9 ANIMAL LIFE 
on a glass slip or in a watch glass and examined with a 
compound microscope, there will be seen a number of ex- 
tremely small creatures which swim about in the water-drop 
by means of fine hairs, or crawl slowly on the surface of the 
glass. These are among our simplest animals. There are, 
as already said, many kinds of these “simplest animals,” 
although, perhaps strictly speaking, only one kind can be 
called simplest. Some of these kinds are spherical in 
shape, some elliptical or football-shaped, some conical, some 
flattened. Some have many fine, minute hairs projecting 
from the surface; some have a few longer, stronger hairs 
that lash back and forth in the water, and some have no 
hairs at all. There are many kinds and they differ in size, 
shape, body covering, manner of movement, and habit of 
food-getting. And some are truly simpler than others. 
But all agree in one thing—which is a very important 
thing—and that is in being composed in the simplest way 
possible among animals. 
2. The animal cell.—The whole body of any one of the 
simplest animals or Protozoa is composed for the animal’s 
whole lifetime of but a single cedd. The bodies of all other 
animals are composed of many cells. The cell may be 
called the unit of animal (or plant) structure. The body 
of a horse is complexly composed of organs and tissues. 
Hach of these organs and tissues is in turn composed of a 
large number of these structural units called cells. These 
cells are of great variety in shape and size and general 
character. The cells which compose muscular tissue are 
very different from the cells which compose the brain. 
And both of these kinds of cells are very different from 
the simple primitive, undifferentiated kind of cell seen in 
the body of a protozoan, or in the earliest embryonic 
stages of a many-celled animal. 
The animal cell is rarely typically cellular in character 
—that is, it is rarely in the condition of a tiny sac or box 
of symmetrical shape. Plant cells are often of this char- 
