AMPHIBIOUS VERTEBRATES 457 



(Cryptobranchus later alis] of the Mississippi, the gills are said to 

 be " internal ", being in the form of folds on the walls of gill- 

 pouches, much like those of many fishes, and a gill-opening is 

 present on the left side of the neck. These gills are evidently on 

 the down-grade, and are still further reduced in the Giant Sala- 

 mander (Megalobranchus maximus) of Japan and China, where the 

 gill-pouches are fewer in number and there is no external gill- 

 aperture. 



A few adult Amphibia supplement their lungs by tufted ex- 

 ternal gills growing out from the sides of the neck. One of 

 these forms is the Olm (Proteus anguineus), found in the under- 

 ground waters of Carniola. In shape it resembles an eel, but 

 possesses small fore- and hind-legs. A somewhat similar form, 

 the Siren Salamander (Siren lacertina), inhabits the pools of 

 swampy districts in the south-east of the United States. In this 

 species the hind-legs have disappeared altogether. 



The large majority of adult Amphibians, such as ordinary 

 Salamanders, Newts, Toads, and Frogs, breathe only by means 

 of lungs (and skin) in the adult condition. Like all other Am- 

 phibia, however, they begin life as fish-like tadpoles, which possess 

 gills, and are at first devoid of limbs. If they did not develop 

 beyond this stage they would undoubtedly be classified as fishes. 

 This remarkable life-history is the most remarkable characteristic 

 of Amphibians, to which, indeed, their name has reference (Gk. 

 amphi, both; bios, life). Just as the Lung-Fishes bridge the gap 

 between ordinary Fishes and Amphibia, and demonstrate how 

 lungs have been evolved from swim-bladders, so do the Amphi- 

 bians lead up to the thorough-going land-groups of Reptiles, 

 Birds, and Mammals, which at no period of life possess gills, 

 although in the early stages of their existence gill-arches and 

 gill-clefts remain as a testimony to their aquatic ancestry. 



If we follow the development of a common Grass Frog (Rana 

 temporaria) we shall find that the young tadpole breathes by three 

 pairs of plume-like external gills, much like those present in the 

 adult Olm and Siren. Later on these are replaced by "internal" 

 gills, in the forms of folds on the outer sides of the gill-arches. 

 These are not strictly comparable to the gills of ordinary fishes, 

 but serve the same purpose. After a time a fold of skin grows 

 back over the gills, leaving only an opening to the exterior on 

 the left side of the body, and ultimately these organs gradually 



