io6 Phylum Protozoa 



tensive work of Phelps (1934)- It is not clear why Phelps found other 

 particles (lettuce) in addition to living bacteria in nutrient medium 

 necessary for the growth of Paramecium, but it is a general rule that a 

 few drops in an isolation dish is not an ideal volume for developing 

 optimum culture media. 



The writer has added sterile Didinium to a culture of Paramecium 

 caudatum on Acr. aerogenes (= "dreigliedrige Kultur") in liter flasks. 

 The cycles of growth, perfectly visible to the unaided eye, are rather 

 striking, and are duplicated indefinitely with subcultures. Such cultures 

 are easy to maintain; a minute or two for transfer once a month suffices. 



In no case has the writer found that a combination of two or more 

 bacteria is better than a suitable single strain for the nutrition of ciliate 

 Protozoa. 



Colpidium colpoda, Paramecium caudatum or P. aurelia, Pleurotricha 

 lanceolata (Penn, 1935), and Urocentrum turbo (Hetherington, 1934b, 

 p. 637), will not grow in an inorganic medium (for instance Peters' 

 medium or spring water) -f washed living bacteria. The writer there- 

 fore cannot agree with Phelps (1934) that some labile substance in 

 the bacterial cell is destroyed upon killing (regardless of method used — ■ 

 heat, ultraviolet light, HC1, H 2 2 , (NH 4 ) 2 S0 4 , and toluene), and that 

 this is the reason pure culture may not be achieved. It would appear 

 rather, since even living bacteria do not suffice when their metabolites are 

 removed, that labile substances pass into the medium which "condition" 

 it in some way, and that these substances must be constantly supplied by 

 metabolizing bacteria. The idea that they are reducing bodies was tested 

 by Hetherington (1934b, p. 636), with negative results. Before growth 

 on dead bacteria may be achieved, unheated liver (Eli Lilly No. 343) 

 and fresh, sterile kidney must be added (Glaser and Coria, 1935) . Para- 

 mecium may be the simplest type of animal to the taxonomist, but its 

 dietary requirements remind us more of mammals! 



Such is the setting for a consideration of the growth requirements of 

 the largest ciliates, which typically eat other Protozoa. 



CULTURE ON SINGLE STRAINS OF PROTOZOA 



The large and beautiful Stentor coeruleus (200-500/x) and Bursaria 

 truncatella (300-400^) are known to eat a variety of foods: bacteria, 

 green and colorless flagellates, diatoms, ciliates, rotifers, and even smaller 

 individuals of their own kind. 



Inorganic medium. Of all the physiological media tested, including 

 a variety of spring waters, Peters' medium has proved almost ideal for 

 freshwater ciliates* : 



♦Preparation is described in the author's 1934a work, p. 316. 



