280 COMPARATIVE ANATOMY 



ting the pharynx. The former thus show a more primitive con- 

 dition, while in the latter these structures have become reduced. 

 There is always only a single external opening covered over by an 

 opercular-like fold of skin. 



The usually feather-like external gills present at first in Anura 

 soon disappear, and their place is taken by internal gills, the epi- 

 thelium covering which is also said to be ectodermal in origin. By 

 the growth of the opercular folds, which contain no skeletal parts, 

 the external respiratory aperture of either side becomes gradually 

 reduced in size, and the two branchial chambers usually open 

 eventually by a single aperture, which is situated either in the 

 median ventral line, or laterally. 



The larvae of the Gymnophiona also possess external gills, which 

 vary much in form in the different genera (Fig. 224). 



In certain Batrachia in which there is no free larval stage it appears 

 that respiration may take place before hatching in the broad and vascular 

 tail (Hylodes martinicensis), in folds of the ventral body wall (Rana 

 opisthodon), or in peculiarly modified external gills (Nototrema^). 



Except in the Perennibranchiata, the gills disappear at meta- 

 morphosis and the respiratory apertures close up. In the Derotre- 

 mata, however, the gill cleft between the third and fourth branchial 

 arches persists. 



II. AIR-BLADDER AND LUNGS. 

 1. THE AIR-BLADDER. 



As already mentioned (p. 273), the lungs and swim-bladder are 

 developed in a similar manner, and only differ from one another in 

 the fact that the lungs always arise from the ventral side of the 

 pharynx, while this is only exceptionally the case as regards the 

 air-bladder (e.g., Polypterus, Calamoichthys), which usually arises 

 on the dorsal side. The exact point of origin of the air- 

 bladder from the alimentary canal varies, 1 and its duct (ductus 

 pneumaticus) may either remain open throughout life, as in all 

 Ganoids and some Teleosts (Physostomi), or it may later become 

 reduced to a solid fibrous cord or even entirely obliterated, as in 

 other Teleosts (Physoclisti). In the latter case there is no com- 

 munication between the swim-bladder and the external air, and it is 

 probable that the contained gas is given off from the walls of the 

 swim-bladder itself. A vascular organ (the so-called " rete mirabile "),. 

 consisting of numerous glands and capillaries, is present in the walls 

 of the swim-bladder in the Physoclisti, and in certain Physostomi a 



1 In Erythrinus it arises laterally, and in some Physostomi (e.y., Herring) it 

 opens further back into the stomach. 



