24 INVERTEBRATE ZOOLOGY 



animals are very frequently the result of protozoan parasites. 

 Malaria, sleeping sickness, dysentery, Rocky Mountain spotted 

 fever, Texas fever are some of the diseases for which the causal 

 agent is a protozoan. 



Food Habits. — In food habits, the Protozoa display great 

 diversity. Many of the chlorophyll-bearing Mastigophora are 

 plantlike in their metabolism. Under the influence of sunlight 

 their chlorophyll synthesizes food substances in a purely plant- 

 like manner. These forms in which the food is built up from 

 simple compounds by the processes of photosynthesis are 

 designated as autotrophic. 



Most of the Protozoa are holozoic in their metabolism and require 

 complicated organic compounds as foods. These organic foods 

 may be ingested as solid particles either through the action of 

 pseudopodia as in Amoeba and other Rhizopoda or through a 

 cytostome as in Parameciuin and many other ciliates and many 

 flagellates. In the endoplasm, these solid food particles undergo 

 digestion in food vacuoles before they are assimilated. In the 

 instances just cited the food material may be either living or 

 dead plant or animal matter. However, in some species the 

 predaceous habit is rather firmly fixed. Thus Didinium lives 

 largely upon paramecia the bodies of which it ingests through a 

 highly specialized cytostome located at the tip of a proboscis. 

 The Suctoria through their hollow tentacles are enabled to 

 suck out the protoplasm from other organisms and utilize it 

 as food. 



Organic matter may be absorbed through the body surface in 

 many Protozoa. This is especially true of forms which lack a 

 cytostome. These organisms dependent upon the absorption 

 of elaborated food stuffs may be either parasitic upon living 

 organisms or may utilize decomposing organic matter. In the 

 latter instance, they are said to be saprophytic. 



Cultures. — Under usual conditions. Protozoa are present in 

 stream, pond, or lake water in relatively small numbers. Various 

 factors in the environment cooperate in keeping the numbers of 

 any given species from becoming excessive. If individuals of a 

 given species multiply unusually, forms which feed upon this 

 species, or depend upon it in other ways, will naturally increase, 

 thereby tending to reduce the excessive numbers. Thus the 

 "balance of nature" works here even as among the higher forms 

 of life. If food is present in excess and natural enemies are lack- 



