274 CLASSIFICATION OF VERTEBRATES. 



CLASS II. AMPHIBIA (BATRACHIA). 



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Ichthyopsida in which lungs are present and the gills are 

 usually lost in the adult. Median fins never supported by 

 dermal rays ; paired appendages in the shape of legs ; body 

 without scales except in caecilians and stegocephals ; a stapes 

 always present, and an Eustachian tube in the higher forms ; 

 nostrils communicating posteriorly with the mouth ; a post- 

 cava always present. 



The amphibia are readily distinguished from the fishes by 

 the absence of paired fins, their place being taken in most 

 forms by legs built upon the pentadactyl type, like those of 

 amniotes. Occasionally, as in Siren, one pair of limbs may 

 be absent, or again, as in the gymnophiona and, among the 

 stegocephals, the aistopoda, both pairs are lacking. Median 

 fins, confined to the caudal region, occur in the young of all, and 

 in the adults of many aquatic species, but these are never sup- 

 ported by dermal rays, while the tail is diphycercal in character. 



The skin is largely without cuticular structures, but its 

 outer layers become cornified and are periodically shed. The 

 deeper layers contain numerous glands ; the secretions of some 

 of these are acrid, and in some cases poisonous ; upon these 

 depends the safety of these otherwise unprotected animals. In 

 some cases the skin is smooth, in others it is roughened and 

 covered with warts, in part due to local thickenings, in part to 

 the presence of these defensive glands. Epidermic nails occur 

 on the toes of a few forms. 



In a few living species of anura, calcareous deposits occur 

 in the dermis, and occasionally (Ceratophrys, etc.) bony dorsal 

 plates may be developed in the same layer. In most gymno- 

 phiona semicircular dermal scales envelop the body, giving it 

 a ringed appearance externally. This dermal skeleton was 

 better developed in the extinct stegocephals, where we usually 

 find from one to three large ventral bony plates and a number 

 of smaller ventral scales, but occasionally this armor extended 

 over the back and limbs. 



The mouth is always terminal ; and teeth, when present, 

 occur on the margins of the jaws (premaxillaries and maxilla- 



