ANIMAL LIFE 
CHAPTER I 
THE LIFE OF THE SIMPLEST ANIMALS 
1. The simplest animals, or Protozoa.—The simplest ani- 
mals are those whose bodies are simplest in structure and 
which do the things done by all living animals, such as 
eating, breathing, moving, feeling, and reproducing in the 
most primitive way. The body of a horse, made up of 
various organs and tissues, is complexly formed, and the 
various organs of the body perform the various kinds of 
work for which they are fitted in a complex way. The 
simplest animals are all very small, and almost all live in 
the water; some kinds in fresh water and many kinds in 
the ocean. Some live in damp sand or moss, and still others 
are parasites in the bodies of other animals. They are not 
familiarly known to us; we can not see them with the. 
unaided eye, and yet there are thousands of different kinds 
of them, and they may be found wherever there is water. 
In a glass of water taken from a stagnant pool there 
is a host of animals. There may be a few water beetles 
or water bugs swimming violently about, animals half an 
inch long, with head and eyes and oar-like legs; or there 
may be a little fish, or some tadpoles and wrigglers. These 
are evidently not the simplest animals. There will be 
many very small active animals barely visible to the un- 
aided eyes. These, too, are animals of considerable com- 
plexity. But if a single drop of the water be placed 
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