392 COMPARATIVE ANATOMY 



cloaca, while in Protopterus a single, apparently blind, canal is 

 present on the same side of the ventral fin as the vent, sometimes 

 to the right and sometimes to the left of the middle line, either 

 within or without the sphincter of the cloaca. 1 



Abdominal pores are not known to occur in Amphibians, Birds, 

 and Mammals, but amongst Reptiles they are perhaps represented 

 by the peritoneal canals of the Chelonia and Crocodilia, which in 

 the former are in close relation with the penis or clitoris, and 

 usually end blindly, while in the latter they open into the cloaca. 



The abdominal pores may possibly correspond to the remains 

 of segmental ducts. At any rate, they and the nephostomes, 

 whichever may be the older phylogenetically, come under the 

 same physiological category, inasmuch as both might serve to 

 remove the products of regressive metamorphosis from the coelome, 

 which to a large extent represents an excretory organ (cf. Fig. 297). 



1 Abdominal pores are apparently wanting in Lepidosiren. 



