CHAPTER IX 
THE SIMPLEST ANIMALS — PROTOZOA 
109. Definitions. — In our study of the grasshopper and 
its insect relatives we considered their behavior and life 
processes. If we had studied the minute structure of any 
of these insects, the grasshopper, for example, and had used 
a microscope to aid us, we should have found that every 
organ was made up of numerous small parts joined together 
in a definite manner. These small parts are called cells. 
Any book on biology uses the word cell again and again. 
The name was first used by the Englishman, Robert Hooke, 
over two hundred years ago, when, with his crude micro¬ 
scope, he examined a piece of bark and found it to be made 
up of little rooms which looked like the cells of the honey¬ 
comb; These spaces he named cells. When better micro¬ 
scopes were made, the living parts of the cell were discovered, 
and it was found that Hooke had seen only the walls of dead 
cells. 
All plants and animals are composed of cells. A cell may 
exist alone, carrying on all the life processes itself, or it may 
exist in connection with a great many other cells, as in all 
large animals and plants. In every case each cell is pro¬ 
duced from another cell. 
There are certain animals that are never more than one- 
celled even when they are full grown. These animals are 
called Protozoa (pro-to-zo'a: Greek, protos, first; zoon, 
animal). 
110. The Protozoan Cell. — The protozoan cell is a single 
mass of living matter, called protoplasm. In a general way 
it carries on the same life processes as the grasshopper, or 
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