1020 



CLASS AMPHIBIA. 



may be greatly reduced. In existing forms there is generally no 

 exoskeleton, and if scutes or scales are developed they are buried 

 in the skin ; but in the Labyrinthodonts bony scutes were very 

 generally present, although frequently restricted to the ventral aspect 

 of the body. 



A marked, although not universal, feature in the class is the 

 change from a respiration by gills to one by lungs ; this change 

 being accompanied by other structural alterations, and termed a 

 metamorphosis. In some instances only external gills are developed, 

 which form a plume on either side of the neck ; and it is these gills 

 which persist in such forms as do not undergo a metamorphosis. 

 In those groups, however, in which a metamorphosis takes place 

 internal gills may be developed for a short period. 



That the Amphibia have taken their origin from primitive Fishes 

 allied to the Dipnoi and Ganoidei is pretty evident. Evidence of 



Fig. 950. — Enlarged view of the upper surface of the cranium of Protriton, with the charac- 

 teristic Labyrinthodont bones shaded ; from the Permian of Bohemia. N, Nasal ; F, Frontal ; 

 P. a, Parietal; S.O, Supraoccipital ; E.fi, Epiotic ; S.T., Supratemporal ; S.g, Squamosal; 

 P.t.f, Postfrontal; Ju, Jugal ; P.t.o, Postorbital ; im, Premaxilla ; Jl'I.S, Maxilla; QJ, 

 Quadratojugal ; Q, Quadrate. (After Fritsch.) 



affinity with the primitive Ganoids is indeed very clearly shown by 

 the so-called labyrinthic structure of the teeth of nearly all the 

 Palaeozoic Amphibia, since we find a similar type of dental structure 

 obtaining in many of the early Ganoids, and nowhere else in the 



