SOIL PROTOZOA 317 



species of protozoa have been demonstrated in the air in the form of 

 cysts. 16 



The soil amoebae and closely related forms select their food instead 

 of ingesting all smaller organisms indiscriminately. Micrococci and 

 bacteria are eaten most readily, bacilli less readily, while yeasts 

 and actinomyces are not consumed at all. 16a Different species of 

 amoebae may prefer different organisms, Gram-negative forms being 

 consumed more readily than Gram-positive forms, young cells more 

 readily than old cells (Oehler). When an amoeba is once accustomed 

 to feed on a certain bacterium, it will continue to select the particular 

 organism in preference to others. 



Facts are on record concerning the ability of various protozoa to 

 utilize various soluble organic substances formed in the soil by bacterial 

 action. 17 This is true of flagellates and even of certain ciliates, which are 

 said to have been cultivated free from bacteria, and to be able to derive 

 their nutrients from various soluble organic and inorganic materials. 18 

 Under natural conditions, however, Colpidium colpoda may feed entirely 

 by phagocytosis, although, under artificial conditions, they may be able 

 to obtain their food from solution. 19 These results need further con- 

 firmation, the evidence submitted being insufficient to be accepted. 



Media for the cultivation of protozoa. The media commonly employed 

 in the cultivation of protozoa consist of complex organic substances. 

 For the cultivation of chlorophyll-bearing protozoa, as well as many 

 heterotrophic forms, purely synthetic media can be used. 



For the growth of Euglena, a medium consisting of 0.5 gram peptone, 0.5 

 gram glucose, 0.2 gram citric acid, 0.2 gram MgS0 4 -7H 2 0, 0.05 gram K 2 HP0 4 

 in 100 parts of water has been suggested. 20 The concentration of citric acid and 

 peptone may be doubled and 0.05 per cent NH4NO3 may be added. 



16 Puschkarew, B. M. fiber die Verbreitung der Suszwasserprotozoen durch 

 die Luft. Arch. Protistend., 28: 323. 1913. 



iea Severtzoff, L. B. Method of counting, culture medium and pure cultures 

 of soil amoebae. Centrbl. Bakt. I, Orig., 92: 151-158. 1924. 



17 Thornton, H. G., and Smith, G. On certain soil flagellates. Proc. Roy. 

 Soc. B., 88: 151-165. 1914; Alexeiev, A. G. Protistic Coprology, as a special 

 branch of protistology, and a description of several new species of protists — 

 "coprocolae." Russ. Jour. Microb., 4: 97-134. 1917. 



18 Peters, R. A. The substance needed for the growth of a pure culture of 

 Colpidium colpoda. Jour. Physiol., 55: 7-32. 1917. 



19 Lwoff, A. Sur la nutrition des infusoires. Compt. Rend. Acad. Sci., 176: 

 928-930. 1923. 



20 Zumstein. Zur Morphologie und Physiologie der Euglena gracilis Klebs. 

 Jahrb. wiss. Bot., 34: 149. 1900. 



