GENERAL CHARACTERS 159 



present as in fishes and many reptiles. The absence of epider- 

 mic scales does not apply to the extinct Amphibia called Ste- 

 gocephali or Labyrinthodonts, and thus neither the epidermis 

 nor the derma affords a completely diagnostic character. 



There are, in addition to the above, many peculiarities of 

 internal structure in Amphibia which must be briefly de- 

 scribed. The skull articulates with the vertebral column by 

 two distinct condyles, a terrestrial character in which they 

 differ from fishes where there is no movable articulation at all, 

 and from all the higher classes except mammals — reptiles and 

 birds having only a single condyle. Huxley regarded this 

 similarity between Amphibia and mammals as indicating that 

 the latter sprang directly from the former, but later discoveries 

 show that the mammals are descended from primitive reptiles. 

 Like fishes the Amphibia in the aquatic larval stage possess 

 sense-organs in the skin of the head and of the sides of the 

 body, forming in the latter position not one lateral line but two 

 or more ; the organs are on the surface not enclosed in tubes. 

 After the metamorphosis the organs become covered by epi- 

 dermis, and in the Anura disappear altogether, but in some 

 Urodela they are found again on the surface of the skin when 

 the animals return to the water in the breeding season. With 

 regard to the gill-slits they resemble those of fishes and are se- 

 parated by similar gill-arches ; the number however is reduced 

 to four, not including the spiracle ; the external gills which are 

 developed on the first three gill-arches are similar to those of 

 larval lung-fishes and Polypterus. In the tailed Amphibia or 

 Urodela the gill-slits are never covered by an operculum, and 

 no internal gills are developed, but in the tail-less forms or 

 Anura, such as the common frog, the gills first formed disap- 

 pear and other smaller gill-processes are developed on the 

 arches which are then enclosed by the operculum or gill-cover. 

 Formerly these enclosed gills were called internal gills and sup- 

 posed to be homologous with those of fishes, but according to 

 Dr. Gadow they are really developed on the exterior of the 

 arches and are of the same nature as the original external gills. 



The lungs, like those of Polypterus and the lung-fishes, are 

 a pair of sacs arising by a single ventral aperture from the 

 pharynx or throat, immediately behind the gill-slits ; they pro- 

 ject freely into the abdominal cavity. In typical cases the gill- 



