332 AMPHIBIA xn. 14- 



(serrations of the jaw elements) frequently occur on the dentary and 

 pre-articular (e.g. Amphodus) but the only modern form to possess 

 true teeth on the lower jaw is Amphignathodon. No recent frog retains 

 the large parietal foramen so typical of the fossil amphibia but some 

 leptodactylids and the aquatic xenopids have a small canal perforating 

 the fronto-parietal, through which runs a fibro-nervous tract from the 

 pineal organ to the habenular ganglion (Griffiths, 1954). The anuran 

 skull is always easily distinguished from those of all other Amphibia 

 by the fact that the frontals are fused with the parietals. 



Urodele skulls are, in some respects, less specialized than those of 

 Anura. The frontals and the parietals remain discrete and in certain 

 species both lacrimals and prefrontals are present. In other respects 

 they are clearly more degenerate (or paedomorphic ?). No urodele has 

 either a jugal or quadrato-jugal (except Tylotriton) and in perenni- 

 branchs even the maxillaries and nasals are lost. Urodeles are further 

 distinguished from frogs (but not from caecilians) by the great size 

 of the prevomers (each consisting really of a prevomer+ palatine) and 

 by the possession of a tooth-bearing coronoid, as well as a dentary 

 and a prearticular. 



The apodan skull is a much more rigid structure than that of either 

 of the above subclasses and, at first sight, approaches more closely to 

 the ancestral pattern. The number of bones present, however, is no 

 greater than in any of the other modern groups. The overall compact- 

 ness is effected particularly by the expansion of the nasals and of the 

 marginal elements of the upper jaw and is probably correlated with the 

 burrowing habits of the group. Lower as well as upper jaws carry 

 teeth and a toothed coronoid is present in the mandible. 



15. Respiration in Amphibia 



The new problems presented by life on land have led to the produc- 

 tion of very varied means of respiration among amphibia. In a ter- 

 restrial habitat oxygen is available in plenty; the difficulty is evidently 

 to arrange for a regular interchange of air in contact with adequately 

 moistened surfaces. The interchange is provided for in most cases 

 by modifications of the apparatus used in fishes, but pumping air 

 presents new problems and it seems that these are not easily solved, 

 since in many amphibians the skin is used as an accessory respiratory 

 mechanism. The retention of moisture becomes more difficult as the 

 ventilation becomes efficient; probably for this reason air is often 

 only transferred to the lungs after it has remained for some time in the 

 mouth. We see again that the new way of life, in a medium remote 



