DIVISION I. ICHTHYOPSIDA. 



CHAPTER LX. 



CLASS IL— AMPHIBIA. 



The class Amphibia comprises the Frogs and Toads, the 

 Salamandroids, the Cmcilice, and the extinct Labyrinthodonts, 

 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 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 

 tins ; and when median fins are present, as is sometimes the case, 

 these are never furnished with fin-rays. The limbs, when pre- 

 sent, 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 theyundergo a metamorphosis after their exclusion 

 from the eg:g. 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 ex- 

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

 internal chamber as in Fishes. In some cases the external 

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

 which are retained in those forms -in which the branchite are 

 permanent {Perennibranchiata). In the tailed Amphibians 



