578 



TELEOSTEI. 



morphosis complete atrophy, and is physiologically replaced by a 

 posterior series of segmental tubes, opening into the hiudermost 

 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 (fi<. 



FIG. 3S4. PORTIONS OF THE 



MESONEPHROS OF MYXINE. (From 



Gegenbaur; after . I. Miiller.) 



a. segmeutal duct ; b. segmen- 

 tal tube; c. glomerulus; d. afferent, 

 e. efferent artery. 



B represents a portion of A 

 highly magnified. 



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 mesonephros coexist 

 through life, and their products are 

 carried off by a duct, the nature of 

 which is somewhat doubtful, but which 

 is probably homologous with the meso- 

 nephric duct of other types. 



The system commences in the em- 

 bryo (Rosenberg, Oellacher, Gotte, 

 Fiirbringer) 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 di- 

 rections. 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 funnel there is 

 formed a glomerulus, projecting into the 

 body cavity ; and at the same time that 

 this is being formed the anterior end 

 of the canal becomes elongated and 

 convoluted. The above structures con- 

 stitute a pronephros, while the pos- 

 terior part of the primitive canal forms 

 the segmental duct. 



The portion of the body cavity with 

 the glomerulus and peritoneal funnel 



