CHARACTERS OF AMPHIBIA. 381 



DIVISION I. ICHTHYOPSIDA. 



CHAPTER LX. 



CLASS IL AMPHIBIA. 



THE class Amphibia comprises the Frogs and Toads, the 

 Salamandroids, the Cacilia, 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, branchia or filaments adapted for breathing air 

 dissolved in water are always developed iipon the visceral arches 

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

 bians 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 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 they undergo a metamorphosis after their exclu- 

 sion 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 



