334 AMPHIBIA xh. 16- 



exchange in the lungs. The rate of breathing depends, as in mammals, 

 on the effect of the carbon dioxide tension of the blood on a respira- 

 tory centre in the medulla. There is also a vasomotor control of the 

 blood-supply to the lungs and, through the vagus nerve, of the state 

 of contraction of the latter. By such means the rate of respiratory 

 exchange is greatly increased during the breeding season, and made 

 to vary with the activity of the animal. 



17. Respiratory adaptations in various amphibians 



The skin and the lungs show many variations according to the 

 habitat of the species, special devices being adopted to enable the 

 animals to live in particular environments. The lungs vary from the well 

 vascularized sacs with a highly folded surface found in the frogs, 

 and especially in the drier-skinned toads, to small simple sacs in some 

 stream-living amphibia. The lung will serve to lift the animal in the 

 water; for this reason it is reduced in the frog Ascaphus, which lives in 

 mountain streams in the eastern U.S.A. In newts this hydrostatic 

 function of the lungs is predominant and the inner surface is often 

 quite simple. The lung is entirely lost in stream-living salamanders, 

 such as the European alpine S. atra. The coldness of the water reduces 

 activity and lowers the need for respiratory exchange to a level at 

 which it can be fully met by the skin. The skin shows increased vascu- 

 larity in these forms with reduced lungs, capillaries reaching nearly 

 to the outermost layers of the epidermis. In the African frog Astylo- 

 sternus, in which the lungs are vestigial, the male develops vascular 

 papillae on the waist and thighs during the breeding season. 



Gills are present in amphibian larvae, and also in certain adult 

 urodeles that may be considered as larvae that have failed to undergo 

 metamorphosis (p. 364). The gills are extensions of the branchial 

 arches, and carry branched villi, richly supplied with blood. Where 

 the main trunk is long the gill projects and is 'external', whereas in 

 other cases, as in the later frog tadpole, the filaments are directly 

 attached to the arches and are called 'internal'. There are no profound 

 differences between the two types. 



18. Vocal apparatus 



Sound is produced as a protective (fear) response and by the male 

 frog as a call to attract the female. Both sexes have vocal organs, but 

 those of the female are much the smaller. The noise is produced by 

 the vibration of the elastic edges of a pair of folds of epithelium of the 

 laryngeal chamber, the vocal cords. Air is passed backwards and 



