8 PROTOZOOLOGY 



as larvae of insects, small crustaceans, annelids, etc., all of which 

 depend largely upon Protozoa and Protophyta as sources of food 

 supply. Thus the fish are indirectly dependent upon Protozoa as 

 food material. On the other hand, there are numbers of Protozoa 

 which live at the expense of fish. The Myxosporidia are almost 

 exclusively parasites of fish and often cause death to large num- 

 bers of commercially important fishes. Success in fish-culture, 

 therefore, requires among other things a thorough knowledge of 

 Protozoa. 



Since Russel and Hutchinson suggested some thirty years ago 

 that Protozoa are probably a cause of limitation of the numbers, 

 and therefore the activities of bacteria in the soil and thus tend 

 to decrease the amount of nitrogen which is given to the soil by 

 the nitrifying bacteria, several investigators have brought out the 

 fact that in the soils of temperate climates Protozoa are present 

 commonly and active throughout the year. The exact relation 

 between specific protozoans and bacteria in the soil is a matter 

 which still awaits future investigations, although numerous ex- 

 periments and observations have already been made. All soil in- 

 vestigators should be acquainted with the biology and taxonomy 

 of free-living protozoans. 



It is a matter of common knowledge that the silkworm and 

 the honey bee suffer from protozoan infection known as micro- 

 sporidiosis. Sericulture in southern Europe suffered great damages 

 in the middle of the nineteenth century because of the "pebrine" 

 disease, caused by the microsporidian, Nosema hombycis. During 

 the first decade of the present century, another microsporidian, 

 Nosema apis, was found to destroy a large number of honey bees. 

 Methods of control have been developed and put into practice 

 so that these microsporidian infections are at present not serious, 

 even though they still occur. On the other hand, other Micro- 

 sporidia are now known to infect certain insects, such as mosqui- 

 toes and lepidopterous pests, which, when heavily infected, die 

 sooner or later. Methods of destruction of these insects by means 

 of chemicals are more and more used, but attention should also 

 be given to utilization of the parasitic Protozoa and Protophyta 

 for this purpose. 



While the majority of Protozoa lack permanent skeletal struc- 

 tures and their fossil forms are unknown, there are at least two 

 large groups in the Sarcodina which possess conspicuous shells 



