DI VISION L ICHTHYOPSIDA . 



CHAPTER LX. 

 CLASS IL AMPHIBIA. 



THE class Amphibia comprises the Frogs and Toads, the 

 Salamandroids, the Ccecili(z, and the extinct Labyrinthodonts, 

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

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

 the urinary bladder is the only representative of the allantois./ 

 As in Fishes, also, 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 Jin-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 ,ana 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 i 

 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 

 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 

 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 



