AMPHIBIA. 



231 



nished with teeth, and the lower jaw sometimes, but there 

 are no teeth in the Toads. The lungs are well developed, 

 comparatively speaking ; and, as there are no ribs by which 

 the cavity of the chest can be expanded, the air is taken into 

 the lungs by a process nearly akin to that of swallowing. 

 There can be no doubt, also, that the skin plays a very im- 

 portant part in the aeration of the blood, and that the frogs, 

 especially, can carry on their respiration by means of the skin 

 without the assistance of the lungs for a very lengthened 

 period. This, however, should not lead to any credence being 

 given to the often-repeated stories of frogs and toads being 

 found in closed cavities in solid rock, no authenticated instance 

 of such an occurrence being known to science. The ova of 

 the frogs and toads are deposited, in masses or strings, in 

 water, and the young or larvae are familiar to every one as 

 tadpoles. Upon its escape from the egg, the young frog (Fig. 



Fig. 115. — Development of the common Pro^. a Tadpole, viewed from above, showiDg the 

 external branchiie (g) ; b Side view of a somewhat older specimen, showing the fish-like 

 tail; c Older specimen, in which the hiDd-leg:s have made their appearance; d Specimen 

 in which all the limbs are present, but the tail has not been wholly absorbed. (After 

 Bell.) 



115) presents itself as a little fish-like creature with a broad 

 head, a sac-like belly, and a long, compressed tail with which 

 it swims actively. It breathes by means of gills or branchiae, 

 of which there are two sets, one external, and the other in- 

 ternal ; at first there are no limbs ; but, as development pro- 

 ceeds, the limbs make their appearance — ^the hind-legs first, 

 and then the fore-legs. The tail, however (Fig. 115), is still 

 retained as an instrument of progression. Ultimately, when 



