248 



CHARACTERS OF VERTEBRATE ANIMALS 



primitive characters are present, such as biconcave centra to the 

 vertebrse, eyelids represented by a circular fold, and (except in 

 the Giant Salamander itself) persistent remains of a gill-breathing 

 condition, seen, it may be, in the actual retention of gills along 

 with the lungs, or of a pair of gill-slits in the throat. 



Fig. 154.— The AxolotI 

 Adult or salamander stage {Amhlystoma tigrinum)^ above; larval or axolotl stage {Siredon Mexzcanus]^ below. 



The Hell-Bender, or Salamander of the Mississippi (Crypto- 

 branchus lateralis), has a pair of gill-clefts, or sometimes one 

 on the left side only. The Three-toed Salamander {Ampkiuma 

 means), from the same river, is eel-like in shape, with very small 

 weak limbs, terminating in three or it may be only two digits. 

 A pair of gill-slits are present. 



