254 VERTEBEATE AKIMALS. 



tail, and breathes wholly by lungs, and through the skin. Both 

 pairs of limbs are always developed in the full-grown animal, and 

 the hind-limbs are usually considerably longer than the fore-limbs, 

 and generally have the toes webbed, whilst those of the fore-limbs 

 are free. The skin is soft, and there are rarely any traces of any 

 integumentary skeleton. The spinal column is short ; the dorsal 

 vertebra; are very long ; and the ribs are quite rudimentary, their 

 place being taken by greatly-developed transverse processes. The 

 bodies of the vertebrae are hollow in front and convex behind {pro- 

 ccelous). The bones of the fore-arm {radius and vlnu), and those of 

 the shank {tihia and Jihwla), are united together to form single bones. 

 The upper jaw is usually furnished 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 



Fi.;, 1S3. — The common Frog (Rana temporaria). 



which the cavity of the chest e.in 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 important 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 larvte are familiar to every one as tadpoles. Upon its 

 escape from the egg, the young frog (fig. 184) presents itself as a 

 little fish-like creature with a broad head, a sac-like bellv, and a 



