GENERAL CONSIDERATIONS 5 



not be too closely limited. We speak, for example, of the water-dwelling 

 forms, having reference to the great multitude of types which may be 

 found in exposed waters of the earth's surface; or of an even greater 

 number of species which live in the sea at all depths, from the surface 

 down to 3,000 feet, while some Radiolaria are present in the most ex- 

 treme depths of the ocean. Where salt and fresh waters mix we find 

 special groups of brackish water fauna which are represented by thou- 

 sands of species. 



The fresh-water species are so numerous and so varied that some help 

 is gained by grouping them in "habitat groups" which are adapted more 

 or less to similar environmental conditions. An attempt to classify such 

 fresh-water species on an ecological basis was made by Kolkwitz ( 1909) , 

 although this classification has never been widely used in protozoan 

 taxonomy. It was based, in the main, on the amounts and conditions of 

 organic matter and oxygen present in the water. The habitat groups 

 were described by him under the terms katharobic, oligosaprobic, meso- 

 saprobic, and polysaprobic types. The first are relatively rare types, being 

 found in fresh-water springs, running rivers and streams, and wells 

 which have little organic matter but are rich in oxygen. 



Oligosaprobic types are characterized by a small amount of organic 

 matter but are rich in minerals. Hence lakes and reservoirs become the 

 haunts of chlorophyll-bearing forms in particular, which often accumu- 

 late to incredible numbers and frequently cause disagreeable odors and 

 tastes in drinking waters and render them unpotable. 



Mesosaprobic types are the most common of all fresh-water Protozoa, 

 for here active oxidation is going on and organic matter, in the presence 

 of sunlight, is in all stages of decomposition. Here the microscopist finds 

 his richest collecting place. 



Polysaprobic types, finally, live in waters with little or no free oxygen, 

 but with an abundance of sulphureted hydrogen, carbon dioxide, and 

 other products of putrefaction which are advertised by their foul odors, 

 due to the gases which are formed. Here belong Lauterborn's (1901) 

 "sapropelic fauna," which live, for the most part, as anaerobes, and 

 which are often characterized by fantastic shapes and inability to live 

 under aerobic conditions. 



Another group have an almost terrestrial habitat and may be found in 

 damp moss, sphagnum, or similar environments. A few types of ciliates 



