90 PROTOZOOLOGY 



in Paramecium caudatum (Fig. 36) by using phenol red, neutral red, 

 Congo red, and litmus, and found that when the organism is kept 

 in a medium with pH 7, its food vacuoles are first alkaline (pH 7.6), 

 soon reach a maximum acidity (pH 4.0), while still in the posterior 

 half of the body. Later, the vacuoles show a decreased acidity, finally 

 reaching pH 7.0. In Vorticella sp. and Stylonychia pustulata, the 

 range of pH observed in the food vacuoles was said to be 4.5- 

 7.0 and 4.8-7.0 respectively. The food vacuoles of Actinosphaer- 

 ium, according to Howland (1928), possess at the beginning pH 

 6.0-7.0 for 5 to 10 minutes, but this soon changes to acid (pH 4.3) 

 in which digestion appears to be carried on. In older food vacuoles 

 which are of less acid (pH 5.4-5.6), the digestion appears to be at 

 an end. In the species of Bresslaua, Claff, Dewey and Kidder (1941) 

 noted that a Colpoda taken into the food vacuole is instantly killed 

 with a sudden release of an acid which shows pH 3.0-4.2. During 

 digestion the protoplasm of the prey becomes alkaline and the un- 

 digested residue becomes acid before extrusion. Mast's recent obser- 

 vations (1942) on the food vacuoles in Amoeba proteus and A. duhia 

 containing Chilomonas or Colpidium indicate: (1) the fluid in the 

 vacuoles becomes first acid and then alkaline; (2) the increase in 

 the acidity of the fluid in the vacuole is not due to cytoplasmic secre- 

 tion, but is probably due to respiration in the ingested organisms, 

 chemical changes associated with their death, etc. ; and (3) the death 

 of the organisms taken in the food vacuoles is probably caused by 

 the decrease in oxygen in the vacuoles, owing to the respiration of 

 the organisms in them. 



Just exactly what processes take place in the food vacuole have 

 been observed only in a few cases. Nirenstein noticed the appear- 

 ance of numerous neutral red-stainable granules around the food 

 vacuole which pass into the interior of the vacuole, and regarded 

 them as carriers of a tryptic ferment, while Roskin and Levinsohn 

 demonstrated the oxidase reaction in these granules. A number of 

 enzymes have been reported in the Protozoa, some of which are 

 mentioned in Table 3. 



These findings suffice to indicate that the digestion in Protozoa 

 is carried on also by enzymes and its course appears to vary among 

 different Protozoa. The albuminous substances are digested and de- 

 composed into simpler compounds by enzymes and absorbed by the 

 surrounding cytoplasm. The power to digest starch into soluble 

 sugars is widely found among various Protozoa. It has been re- 

 ported in Mycetozoa, Foraminifera, Pelomyxa, Amoeba, Enta- 

 moeba, Ophryoscolecidae and other ciliates by several investigators. 



