AMPHIBIANS 249 



The flesh-coloured Olm (Proteus anguineus) (fig. 153) is a 

 remarkable eel-like form, about 10 inches long, which inhabits 

 the underground lakes and streams of certain caves in Dalmatia, 

 Carniola, and Carinthia. Both fore- and hind-limbs are very 

 small, and furnished respectively with three and two digits. 

 Coral-coloured external gills are present as well as two pairs of 

 gill-slits. The eyes are small and concealed beneath the skin. 

 Much longer (28 inches) than the Olm is the Siren (Siren 

 lacertina) of the south-east part of the United States. The 

 dark-coloured body is extremely eel-like and the hind-limbs are 

 entirely absent, while the four -toed fore -limbs are very small. 

 External gills and three pairs of gill-slits are present. 



One of the most interesting Urodeles is the Mexican Axolotl 

 (fig. 154), which in what must be called the adult state is a 

 thorough -going Salamander, known as Amblystoma tigrinum, 

 and in the United States, to which the range of the animal 

 extends, this adult condition is reached. In the lake surrounding 

 the city of Mexico, however, this is not the case, and the Axolotl 

 permanently remains in the larval state, resembling an enormous 

 newt tadpole. Gill-slits and external gills are present, and eggs 

 are laid just as by adults. We have here the astonishing pheno- 

 menon of an animal which, so to speak, is dropping the adult 

 stage out of its life-history, the conditions being unfavourable 

 for its development. We only know this by accident, and the 

 precocious tadpoles were, when first described, thought to be adult, 

 receiving the name of Siredon Mexicanus. There are doubtless 

 other still undetected cases of this remarkable phenomenon in the 

 animal kingdom. 



Order 2. TAILLESS AMPHIBIANS (Anura) 



While the Tailed Amphibia are confined to the Northern 

 Hemisphere, the Frogs and Toads, which constitute the group 

 of Tailless Amphibia, are cosmopolitan. The Grass Frog (Rana 

 temporaries), which is one of the most familiar British Vertebrates, 

 is a good type of one of the most widely-distributed families of the 

 order, and a brief description of its more obvious external char- 

 acters (fig. 155) will illustrate the differences which distinguish 

 animals of the sort from Salamanders and their allies. 



The flattened triangular head, with rounded snout, passes 



