CLASS AMPHIBIA. 87 



nished with movable eyelids, and two ears, which can 

 be detected externally. 



The skeleton of the Frog (fig. 39) is of a much higher 

 type than that of a fish ; but there are only three points 

 W'hich need be particularly pointed out. Firstly, the 

 broad and flat skull is jointed to the backbone by two 

 distinct joints. Secondly, the joints or vertebrce of the 

 back (d) carry long side-processes, but the ribs are quite 

 rudimentary and merely consist of gristle. Thirdly, the 

 limbs are not in the least like the "fins" of fishes, but 

 they exhibit the same bones as are present in the limbs of 

 the higher animals. 



The common Frog is of a reddish-brown or yellowish- 

 brown colour, spotted with black ; but it has, to a certain 

 extent, the power of altering its colour according to the 

 intensity of the light to w^hich it may be exposed. It 

 feeds upon small slugs, worms, and insects, and is in turn 

 largely eaten by various animals, especially by Owls. It 

 is an excellent swimmer, and mostly passes the wdnter 

 under water, in a torpid condition. Owing to the length 

 of its hind-legs, it is also a capital jumper; and it can 

 produce the peculiar sound which is know^n as " croak- 

 ing." 



Recapitulation of Essential Characters. — The 

 animal when young is a water-breather, and is provided 

 with gills. In its adult state it breathes by means of 

 lungs (with or without the original gills). Both pairs of 

 limbs are usually jjresent, and they do not present the 

 form of "fins," possessing, on the contrary, the same 

 bones as in higher animals. The blood is cold. The 

 skull is jointed to the backbone by two joints, and the 

 nose opens behind into the throat. The heart is three- 

 chambered in the adult, but two-chambered in the young. 

 The skin is almost universally soft and destitute of scales. 

 These characters distinguish the class Amphibia as a 

 whole. 



