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DIVISION LICHTHYOPSIDA. 



CHAPTER LVIL. 

 CLASS IL AMPHIBIA. 



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

 mandroids, the Ccecilice, and the extinct Labjrinthodonts, and 

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

 Fishes, the embryo is not furnished with an amnion, and the 

 urinary bladder is the only representative of the allantois. As 

 in Fishes, also, branchice or filaments adapted for breathing air 

 dissolved in water are always developed tipon the visceral arches 

 for a longer or shorter time. On the other hand, the Amphibi- 

 ans 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 (rarely concealed 

 or inconspicuous) 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 disappearing 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 



