18 PROTOZOOLOGY 



overcooling (not lower than — 9°C.) no injury is brought about. 

 At 0°C., Paramecium was able to multiply once in about 13 days. 

 Wolfson (1935) studied Paramecium sp. in gradually descending 

 subzero-temperature, and observed that as the temperature de- 

 creases the organisms often swim backward, its bodily move- 

 ments cease and its cilia finally stop beating. If the low tempera- 

 ture exposure has not been of sufficient intensity or duration, 

 warming induces a resumption of movement. Kept for 10-15 

 minutes at 10°C., the organism increases its body volume and be- 

 comes rounded, from which condition it may recover if the tem- 

 perature rises, but which otherwise is followed rapidly by a com- 

 plete disintegration. When the water in which the ciliates are 

 kept freezes, the organisms do not survive. 



Light. In the Phytomastigina which include chromatophore- 

 bearing flagellates, the sun light is essential to photosynthesis 

 (p. 92). The sun light further plays an important role in those 

 protozoans which are dependent upon chromatophore-possessing 

 organisms as chief source of food supply. Hence the light is an- 

 other factor concerned with the distribution of free-living pro- 

 tozoans in the water. 



Chemical composition of water. The chemical nature of the 

 water is another important factor which influences the very exist- 

 ence of Protozoa in a given body of water. Different Protozoa 

 show different morphological as well as physiological character- 

 istics. As numerous cultural experiments indicate that individual 

 protozoan species requires a certain chemical composition of the 

 water in which it is cultivated under experimental conditions, al- 

 though this may be more or less variable among different forms 

 (Needham et al.). 



In their "biological analysis of water" Kolkwitz and Marsson 

 (1908, 1909) distinguished four types of habitats for many aquatic 

 plant, and a few animal, organisms, which were based upon the 

 kind and amount of inorganic and organic matter and amount 

 of oxygen present in the water: namely, katharobic, oligosapro- 

 bic, mesosaprobic, and polysaprobic. Katharobic protozoans are 

 those which live in mountain springs, brooks, or ponds, the water 

 of which is rich in oxygen, but free from organic matter. Oligosa- 

 probic forms are those that inhabit waters which are rich in min- 

 eral matter, but in which no purification processes are taking 

 place. Many Phytomastigina, various testaceans and many cih- 



