173 



CHAPTEE XXXIIL 



AMPHIBIA. 



The class Aiyiphihia comprises the Frogs and Toads, the Sala- 

 mandroids, the Ccecilice, and the extinct Labyrintliodonts, and 

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

 Fishes, tranchim, or filaments adapted for hreathing air dis- 

 solved in water, are always developed upo7i the visceral aixhes 

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

 bians differ from the Fishes in the fact that true lungs are 

 cdways present in the adidt ; the limbs are never converted into 

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

 case, these are never furnished ivith fin-rays. Tlie limbs, when 

 present, exhibit in their skeleton the same parts as do the limbs 

 of the higher Vertebrates. The skidl always artieidates ivith 

 the vertebral column by means of tivo 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 repo'-oductive organs open into a com- 

 mon 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 branchise ; 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- 

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



