184 ZOOLOGY 



of movement is not alone sufficient to distinguish plants from 

 animals. Both one-celled plants and animals require oxygen to 

 maintain life, as has been shown by repeated experiments. The 

 protoplasm of which the ceil is composed reacts to the same 

 stimuli in both plants and animals. The one distinction that 

 seems to exist between a plant and animal cell is that in the 

 plant cell food is manufactured by means of the chlorophyll con- 

 tained in it. Animal cells contain no chlorophyll and require 

 organic food. 



Habitat of Protozoa. — Protozoa are found almost everywhere in 

 shallow water, seemingly never at any great depth. They appear to be 

 attracted near to the surface by light and the supply of oxygen. Every 

 fresh-water lake swarms with them, the ocean contains countless myriads 

 of many different forms. 



Use as Food. — They are so numerous in lakes, rivers, and the ocean as 

 to form the food for many animals higher in the scale of Kfe. Almost all 

 fish that do not take the hook and that travel in " schools," or com- 

 panies, migrating from one place to another, live partly on such food. 

 Many feed on slightly larger animals, which in turn eat the Protozoa. 

 Such fish have on each side of the mouth attached to the gills a series of 

 small structures looking like tiny rakes. These are called the gill rakers, 

 and are used by the fish to collect tiny organisms out of the water as it 

 passes over the gills. The whale, the largest of all mammals, strains pro- 

 tozoans and other small animals and plants out of the water by means of 

 hanging plates of whalebone, the slender filaments of which form a sieve 

 from the top to the bottom of the mouth. 



Skeleton Building. — Some of the Protozoa build elaborate skeletons. 

 These may be formed outside of the body, being composed of tiny micro- 

 scopic grains of sand, or other materials. In some forms the skeleton is 

 internal, and may be made of lime which the animals take out of the water. 

 Still other Protozoa construct shells which house them for a time; then, 

 growing larger, they add more chambers to their shell, forming ultimately a 

 covering of great beauty. These shells or skeletons of Protozoa, falling to 

 the sea bottom, cover the ocean floor to a depth of several feet in places. 



The Protozoa have also played an important part in rock building. The 

 chalk cliffs of England and other chalk formations are made up to a large 

 extent of the tiny skeletons of Protozoa, called Foraminifera. Some lime- 

 stone rocks are also composed in large part of such skeletons. 



Flagellates. — Some cells show characters which are like both plants 

 and animals. Such are the group of organisms known as flagellates. All 

 flagellates move through the water by means of one or two (rarely more) 

 Jong threads of protoplasm, or cilia. Some flagellates are provided with 



