AMPHIBIA. 249 



ICHTHYOPSIDA. 



CHAPTER XXV. 

 CLASS II. AMPHIBIA. 



This class of Vertehrata comprises the Frogs and Toads, the Newts 

 and Land-salamanders, the Gcecilice, and some extinct forms, and it 

 may be briefly defined as follows : In all cases gills or branchiue 

 adapted for aquatic respiration are present during a part or the 

 whole of life ; but in all cases true lungs adapted for breathing air 

 are ultimately developed, even when the gills are retained through j 

 life. All pass through some sort of a metamorphosis after being set 

 free from the egg. The limbs may be absent, or there may be only 

 one pair, but in no case are they ever converted into fins. When 

 median fins are present, as is sometimes the case, these are never 

 furnished with fin-rays or interspinous bones, as in the fishes. The 

 skull always articulates with, or is jointed to, the spinal column by 

 two articular surfaces or condyles. The heart consists of two auricles 

 and a single ventricle. The nasal sacs always open behind into the 

 ' mouth ; and there is a common cavity or " cloaca '' which receives 

 not only the termination of the intestine {rectv/ni), but also the ducts 

 of the kidneys and of the reproductive organs. The skin is soft and 

 glandular, and, as a rule, neither horny nor bony structures are 

 developed in it. 



The great and distinguishing character of the Amphibia is that 

 they invariably undergo some kind of metamorphosis after birth ; 

 though in some rare cases the eggs are retained so long within the 

 body of the parent that there is little or no obvious change. In the 

 great majority of cases, however, the Amphibians commence life as 

 water-breathing larvae, provided with gills ; but in their adult state 

 they possess two air-breathing lungs, the gills sometimes disapjDear- 

 ing when the lungs are developed, but being sometimes retained 

 throughout life. In most cases, and always if retained, the gills are 

 external, placed on the sides of the neck, and not contained in a special 

 cavity, thus differing from the gills of fishes. In the Frogs and 



