PHYLUM PROTOZOA 23 



Habitats.^ — ^Protozoa are encountered in extremely diverse 

 habitats. All classes, with the exception of the Sporozoa, have 

 numerous species which occur as free-living organisms in both 

 fresh and salt water. Soil-inhabiting species are not uncommon. 



Many species are widely distributed over the face of the earth 

 wherever conditions favorable for their existence are found. 

 This broad distribution seems to be associated with the power of 

 entering into an inactive state during which the body is sur- 

 rounded by a protecting wall. This encysted condition may 

 come as part of a definite life cycle or may result from inhospitable 

 conditions, such as the evaporation of the water in which active 

 specimens are living. In this state the inactive cysts may be 

 carried extremely great distances by the wind or other agencies 

 of distribution. Encysted forms are excellent examples of sus- 

 pended animation, for when the cysts fall into favorable con- 

 ditions the cyst wall is lost and the protozoan becomes active after 

 its period of dormancy. By this means many species have 

 attained almost cosmopolitan distribution, so that students in 

 Europe, Asia, Africa, and South America find in their pools and 

 lakes many of the same species that North American students 

 encounter. Forms of the open ocean and of the deep seas seem 

 to be more restricted in their distribution. Even different regions 

 in the same ocean may yield characteristically different lists of 

 protozoan fauna. The limitations of distribution in the ocean are 

 even more striking when we consider that certain species live 

 only at the surface of the water while some genera have never 

 been discovered anywhere except in the extreme depths of the 

 seas. 



Many forms occur as cysts in the soil but some, especially 

 rhizopods and flagellates, remain active in ordinary garden soil 

 and are thought to be important because they feed upon bacteria 

 of the soil. Little is known of the actual role of Protozoa in 

 connection with the problems of soil biology. While active 

 Protozoa doubtless feed upon important soil bacteria, no one has 

 yet proved that they exert an injurious effect upon the soil. 



The parasitic habit has become the exclusive condition among 

 the Sporozoa, but in each of the other classes the same habit is 

 encountered to a greater or less degree. Some species have the 

 faculty of leading either an independent or a parasitic existence, 

 as opportunity is presented to the individual animal. Espe- 

 cially in tropical countries the diseases of man and of his domestic 



