44 LOUIS PASTEUR 



older observers formed a most varied assemblage. 

 Many of them proved to be the minute represen- 

 tatives of higher groups of animals, such as worms 

 and crustaceans. Others, such as the wheel ani- 

 malcules, belong to groups of many-celled forms 

 characterized by their small size. But a large pro- 

 portion of this minute world is found to be com- 

 posed of organisms consisting of a single cell, the 

 one-celled animals being known as the Protozoa, 

 and the one-celled plants as the Protophyta. 



The Protozoa form an extensive group, includ- 

 ing many thousand known species of the greatest 

 diversity of form, size, and behavior. The majority 

 live in water, but some, like the soil Amoebae, live 

 in earth; others are found in decaying organic 

 matter; some species live in the tissues of plants, 

 and many kinds are parasitic within the bodies of 

 animals. In the sea they are represented by very 

 numerous forms, many of which are furnished with 

 beautiful silicious or calcareous skeletons and cov- 

 erings, whose accumulation at the bottom of the 

 ocean is responsible for the formation of chalk, 

 many limestones, and other rocky deposits. To- 

 gether with the unicellular plants of the sea, which 

 are able to build up their living substance out of 

 the salts dissolved in the water, they afford the 



