596 The Animal Kingdom 



has occurred in the circulatory system. The amphibian heart has two 

 auricles, a single ventricle, and a sinus venosus; thus there is some 

 sH'dit separation of oxygenated from unoxygenated blood and an ac- 

 companying increase in efficiency of circulation. The skeleton is formed 

 chiefly of bone, there are two occipital condyles, and the skull is rela- 

 tively weak. There is no control of body temperature, and the amphi- 

 bians usually hibernate deep in the soil or in mud at the bottom of 

 lakes and streams. 



The name "amphibian" refers to the fact that nearly all the mem- 

 bers of this class lead an amphibious life, that is, the larval stages live in 

 water while the adults live in moist situations on land. The adults must 

 return to water to lay the eggs. These eggs are without embyronic 

 membranes and usually hatch into larvae with gills. These larvae meta- 

 morphose into the adult before leaving their aquatic habitat. 



The Classification of the Modern Amphibia. — The living am- 

 phibians are classified as follows : 



Order I. Urodcla. Adult with tail, body with distinct head, trunk, and tail, legs 

 of equal size or lacking. 



Family 1. Proteidae. Mud puppy, Necttirus. 

 Family 2. Amphiumidae. Amphiuma, the congo eel. 

 Family 3. Cryptobranchidae. The hellbender, Cryptobranchus. 

 Family 4. Salamandridae. Newt, Triturits. 



Family 5. Ambystomidae. Tiger salamander, Amhystoma tigrinum. 

 Family 6. Plethodontidae. Red-back salamander, Plcthodon cinercus; two- 

 lined salamander, Eurycea bislineata. 

 Family 7. Sirenidae. Siren. 



Order II. Anura. Adults without neck or tail, head and trunk fused, hind legs 

 well developed, forelegs short. 



Family 1. Liopelmidae. Bell toad, Ascaphus. 



Family 2. Discoglossidae. Midwife toad, Alytcs obstctricans. 



Family 3. Pipidae. Marsupial frog, Pipa pipa. 



Family 4. Bufonidae. Toads, Bufo sp. 



Family 5. Pelobatidae. Spadefoot toad, Scaphiopus. 



Family 6. Hylidae. Cricket frog, Acr'is grylhis; spring peeper, Hyla sp.; 



marsupial frog, Gasfrothcca sp. 



Family 7. Ranidae. Leopard frog, Rana pipiens; bullfrog, R. catesbeiana. 



Family 8. Brevicipitidae. Narrow-m.outhed toads, Gastrophryne. 



Order III. Apoda. Wormlike forms, without legs or girdles. 



Family 1. Coeciliidae. Caecilians, IchtJiyophis and Gymnophis. 



