172 THE ANATOMY OF VEETEBRATED ANIMALS. 



CHAPTER IV. 



THE CLASS AMPHIBIA. 



The IcHTHTOPSiDA. — Class II. — Amphibia. 



The only clearly diagnostic characters of this class as 

 compared with Fishes are the following : — 



1. Amphibia have no fin-rays. 



2. When limbs are present they contain the same skeletal 

 elements as those of the higher Vertehrata. 



Certain other structural peculiarities are common to the 

 whole of the Amphibia, and are very characteristic of them 

 without being diagnostic. Thus : — 



1. The body is usually devoid of any exoskeleton, and 

 when scales, or scutes, are present in recent Amphibia, they 

 are concealed within the skin (Ccecilia, Ephippifer). In the 

 extinct Lahyrinthodonta, the dermal armour is confined to 

 the ventral region of the body. 



2. The vertebi'al centra are always represented by bone. 



3. The sacmm rarely consists of more than one vertebra, 

 though there are individual exceptions to this rule, as in 

 Menopmna. 



4. The suspensorial apparatus of the mandible is con- 

 tinuous with the skull, which has two occipital condyles, 

 and no completely ossified basi- occipital. 



5. There are no sternal ribs. 



The Amphibia are divisible into the following groups : — 

 A. A distinct and often long tail; the vertebise araphicoelous or 

 opisthocuelous ; the proximal elements of the tarsus not 

 elongated. 

 A. Two or four limbs ; no scutes or scales. 



I. Saurobatrachia or Urodela. 

 a. External branchiae or gill-clefts persistent, or disap- 



