104 Laboratory Guide in Zoology 



The members of this and the preceding classes are the 

 lowest of the true vertebrates, not only in structure, but 

 also in strength, intelligence, and sensibility. As a class, 

 however, the fishes are superior to all the others in number 

 and variety of forms. They live always in the water, are 

 cold blooded, and breathe by gills. Their fins are homolo- 

 gous with the limbs of the higher vertebrates. 



Class III. Amphibia. 



The Amphibia are cold-blooded animals, having gills 

 when young, and lungs when adult. In the early stages 

 the tail is modified into a swimming organ. The eggs are 

 deposited in the water or upon moist surfaces, the skin is 

 soft and naked, and the skeleton is bony or ossified. 

 There is no distinct neck. There are a number of orders 

 of Amphibia, the lower including the newts and salaman- 

 ders, or the tailed Amphibians, while the higher include 

 the frogs and toads. 



Class IV. Reptilia. Reptiles. 



These are cold-blooded animals, distinguished from the 

 Amphibia by never having gills, and from birds by the 

 absence of feathers. With the exception of some of 

 the turtles, they are carnivorous. The Reptilia includes 

 the snakes, the lizards, the turtles, and the crocodiles. 



Class V. Aves. Birds. 

 This is the most clearly defined class in the animal king- 

 dom. Its members are air-breathing, egg-laying, feathered 

 vertebrates with the front limbs developed for flying or 

 swimming, and the hind limbs for perching, walking, or 

 swimming. The bones are light and compact, and the neck 

 is usually long. The birds are a group of great interest 

 and of the greatest economic importance, whether we con- 

 sider their direct value to man as food, or their indirect 

 benefits in checking insect and other animal pests. 



