SOIL PROTOZOA 325 



mental work has however failed to support this attractive theory and it 

 appears that internal causes may play at least as great a part as external 

 factors. 47 



Cysts are more resistant than active protozoa to the action of enzymes; 

 also to chemicals, 48 the toxic order being CS 2 > alcohol > acetone > 

 benzol > ether > xylol > chloroform > carbon tetrachloride. The 

 relative toxic effect of acids upon excystation is salicylic > butyric > 

 oxalic > phosphoric > hydrochloric > sulfuric > acetic. Cysts and 

 active organisms acclimatize to higher temperatures. The greater the 

 degree of desiccation, the higher the resistance of cysts to higher tem- 

 peratures and chemicals. The cyst of Colpoda was found 49 to consist of 

 a carbohydrate, which is digested by an enzyme secreted by the enclosed 

 organism during the process of excystation. In other protozoa the 

 structure of the cyst wall is different being siliceous in Monas and 

 related flagellates. Excystation may take place by digestion of the 

 wall or by its rupture or by the emergence of the organism through a 

 preformed pore which is a very characteristic structure in some forms. 



Certain soil amoebae have also flagellate stages. 50 Naegleria gruberi 

 (No. 128, PL XVII) is a soil amoeba found in California soils to a depth 

 of over 20 feet. This amoeba has a biflagellate phase, enflagellates and 

 exflagellates rather quickly on slight provocation under the conditions of 

 laboratory culture. It takes about 70 minutes for a culture of amoebae 

 to change into the flagellate stage, while the reverse process is somewhat 

 more prolonged and less uniform. The addition of water, of fresh 

 culture medium (filtered and sterilized soil and manure infusions) or an 

 excess of air tend to induce enflagellation, but exflagellation is less 

 definite in response to opposite factors. 51 



Occurrence of trophic and encysted protozoa in the soil. The first 

 detailed careful examination of the occurrence of active and inactive 



47 Kofoid, C. A. The life cycle of the protozoa. Science, N. S., 57: 397- 

 408. 1923. 



48 Bodine, J. H. Excystation of Colpoda cucullis. Some factors affecting 

 excystation of C. cucullis from its resting cysts. Jour. Exp. Zool., 37: 115-125. 

 1923. 



49 Goodey, T. The excystation of Colpoda cucullis from its resting cysts, and 

 the nature and properties of the cyst membranes. Proc. Roy. Soc. B, 86: 427- 

 439. 1913. 



60 Wilson, C. W. On the life-history of a soil amoeba. Univ. Cal. Publ. 

 Zool., 16: 241-292. 1915. Kofoid, C. A. On the relative numbers of rhizopods 

 and flagellates in the fauna of soils. Science, N. S., 42: 937-940. 1915. 



61 A detailed discussion of natural history of protozoa is given by Dofflein, F. 

 Studien zur Naturgeschirhte der Protozoen. Zool. Jahrb., 41: 1-112. 1919. 



