INTRODUCTION 



Irritability 



Animals are irritable and respond quickly 

 to changes in their surroundings, such as 

 changes in temperature and in light. Plants 

 respond less quickly, and nonliving things 

 do not respond at all. 



Metabolism 



Animals and plants are "machines" that 

 run themselves. This is due to the processes 

 of metabolism whereby protoplasm is broken 

 down to furnish energy and is built up again 

 out of food. Animals require other animals 

 and plants for food, whereas plants are able 

 to manufacture their own food from non- 

 living materials. The ability to transform 

 environmental material into its own specifi- 

 cally organized and active substance is one 

 thing that distinguishes living from nonliv- 

 ing matter. 



Growth 



Animals and plants grow as a result of the 

 building up of protoplasm within the cells. 

 Nonliving things may increase in size, but 

 the new material is added to the outside. 



Reproduction 



Animals reproduce others of their kind. 

 In general, nonliving bodies cannot repro- 

 duce their kind. 



The unity of animal life is thus clearly 

 c\ident in composition, structure, form, 

 movement, irritability, metabolism, growth, 

 and reproduction. 



ANIMAL HABITATS 



Most areas on the surface of the earth are 

 inhabited by animals. We are familiar with 

 many species that live on land; with fresh- 

 water inhabitants, such as fish and frogs; and 

 with salt-water types, such as seals, whales, 

 and sharks. Parasites that live on or within 

 the bodies of other animals are less well 

 known. The four major habitats of animals 



that are briefly described here are salt water, 

 fresh water, land, and other organisms. A 

 more detailed account of animal habitats is 

 presented in a later chapter on Ecology and 

 Zoogeography. 



Salt-water animals 



About 72 per cent of the earth's surface is 

 covered by the sea; this salt water serves as 

 a home for vast numbers of different kinds 

 of aquatic animals. As a rule salt-water ani- 

 mals cannot live in fresh water or on land. 

 Furthermore, they do not roam over the 

 ocean at will, but are restricted to definite 

 habitats. For example, a large number of 

 animals are found only on the beaches; some 

 live on sand beaches and others on mud 

 beaches; some are attached to rocks and 

 others live among seaweeds. The open ocean 

 is thickly populated with animals; many are 

 able to swim about, but others float near 

 the surface and are carried from place to 

 place by waves and currents. As a rule each 

 species seeks a certain depth and does not 

 move up or down beyond a more or less 

 narrow range. 



Plants and animals that live in the sea 

 usually sink to the bottom when they die. 

 On this account the sea bottom is a favor- 

 able habitat for scavengers, and a distinct 

 group of animals lives in this debris. Each 

 of these sea habitats— the beaches, open 

 ocean, and sea bottom— may be subdivided 

 into several minor habitats, which indicates 

 how restricted animals really are in the 

 character of their environment. The study 

 of the relation of living things to their en- 

 vironment is called ecology. The marine 

 animals alone may be divided into about 

 50 groups, according to the nature of the 

 environments in which they live. 



Fresh-water animals 



Fresh-water animals live in lakes, ponds, 

 pools, rivers, streams, swamps, and bogs. 

 Some prefer flowing water, and others prefer 



