SOIL PROTOZOA 315 



pH 3.5-3.9 on the one hand and 9.75 on the other. 11 Testaceous 

 rhizopods are most numerous in acid peat soils and are very scarce in 

 alkaline soils. 



Many of the common protozoa can tolerate semi-anaerobic con- 

 ditions. Aeration of soil has, therefore, little direct influence upon 

 their development. 



On the basis of their nutrition, the protozoa are divided into auto- 

 trophic (or holophytic forms which synthesize food using the energy of 

 sunlight) and heterotrophic forms; these are subdivided into holozoic, 

 when solid food particles are taken in, and saprophytic, when nourish- 

 ment is absorbed from soluble organic substances by diffusion through 

 the body surface. The great majority of protozoa are holozoic in their 

 nutrition. 



The autotrophic protozoa are usually found among the flagellates; 

 they obtain their nutrition entirely from inorganic substances and the 

 CO2 from the atmosphere ; they are often spoken of as Phytoflagellates 

 and may even be considered with the algae. They contain colored 

 substances (chromophyll), which enables them to utilize the energy of 

 light, similarly to higher plants. These colored substances are seldom 

 diffused throughout the cell, but are usually present in special bodies, 

 termed chromatophores. The most common representative of this 

 group is the genus Euglena. In some cases, protozoa are found to live 

 symbiotically with green algae. Such protozoa have been demon- 

 strated among the amoebae and flagellates. 



The holozoic protozoa, to which the majority of ciliates belong, use as 

 food the various complex materials which form the constituents of liv- 

 ing organisms (bacteria, algae). Some protozoa, like various amoebae, 

 can feed on other protozoa, even on the larger ciliates. 12 The whole 

 organism or part of it is taken into the body of the protozoan, where it 

 is subjected to the chemical action of digestion, with the production 

 of organic substances utilized by the protozoan as food. The process 

 of nutrition takes place in three stages, the taking in of the food, its 

 decomposition, and the excretion of the unutilizable part. Digestion 

 in all free-living protozoa is intracellular, by means of diastatic, proteo- 



11 Nasir cited by Sandon, 1927 (p. 329); Fine, M. S. Chemical properties of 

 hay infusions. Jour. Exp. Zool., 12. 1912. 



12 Beers, C. D. The clearing of ciliates by amoeba. Science, 64: 90. 1926; 

 Brit. Jour. Exp. Biol., 1: 325-341. 1924; Mast, S. O., and Root, F. M. Jour. 

 Exp. Zool., 21: 33-49. 1916; Schaeffer, A. A. Quart. Rev. Biol., 1: 95-118. 

 1926. 



