6 COLLEGE ZOOLOGY 



are, however, of great importance, especially those which cause 

 diseases such as malaria. Protozoa are to be found almost 

 everywhere. If a few dead leaves are placed in a dish of water 

 and left to decay, the scum which forms on the surface will be 

 found to contain thousands of these minute organisms. The 

 simplest animals belong to the Protozoa; among these are 

 Ameba (Fig. 9), Paramecium (Fig. 33), and Euglena (Fig. 22), 

 which will be studied in some detail in Chapter II. 



Few people realize the abundance and variety of animal life. 

 Almost every part of the earth is inhabited by animals of some 

 kind, and these animals are more or less 'restricted to certain 

 kinds of habitats. For example, fishes live in the water, earth- 

 worms in the ground, the polar bear in Arctic regions, the ele- 

 phant in the Tropics, the prairie dog on the prairies, the moun- 

 tain goat on the mountains, and parasites upon or within the 

 bodies of other organisms. Four principal kinds of animals 

 may be recognized according to their mode of existence: (1) ma- 

 rine animals living in the salt waters of the sea, (2) fresh-water 

 animals living in fresh-water streams, ponds, and lakes, (3) ter- 

 restrial animals living on land, and (4) parasites which live on or 

 within the bodies of other animals. 



The oceans are inhabited by millions of animals of all sizes, 

 ranging from the whale to the microscopic floating organisms 

 known as plankton. Salt-water animals are restricted to certain 

 definite regions; some float on or near the surface, and others 

 live at various distances from the surface, until a depth is reached 

 where the light never penetrates. As a rule, animals living in 

 salt water die almost at once if transferred to fresh water; like- 

 wise salt water is fatal to fresh-water animals. 



Every pond, lake, brook, creek, and river is inhabited by a host 

 of living animals. A pond, for example, furnishes a home for 

 the early stages in the life history of the mosquito, whose eggs are 

 laid in a raft-like mass on top of the water, and whose young 

 swim about at or near the surface. Frogs and salamanders find 

 a home amid the vegetation common to ponds. Crayfishes 



