Ichthyopsida 447 



the mouth. The apertures (internal nares or choanae) are in the 



anterior region of the roof of the mouth. 



The skin is more or less important as a respiratory membrane in 

 all amphibians, whether living in water or on land — probably least so 

 in common toads, whose skin is thicker and less mucous than that of 

 other amphibians. Buccopharyngeal respiration probably occurs to 

 varying extents in all amphibians, whether the cavities are occupied 

 by water or by air. Some aquatic salamanders have gills and lungs, 

 both functional, but the air-capacity and blood-supply of the lungs are 

 so small as to indicate that the gills are functionally more important. 

 Most long-bodied tailed amphibians (Urodela) in the adult stage are 

 without gills and have lungs which are more or less efficient in conjunc- 

 tion with the skin and buccopharyngeal surfaces. It is likely that, when 

 the animal is in the water, the lungs may serve also a hydrostatic 

 function, as do the air-bladders of teleostome fishes. 



Still another respiratory complex is found in the lungless sala- 

 manders. There are many species in whose adults gills and lungs are 

 totally absent. Respiration is entirely cutaneous and buccopharyngeal. 

 These vertebrates have therefore reverted functionally to the respira- 

 tory method of such an invertebrate as an earthworm, in which there 

 are no specialized and localized organs for respiration. Some sala- 

 manders have lungs which are, in varying degree, small and weakly 

 developed — 'animals which therefore seem to be intermediate between 

 those that are lungless and those with well-developed lungs. It is 

 probable that absence of lungs is an adaptation to habitat. Most of the 

 lungless salamanders live in regions of swift-running mountain streams. 

 In the absence of lungs, the specific gravity of the body is greater. The 

 animal is therefore less likely to be carried downstream by the current. 



The paired appendages are never finlike. From the beginning of 

 their differentiation in the larva, they are pentadactyl legs — but not 

 always literally "penta-," for the number of digits is often reduced to 

 four and sometimes to three or even two. The pelvic girdle in modern 

 amphibians has skeletal attachment to a single vertebra, the sacral 

 vertebra. In the long-bodied amphibians the legs are short and not 

 strong enough to support the weight of the body, which is therefore 

 dragged rather than carried. In certain much-elongated eel-like am- 

 phibians (Fig. 349) the legs are so small and weak as to seem to be of 



Fig. 345.— (Continued) 

 capsule behind. The prootic bone is the only ossification in the otic capsule and, 

 in the adult frog, is fused with the adjacent exoccipital ossification. On the right 

 side the suspensorium is omitted so as to expose to view the annulus tympanicus 

 which supports the tympanic membrane of the "middle ear." 



