there are present a small number of diverticula, 

 terminating in glomeruli : they are probably to be regarded as anterior 

 segmental tubes. (2) Of a mesonephros, which commences a considerable 

 distance behind the pronephros, and is formed of straight extremely simple 

 segmental tubes opening into the segmental duct (fig. 385). 



The excretory system of Myxine clearly retains the characters of the 

 system as it exists in the larva of Petromyzon. 



Teleostei. In most Teleostei the pronephros and mesone- 

 phros coexist through life, and their products arc carried off by 

 a duct, the nature of which is somewhat doubtful, but which is 

 probably homologous with the mcsonephric duct of other types. 



The system commences in the embryo (Rosenberg, Oellachcr, 

 Gotte, Furbringer) with the formation of a groove-like fold of the 

 somatic layer of peritoneal epithelium, which becomes gradually 

 constricted into a canal; the process of constriction commencing 

 in the middle and extending in both directions. The canal does 

 not however close anteriorly, but remains open to the body 

 cavity, thus giving rise to a funnel equivalent to the pronephric 

 funnels of Petromyzon and Myxine. On the inner side of this 



