134 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



4 (i). Not fishlike; always with lungs (sometimes gills in addition); paired 

 appendages in the form of limbs or absent. 5 



5 (6). Skin naked and slimy; never marine. Class Amphibia 



6 (5). Skin provided with an exoskeleton consisting of hairs, scales, or 

 feathers (if apparently absent, large marine forms). 7 



7 (8, 9). Exoskeleton in the form of scales, never with hair or feathers. 



Class Reptilia 



8 (7, 9). Exoskeleton in the form of feathers and scales. Class Aves 



9 (7, 8). Exoskeleton in the form of hairs (sometimes apparently absent, 

 sometimes scales present also) ; nearly always nourishing the young with milk. 



Class Mammalia 



