INTRODUCTION 25 



digestive tract may have had their initial start toward parasitism 

 when living as such sapropelic forms. 1 



Protozoa are distributed over the entire world. Wherever there 

 is moisture, there will these unicellular animals be found unless 

 conditions of heat or of chemical composition are inimical to life. 

 Oceans and their tributaries, lakes, ponds, pools and ditches, 

 mountain streams and wells contain them, their numerical abund- 

 ance depending on the available food. They are present, not only 

 in permanent waters, but also in casual puddles of field and road, 

 in droplets caught in the axils of leaves or in hollows of rocks, in 

 rain w T ater of roof or pail and in damp moss. In many cases they 

 are active for only an hour or more until their world dries up, when 

 they may be saved again by encystment, but some forms retain 

 their activity in ordinary garden earth where they are supposed to 

 play an important part in connection with Bacteria of the soil 

 (Cutler and Crump, 1920; Goodey, 1916). The majority of such 

 soil-dwelling forms belong to the Sarcodina and Mastigophora, 

 Gruber's Amoeba ierrieola being a typical case, while other genera 

 and species are discovered from time to time (Bodo, Prowazelcia, 

 Spironema, Oicomonas, Cercomonas, D hn a stig amoeba punctata and 

 many others (see Soil-dwelling Protozoa, Chapter X, p. 353). 



While excessive heat kills them, excessive cold does little harm 

 beyond retarding vital activities and the melted ice of glaciers may 

 teem with them. They may live, not only in the exposed waters 

 of the earth's surface, but also as parasites in the fluids of other 

 living protoplasm or its products. They may be found in the warm 

 blood of birds and mammals, or in the cold blood of fishes, amphibia 

 and reptiles; in the digestive tract of every type of animal; in the 

 saliva and urine of different types and in the living protoplasm 

 itself of plants, other Protozoa and of tissue cells. No type of 

 animal life is free from the possibility of association with Protozoa 

 either as commensals, or svmbionts or parasites (see Chapter X, 

 p. 358). 



The common Protozoa of our own ponds and pools are exactly 

 the same in genera and species as those found in similar places in 

 Europe, Asia, Siberia, Africa, South America and Australia; they 

 are cosmopolitan, and the temptation to describe new species because 

 they happen to have been found in some hitherto unexplored local- 

 itv has no justification from the facts of geographical distribution. 

 This is particularly applicable to the fresh water forms but does 



1 The suggestive experiments and conclusions of Avery and Morgan (1924) give 

 reason for the belief that the inability of some organisms to live in free-oxygen hold- 

 ing media is due to the absence in such forms of a peroxidase capable of breaking 

 down hydrogen peroxide. The latter accumulates under ordinary aerobic conditions 

 and is detrimental to forms which are unable to provide the peroxidase. The limi- 

 tation of free oxygen may be the explanation of successful artificial cultivation of 

 forms — for example Spirostomum ambiguum — which grow best under partly anaero- 

 bic conditions (see Bishop, 1923). 



