502 



SYSTEMATIC ZOOLOGY 



modification of the nasal chamber, which now has not only an 

 external opening, but also an internal opening into the mouth 

 or pharynx. 



The eyes are kept moist while the animal is 

 on land by means of structures like eyelids, and 

 the ears in the higher Amphibia show a decided 

 advance in development over the condition in 

 fishes. They are now distinctly organs of hear- 

 ing, and have an eardrum, or tympanic mem- 

 brane, on a level with the surface of the skin, 

 and familiar to every one as the flat circular 

 disc on the sides of a frog's head. The cham- 

 ber within this tympanum connects with the 

 posterior part of the mouth by a tube, the 

 Eustachian tube, just as in all the higher 

 Vertebrata. 



In the lower Amphibia the tongue is rudi- 

 mentary ; in some it is absent. In the higher 

 groups it is attached at the anterior end of the 

 floor of the mouth and its free, posterior end is 

 bifid and can be thrown out to catch inse 

 and other small animals which serve as food. 

 Many can live a long time without food and 

 hibernate during the winter in the mud at the 

 bottom of streams and ponds. The intestine 

 and the urogenital ducts open into a cloaca, so 

 that there is a single opening at the posterior 

 end of the body, the cloacal opening ( Fig. 509). 

 The sexes are always separate. The eggs are 

 usually laid and fertilized in the water, and 

 develop independently without any care from 

 the parents ; the few exceptions will be men- 

 tioned later. The living Amphibia may be 

 divided into three subclasses, or orders, as they 

 are often called. 



SUBCLASS I. URODELA 





FiG. 310. Siren lacer- 

 titia. ( From Parker 

 and Haswell's Text- 

 book.) 



The Urodela (Gr. ovpd, tail, and Sf;Ao?, dis- 

 tinct) include all Amphibia which have a tail in the adult condi- 



