INTERNAL ANATOMY. 67 



entirely rudimentary structure, which puts in a fleeting 

 appearance during the embryonic development, but never 

 functions as a kidney. 



In other cases, as with the Teleostomes, or bony fishes, 

 Amphibians, Crocodiles, and Turtles, the pronephric sys- 

 tem attains a higher grade of development, and actually 

 functions for a time as the sole kidney of the animal. In 

 some of the bony fishes (e.g. Zoarces and Merlucins), it 

 functions as the kidney for an extraordinarily long time, 

 apparently throughout the period of adolescence. In one 

 curious instance of a fish, Fierasfer, which has acquired a 

 semi-parasitic habit, it appears that the development has 

 been arrested to such an extent that the pronephros 

 functions as the principal organ of excretion throughout 

 life, the mesonephros remaining rudimentary (EMERY). 



The most extensive pronephric system which has as yet 

 been described for any craniate Vertebrate, is that repre- 

 sented diagrammatically in Fig. 32. This is the larval 

 excretory system of a remarkable worm-like legless Am- 

 phibian, Ichthyophis glutinosits, belonging to a very primi- 

 tive subdivision of the Amphibia known as the Cceciliani, 

 which occur in the hot regions of South America, Africa, 

 Seychelles, East Indies, and Ceylon. 



We owe our knowledge of this elaborate pronephric 

 system to RICHARD SEMON of Jena. 



It consists of some twelve pairs of irregularly contorted 

 tubules placed dorsal to the general body-cavity in a posi- 

 tion which is described as retro-peritoneal, and arranged seg- 

 mentally, one behind the other, on either side of the dorsal 

 aorta. Broadly speaking, the canals run outwards in a 

 transverse direction. Near their inner extremities they 

 usually divide into two short branches, which terminate 

 each in a funnel-shaped opening into the body-cavity. 



