ANIMAL LIFE 



CHAPTEE 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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