SUB-KINGDOM L VERTEBRATA. 



CLASS IV. AMPHIBIA. 

 BY R. LYDEKKER, B.A., F.R.S., V.P.G.S., &c. 



IN popular natural history nearly every cold-blooded verebrate that breathes 

 atmospheric air by means of lungs is commonly spoken of as a reptile, whether 

 it crawls or creeps on the ground like a lizard, a frog, or a snake, or swims 

 in the water like a newt ; and if we take their adult condition alone into 

 consideration, there is a good deal to be said in favour of regarding frogs 

 and salamanders as reptiles, since they agree in many of their essential 

 characters with the creatures to which the naturalist restricts that term. 

 The case is, however, very materially altered when the developmental history 

 of frogs and salamanders is taken into consideration. As every one knows, 

 these creatures with a few exceptions commence their active existence 

 after leaving the egg in the form of a tadpole. And as a tadpole is an aquatic 

 creature, breathing the air dissolved in water by means of gills, and being 

 provided with a fish-like tail, it is obvious that we have here a very important 

 distinction from reptiles, which, at the commencement of life, are mere 

 miniatures of the adult. It may be said, indeed, that frogs and salamanders 

 commence life as fishes, but turn into reptiles when they become adult, 

 and the existence of this remarkable change or metamorphosis serves to 

 differentiate these creatures from reptiles as a distinct class, for which the 

 name Amphibia is now usually adopted, although the alternative term 

 Batrachia is also extant. It is true that in certain cases no such metamor- 

 phosis exists, the egg developing at once into a perfect air-breathing frog, 

 while in some other instances the gills of the larval state are retained through- 

 out life. In the former of these instances it is, however, evident that the 

 larval stage has been suppressed owing to the exigencies of abnormal condi- 

 tions in the life-history of the particular species. In the latter case, either 

 the creature has now ceased to develop into the air-breathing adult form, or 

 it has never advanced beyond the larval stage at any period of its history. 



Amphibians, then, may be defined as cold-blooded vertebrates usually 

 furnished with external gills at the commencement of life, but before becom- 

 ing adult passing through a metamorphosis during which the gills are ex- 

 changed for lungs ; and such of them as possess limbs having these struc- 

 turally similar to those of reptiles. This change from a fish-like to a reptile-like 

 animal is one of the most marvellous phenomena to be met with among 

 vertebrates, and it is not a little remarkable that while everybody has the 

 opportunity of witnessing this transformation each year of their lives, so many 

 still refuse to believe in the evolution of animal life in general. 



The metamorphosis is, however, by no means the sole feature by which 

 amphibians are distinguished from reptiles. There are many structural 

 peculiarities by which the adults of the former differ from the latter, although 

 these are, unfortunately, deep-seated, and not externally apparent. Com- 



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