144 THE COMMON FROG. [chap. 



Dr. Gunther.i He has conclusively shown that Cera- 

 todus and Lepidosiren are closely allied, and thus 

 finally brought the latter definitively within the class 

 of fishes, for that Ceratodus is a fish no one questions. 

 It is an animal, however, of somewhat amphibious 

 habits, as at night it leaves the brackish streams it 

 inhabits, and wanders amongst the reeds and rushes 

 of the adjacent flats. Vegetable substances constitute 

 its principal food. 



Ceratodus and Lepidosiren together afford the most 

 remarkable evidence of the persistence of the same 

 type of structure in the Vertebrate sub-kingdom. 

 The group to which they both belong reaches back 

 into the very earliest epoch, which has yet afforded 

 us any evidence whatever of the existence of fishes ; 

 while the genus Ceratodus seems to have persisted 

 unchanged from the period of the deposition of the 

 triassic strata. 



The Excretory Organs. 



As has been said, it is a necessary action for 

 every living being, whether plant or animal (in order 

 that it may continue to live) to eliminate certain 

 substances, the most noteworthy being carbonic acid, 

 which is set free by the process of respiration. 



The active processes of life, however, necessarily 

 wear out, by their activity, parts containing the gas 

 nitrogen, which enters into the composition of every 

 animal, and especially into every part exhibiting much 

 vital activity. 



' See Phil. Trans. 1871, p. 511 ; Plates xxx. to xlii. 



