346 AMPHIBIA. 



CHAPTER XXXI. 

 A M P H I B I A. 



THE class Amphibia comprises the Frogs and Toads, the Sala- 

 mandroids, the C&cili<z, and the extinct Labyrinthodonts, and 

 may be briefly defined as follows : As is the case with the 

 Fishes, branchia, or filaments adapted for breathing air dissolved 

 in water, are always developed upon the visceral arches for a longer 

 or shorter time. On the other hand, the Amphibians differ from 

 the Fishes in the fact that true lungs are always present in the 

 adult; the limbs are never converted into fins ; and when median 

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

 with fin-rays. The limbs, when present, exhibit in their skeleton 

 the same parts as do the limbs of the higher Vertebrates. The 

 skull always articulates with the vertebral column by means of two 

 occipital condyles. The heart consists of two auricles and a single 

 ventricle. The nasal sacs communicate posteriorly with the 

 pharynx; and the rectum, ureters, and ducts of the reproductive 

 organs open into a common chamber or " cloaca" 



The great and distinguishing character of the Amphibia is 

 the fact that they undergo a metamorphosis after their exclusion 

 from the egg. They commence life as water-breathing larvae, 

 provided with gills or branchiae ; but in their adult state they 

 invariably possess lungs ; the branchiae in the higher forms dis- 

 appearing when the lungs are developed, but being in other 

 cases permanently retained throughout life. 



In the earliest embryonic condition the branchiae are exter- 

 nal, placed on the side of the neck, and not situated in an 

 internal chamber as in Fishes. In some cases the external 

 branchiae only are present, and they are, in any case, the gills 

 which are retained in those forms in which the branchiae are 

 permanent (Perennibranchiata). In the tailed Amphibians 

 ( Urodela) and in the Frogs and Toads (Anoura) two sets of 

 gills are developed an external set, which is very soon lost, 

 and an internal set, which is retained for a longer or shorter 

 period. As maturity is approached, true lungs adapted for 

 breathing air are developed. The development, however, of 

 the lungs varies with the completeness with which aerial respi- 

 ration has to be accomplished ; being highest in those forms 

 which lose their gills when grown up (Caducibranchiata), and 

 lowest in those in which the branchiae are retained throughout 

 life (Perennibranchiata). 



The class Amphibia is divided into the four orders of the 



