kind of symmetry. The freely floating protozoans, 

 such as the radiolarians, are likely to be spherically 

 symmetrical, with organs of locomotion and feeding, 

 or protective spines, projecting from the whole sur- 

 face and meeting life in every direction. Bottom- 

 living forms that grow attached by a stalk are usu- 

 ally radially symmetrical, with a mouth at the free 

 end surrounded by a ring of food-trapping organs. 

 The fast swimmers, various ciliates and flagellates, 

 are usually bilaterally symmetrical, with front and 

 rear end, top and bottom surfaces. They may have an 

 asymmetrical spiral twist at the front. Finally, many 

 protozoans can be described only as asymmetrical. 



Habitats 



Protozoan habitats are all essentially aquatic, 

 though the amount of water required by a micro- 

 scopic animal may sometimes be no more than the 

 merest film between particles of damp soil or of rain- 

 moistened desert sand. Parasitic protozoans find ad- 

 equate moisture between or within the living cells of 

 their plant or animal hosts. The free-living protozo- 

 ans abound in all bodies of water, large or small: in 

 puddles of standing rain water and in rain-filled tree 

 holes or hollow stumps; in bird baths or flower urns; 

 in ditches and canals; in brooks and rivers; in 

 swamps, ponds, and lakes; and in all the seas of the 

 world. Even the melting surfaces of icebergs, gla- 

 ciers, and snow banks have active populations of 

 flagellates, as one can tell at a distance by the green- 

 ish or reddish cast of such snow. At the other tem- 

 perature extreme are the protozoans that live in hot 

 springs (at up to 133°F. in one place in Japan). 

 This is highly exceptional, of course, and most pro- 

 tozoans die when their external environment reaches 

 temperatures between 97°F. and 104°F. They lack 

 the internal controls that enable a warm-blooded 

 (really temperature-constant) animal like man to 

 keep his body temperature from rising much above 

 98.6°F. even when his surroundings rise to tempera- 

 ture levels that kill living protoplasm. The optimum 

 temperature range for activity and growth of proto- 

 zoans seems to be between 6 1 °F. and 77 °F. 



Distribution 



The common protozoan species are ubiquitous. A 

 schoolboy who scoops up pond water in Australia is 

 likely to find the same species of Paramecium as will 

 a boy in Germany or California. Soil samples all 

 the way from Greenland to Argentina have yielded 

 Amoeba proteus. Apparently, animals that are as 

 small as protozoans and have the habit of encysting 

 (encasing themselves in a dormant condition within 

 a waterproof, resistant wall) are readily transported 

 about the world by wind, animals, and the slightest 

 movement in bodies of water. Thus protozoans, and 

 especially those that live in large bodies of water, 



show very little of the limitations in geographic dis- 

 tribution that are due to mechanical barriers and 

 that we expect in dealing with the larger animals. 

 This does not mean that protozoan species do not 

 differ where conditions of life make different de- 

 mands. In the seas there are characteristic species 

 of warm and of cold waters, of shallow and of deep 

 waters, of surface waters and of sandy or muddy 

 bottoms. Where fresh waters move rapidly, protozo- 

 ans are sparse; but where such water is slow-mov- 

 ing or stagnant, especially if there is much organic 

 matter present, protozoans come into their own. 



The numbers of protozoans, contrary to what 

 many people suppose, are greatest in arctic and ant- 

 arctic waters, which are richest in the nitrogenous 

 compounds necessary for protoplasmic growth. 

 Tropical seas are home to a great variety of species, 

 including most of the really bizarre protozoans, but 

 these do not occur in the dense populations that 

 make cold waters a kind of protozoan soup. 



The salt content (salinity) of waters also deter- 

 mines what species will be found there. Especially 

 versatile species are at home in marine, brackish, 

 and fresh waters; but most are restricted to one of 

 these habitats, or even to a particular level of salt 

 content. Brine pools or large salty bodies such as 

 Great Salt Lake contain some species of flagellates, 

 amebas, and ciliates that are not found elsewhere. 



More critical than salt content for many protozo- 

 ans is the acidity or alkalinity of the water or moist 

 soil in which they must thrive. A few species are reg- 

 ularly found in extreme situations, such as in the 

 highly acid drainage from mines, but most grow best 

 under conditions that hover close about the neutral- 

 ity point. If grown above or below their most favor- 

 able range, some species are not only smaller but 

 have a very different body shape. 



In soils protozoans live mostly within six inches of 

 the surface, but they can be found in small numbers 

 even at depths of several feet. Their numbers vary 

 mainly with the supply of bacterial food, and in 

 moist, rich soils the density of amebas and flagel- 

 lates may reach a million per gram of soil, even 

 though you cannot see anything alive about the soil 

 as you pass it through your fingers. Whether proto- 

 zoans play a role in enriching the soil, or whether 

 they are harmful to the soil by destroying soil-enrich- 

 ing bacteria, we do not really know, even though this 

 is a matter of great economic importance. 



Encystment 



Where the sun beats down on desert sands proto- 

 zoans are scarce. They stay quietly within their cyst 

 walls except immediately after a rain, when they 

 emerge to feed actively — perhaps for no more than 

 a single hour during a whole year. As the sand dries 

 the protozoan rounds up and appears to lose its spe- 



[19 



