250 THE SCIENCE OF LIFE. 



Order 6. Dipnoi, (dis, twice ; pnce, breath,) comprises 

 the Mud-fishes (Lepidosircn) of tropical rivers. (Fig. 148.) 



FIG. 148. Lepidosren. 



They have eel-like bodies covered with cycloid scales. 

 Both ventral and pectoral fins are present, but are small 

 filiform organs, nowise resembling ordinary fins. They 

 have rudimentary external gills, and internal ones which 

 communicate with the exterior by a single slit. They 

 also possess true lungs, which communicate with the 

 gullet by a tube or trachea. The heart has two auricles 

 and one ventricle. They are quite Amphibian in struct- 

 ure, and live long out of the water. 



3. CLASS II, AMPHIBIA, (0w//, . both ; bios, life,) re- 

 ceives its name from the animals it contains being able 

 to live both on land and water. They are cold-blooded 

 Vertebrates which breathe by gills during some part of 

 their life, but sooner or later possess lungs. Some retain 

 their gills through the whole of their life, as the Proteus, 

 Siren, and Axolotl ; others lose their gills after a time, 

 and breathe by lungs only, as Frogs, Toads, and Newts. 

 All undergo metamorphoses after leaving the egg, pass- 

 ing through the " tadpole" state,, in which they resemble 

 Fishes in their respiration, circulation, and locomotion. 



Order i. Urodela, (oura, a tail ; delos, visible,) the tailed 

 Amphibia. They have a naked skin, and two to four 



