INTRODUCTION 5 



than excessive heat. In the polar regions or at extremely high 

 altitudes, certain Protozoa occur at times in fairly large num- 

 bers. Although the majority are incapable of living in water 

 containing large quantities of chemical substances, some forms 

 such as the members of Epalcidae are said to live in water rich 

 in sulphurous substances produced by decaying and decompos- 

 ing organic matter. Acid or alkaline contents of the water 

 influence also the distribution of Protozoa in various ways. 



Numerous species of Protozoa are cosmopolitan in their 

 occurrence, partly because of their early appearance as living 

 organisms. Amoeba proteus, Paramecium caiidatum, etc., have 

 been observed in fresh waters nearly everywhere in the world. 

 The wide distribution of Protozoa is in part to be attributed also 

 to their ability to encyst. With a small number of exceptions, 

 the vast majority of free-living protozoans become more or less 

 rounded and inactive, and differentiate or secrete a resistant 

 envelope around themselves to withstand temporary unfavor- 

 able conditions. The factors involved seem to be low or high 

 temperature, lack of food material, evaporation and chemical 

 changes of the water in which they live. In some cases, the 

 organism encysts temporarily in order to undergo nuclear re- 

 organization and multiplication. Because of this change and 

 also of the failure of causing certain protozoans to encyst under 

 experimental conditions, it is supposed that some internal fac- 

 tors may play as great a part as the environmental conditions 

 in the phenomenon of encystment. 



Ordinarily a single cyst wall seems to be sufficient to pro- 

 tect the protoplasm against unfavorable external conditions. 

 In some cases, however, there may be a double cyst wall, the 

 inner one usually being more delicate. The cyst wall, as a 

 rule, is composed of homogeneous substance, but it may con- 

 tain calcareous scales, as in Euglypha (Fig. \). While chitin is 

 the usual material of which the cyst wall is composed, cellulose 

 makes up the cyst envelope of numerous Phytomastigina. 



Some protozoans inhabit soil of various types and localities. 

 Under ordinary circumstances, they occur near the surface, 

 their maximum abundance being found at a depth of about 10 

 to 12 centimeters (Sandon). It is said that a very few occur in 

 the subsoil. Here also one notices a very wide geographical dis- 



