ORDERS OF AMPHIBIA, 



339 



preventing the performance of the above-mentioned actions. 

 There can be no doubt, also, that the skin in these animals 

 plays a very important part in the aeration of the blood, and 

 that the frogs especially can carry on their respiration cutane- 

 ously, without the assistance of the lungs, fo,r a very lengthened 

 period. This undoubted fact, however, should not lead to any 

 credence being given to the often-repeated stories of the occur- 

 rence of frogs and toads in cavities in solid rock, no authen- 

 ticated instance of such a phenomenon being as yet known to 

 science. 



Fig. 151. Development of the common Frog (Rana temporaries), a Tadpole, viewed 

 from above, showing the external branchiae (g) ; b Side view of a somewhat older 

 specimen, showing the fish-like tail ; c Older specimen, in which the hind-legs have 

 appeared ; d Specimen in which all the limbs are present, but the tail has not been 

 wholly absorbed. (After Bell. ) 



The young or larvae of the Frogs and Toads are familiarly 

 known as " Tadpoles." The ova of the Frog are deposited in 

 masses in water, and the young form, upon exclusion from the 

 egg, presents itself as a "tailed" Amphibian, completely fish- 

 like in form, with a broad rounded head, a sac-like abdomen, 

 and a compressed swimming-tail (fig. 151, a). There are at 

 first two sets of gills, one external and the other internal. The 

 external branchiae (fig. 151,0) have the form of filaments at- 

 tached to the side of the neck, and they disappear very shortly 

 after birth. The internal branchiae are attached to cartilaginous 

 arches, which are connected with the hyoid bone, and they are 

 contained in a gill-cavity, protected by a flap of integument, 

 which differs from the gill-cover of fishes in never developing 

 any opercular bones or branchiostegal rays. Within the bran- 

 chial chamber thus formed the fore-limbs are budded forth, but 

 the hind-limbs are the first to appear, instead of the fore-limbs, 



