304 HANDBOOK OF NATURE STUDY 



with, horny plates. The smaller ones are well adapted to 

 live in holes or in rock crevices. The blood of fishes, am- 

 phibians, and reptiles has about the temperature of the air 

 or the water in which they live, and they always feel cool 

 to our touch, and for this reason they are often called cold- 

 blooded animals. 



Higher than the reptiles stand the Birds and the Mam- 

 mals. The blood of these two classes is kept at a uniformly 

 high temperature winter and summer; they are therefore 

 often spoken of as warm-blooded animals. For reasons given 

 under "Review of Birds and Review of Mammals," the 

 latter must be placed at the head of the animal kingdom. 



This brief sketch is not even an attempt at a complete 

 classification ; in fact, many important classes of marine 

 animals have not been mentioned at all. 



b. Conditions for animal life. Animals depend for food 

 either upon plants or upon other animals. All of them need 

 air, which they breathe directly by means of lungs, or absorb 

 it from the water by means of gills. Nor can one of them 

 live entirely without water, although some find a sufficient 

 amount of it in the food which they eat. Warmth and 

 light, as* you can easily prove, are also important factors in 

 animal life. 



