2 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 cell. 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. 

 Each 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- 



