40 AMPHIBIA CHAP. 



EESPIRATORY ORGANS 



A very important and characteristic feature of the Amphibia 

 is the development of two sets of respiratory organs : Gills and 

 Lungs. It is as well to give definitions of these organs. Lungs 

 are hollow evaginations from the ventral wall of the pharynx, 

 and their thin, vascularised walls enable the blood to exchange, 

 by osmosis, carbon dioxide for oxygen from the air which enters 

 the lungs by the mouth or the nostrils, and the windpipe. The 

 latter is unpaired, the lungs themselves are paired. Gills are 

 highly vascularised, more or less ramified excrescences, covered 

 by a thin epithelium of ecto- or endo-dermal origin, which permits 

 of the exchange of carbon dioxide for oxygen from the air which is 

 suspended in the surrounding water. It is obvious that this 

 definition applies to all sorts of well-vascularised organs whose 

 thin surface comes into contact with the water. Various recesses 

 of the pharyngeal cavity, the dorsal and ventral folds of the 

 tail-fin, nay, even any part of the skin of the body can, and does 

 occasionally, assume additional respiratory functions. The proper 

 definition of gills, in Vertebrates, requires, therefore, the restric- 

 tion that they must be developed upon and carried by visceral 

 arches. 



The general statement that the Amphibia breathe by lungs, 

 and, at least during some stage of their life, also by gills, requires 

 various restrictions. As a rule the majority of Amphibia first 

 develop gills, later on also lungs, whereupon, during the meta- 

 morphosis, the gills are gradually suppressed, so that the perfect 

 animal breathes by lungs only (see p. 61). But a number of 

 Urodela retain their gills throughout life, although the lungs are 

 also functional. These are the Perennibranchiata, not a natural 

 group, but a heterogenous assembly, Proteidae and Sirenidae. 

 Some species of Amblystoma remain individually Perenni- 

 branchiate (cf. Axolotl, p. 112). On the other hand, in some 

 Anura the gills are almost or entirely suppressed, or restricted 

 to the embryonic period only. Lastly, a considerable number of 

 Salamandridae have lost their lungs ; they breathe by gills until 

 their metamorphosis, but have in the adult state to resort to 

 respiration by the skin (cf. p. 46). 



The general plan of the development of the branchial re- 

 spiratory apparatus is as follows: The six visceral arches, 



