I C H T H TOPS ID A. 

 CHAPTER XXVI. 



CLASS II. AMPHIBIA. 



THIS class of Yertebrata comprises the Frogs and Toads, 

 the Newts and Land-salamanders, the Ccecilice, and some ex- 

 tinct forms, and it may be briefly defined as follows : In all 

 cases gills or branchiae adapted for aquatic respiration are pres- 

 ent 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 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 condoles. The 

 heart consists of two auricles and a single ventricle. The na- 

 sal sacs always open behind into the mouth ; and there is a 

 common cavity or " cloaca " which receives not only the ter- 

 mination of the intestine (rectum), but also the ducts of the 

 kidneys and of the reproductive organs. 



The great and distinguishing character of the Amphibia 

 (Gr. amphi, both ; bios, life) 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 larva?, provided with gills ; but, in their adult 

 state, they possess true air-breathing lungs, the gills sometimes 

 disappearing when the lungs are developed, but being some- 



