460 Physiology 



tion, producing succinate and fumarate from labeled acetate or ethanol 

 with essentially quantitative recovery of radioactive carbon, indicating no 

 decarboxylation of intermediate C^ acids (131). In relation to hetero- 

 autotrophic nutrition of phytoflagellates, it is interesting that a mutant 

 form of Azotobacter agilis has lost the ability to use glucose, lactate, pyru- 

 vate, and various Krebs-cycle acids, but retains the ability to use acetate 

 and ethanol (269, 270). 



Evidence for the occurrence of the tricarboxylic acid cycle in certain 

 ciliates seems conclusive. Tefrahyynena pyriformis takes up carbon dioxide 

 with formation of succinate in the anaerobic dissimilation of glucose 

 (561). Studies on oxygen consumption show that pyruvate, succinate, 

 a-ketoglutarate, fumarate, malate, and oxalacetate are utilized, and that 

 malonate produces typical inhibition (513, 528). It is interesting that 

 malonate in low concentrations (5 [xg/ml) serves as a substrate for T. 

 pyriformis (528), although it is distinctly inhibitory at high concentra- 

 tions. With the exception of citrate and r/5-aconitate, the various inter- 

 mediates of the cycle are readily utilized by Plasmodium gallinaceum 

 (407, 535), and fumarate, pyruvate, and succinate are known to be oxi- 

 dized by P. lophurne (21). The evidence for such a cycle in phytoflagel- 

 lates is not yet conclusive. Added to a peptone medium, malate stimulates 

 growth of Astasia longa, Euglena gracilis, and Polytoma ocellatinn, while 

 succinate stimulates growth of these and six other species (565), and also 

 accelerates growth of E. gracilis in darkness (244). In addition, fumarate, 

 malate, and succinate are satisfactory substrates for E. gracilis var. bacil- 

 laris at pH 3.0-3.6 (228). Occasional failures to use Krebs-cycle acids have 

 been observed but these cases probably should be reconsidered. Experi- 

 mental data for Tetrahymena pyriformis (516) indicate that permeability 

 of the surface to the substrate is a major factor to be considered. Such a 

 factor may explain the reported inability of Polytoma uvella to use 

 malate and pyruvate (456), and that of Astasia klebsii to oxidize succinate 

 or citrate at a significant rate (83). The need for carbon dioxide, estab- 

 lished for several phytoflagellates (471), might suggest that carboxylation 

 occurs as an essential part of the cycle but such an assumption is yet to 

 be confirmed experimentally. The data for Zoomastigophorea also are 

 fragmentary. Trypanosoma lewisi oxidizes several of the intermediates 

 rather slowly (403), although the basal "medium" used for such tests may 

 not be the most favorable one for reactions which depend upon a variety 

 of giowth-f actors. Trypanosoma cruzi and the species of Leishmania from 

 man form succinate as one of the products in oxidation of glucose and 

 levulose (57). 



DIGESTION 



Digestion in holozoic species occurs typically within vacuoles which 

 enclose the food after ingestion. The mechanical features of ingestion vary 



