REVIEW OF ANIMAL LIFE 303 



hearing. The highest animals among the articulates are 

 the insects. 



The Molluscs form another class, with which you are quite 

 familiar. The oyster, the clams, the different kinds of snails 

 and slugs, belong to this class. Most of them live in hard 

 calcareous shells, which protect their soft bodies. 



Worms, articulates, and molluscs have no bony skeleton. 

 Fishes, frogs, snakes, birds, and mammals do have a bony 

 skeleton, whose principal part is the backbone, or vertebral 

 column, which consists of a large number of bones called 

 vertebrae. 



Worms, articulates, and molluscs belong to the great division 

 of invertebrates ; fishes, frogs, snakes, birds, and mammals 

 are vertebrates. 



The lowest among the vertebrates are the Fishes. The 

 structure of their breathing organs, the gills, restricts them 

 to a life in the water. Fins are their limbs of locomotion, 

 their sense organs are not highly developed, and their brain 

 is very small. 



The next higher class, the Amphibians, comprises such 

 animals as frogs, toads, and salamanders. These animals 

 lay their eggs in water like fishes, but the young do not, 

 at first, resemble the adult, and pass through a metamor- 

 phosis. During the polliwog state they can breathe only 

 in water by means of gills. After some time their lungs 

 develop ; they can now breathe in the air, and they acquire 

 the form of the adults. Amphibians are covered with a soft, 

 naked skin ; they live in or near water, and in moist, shady 

 places. 



The next higher class, the Reptiles, deposit their eggs in 

 the ground, where they are hatched by the warmth of the 

 sun. The young breathe at once by means of lungs, and 

 resemble the adults. To this class belong snakes, lizards, 

 turtles, and alligators. Reptiles are covered with scales or 



