INTRODUCTION 25 



(Dactylochlamys yisciformis, Lauterb., Saprodinium dentatum, 

 Laiiterb., Discomorpha pectinata, Levand., Pelodinium reniforme, 

 Lauterb.). Doflein, following the suggestion made earlier by Bunge, 

 believes that the anaerobic parasitic forms of the digestive tract 

 may have had their initial start towards parasitism when living as 

 such sapropelic forms. ^ 



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 inimicable 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 

 water of roof or pail, and in damp moss. In many cases they are 

 active for only an hour or more until their Tvorld dries up when they 

 may again encyst, 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 Amoeha terrlcola 

 being a typical case, while other genera and species are discovered 

 from time to time (Bodo, Prowazekia, Spironema, Oicomonas, Cerco- 

 monas, Ndgleria punctata and many others. 



While excessi\'e heat kills them, excessive cold does little harm 

 beyond retarding vital activities and the melted ice of glaciers may 

 teem with them, and some species are not harmed by exposure to 

 liquid air. 



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. The\' 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 

 symbionts or parasites. 



The common Protozoa of our own ponds and pools are exactly 

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



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

 reason for the beUef 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). 



