254 CHARACTERS OF VERTEBRATE ANIMALS 



and skin for purification and the latter to the head, while the 

 greater part of the body has to put up with mixed blood. There 

 are, however, numerous minor differences, one of the most striking 

 being the presence of three pairs of aortic arches as against four, 

 those corresponding to the third pair in the Salamander having 

 disappeared. 



The lungs are similar in nature to those in the Salamander 

 (see p. 244), being a pair of bags, the linings of which are raised 

 into a honeycombing of ridges. The supply of air of the lungs 

 is renewed in the same way, by a pumping action in the mouth- 

 cavity, the floor of which is alternately raised and lowered. 



The only points in the nervous system and sense organs which 

 need be noticed are the extreme shortness of the spinal cord and 

 the presence of a sound-conducting middle ear not unlike that 

 present in the Sand Lizard (see p. 192). 



The life-history of the Frog is extremely interesting. From 

 the eggs are hatched fish-like limbless tadpoles, in which the 

 breathing organs are at first external gills, like those of the 

 larval Salamander (see p. 240). After a time, however, these 

 begin to shrivel and are replaced by the so-called internal gills, 

 consisting of folds on the walls of the four pairs of gill -clefts, 

 quite unlike anything found in the Tailed Amphibia but closely 

 resembling the gills of some fishes. As these gills develop, a 

 fold of skin grows backwards over the gill-slits and unites to 

 the body behind them, leaving only a small round hole on the 

 left side through which is expelled the water that has been taken 

 in at the mouth and passed through the gill-slits. The adult 

 form is reached by growth of limbs, accompanied by loss of tail 

 and closure of the gill-slits, the lungs assuming their adult function. 

 There is also a change in the nature of the food, for a tadpole 

 is a vegetarian while the adult frog lives on insects and other 

 small animals. 



The Grass Frog has a wide distribution through Europe 

 and non-tropical Asia, while the family (Ranidae) to which it 

 belongs is represented in all parts of the world except New 

 Zealand and Polynesia. The well-known Edible Frog of the 

 Continent {Rana esculentd) is common in the east of England, 

 but has most likely been introduced. Much larger species of 

 the same family are the Bull Frog [Rana Catesbyand), inhabiting 

 the east of North America and attaining a length of over 7 inches, 



