GENERAL PHYSIOLOGY 195 



the cytopharynx which varies in size according to the abundance 

 of food particles present. In Paramecium caudatum the vacuole, 

 when formed, becomes spindle-shape as though pulled away from 

 the gullet by endoplasmic force, but it soon becomes spherical as it 

 moves about in the fluid endoplasm (Nirenstein, 1905). With the 

 ingestion of larger food bodies such as infusoria, flagellates of larger 

 size, diatoms, rotifers, etc., comparatively little water accompanies 

 the prey. Paramecium caudatum when eaten by Didinium na.su- 

 tum, for example, lies in close contact with the protoplasm of its 

 captor and no water at all can be made out (Fig. 98). In such cases 

 the ingested organism is paralyzed and therefore motionless when 

 swallowed, but it very often happens that resistant food bodies 

 continue to struggle after they have been taken into the protoplasm; 

 rotifers, for example, are usually not motionless when engulfed by 

 Amoeba proteus. In such cases a considerable volume of water 

 gives the prey ample room to move without danger to the make up 

 of the captor. In other cases in which water does not appear to 

 be taken in with the food, the latter becomes surrounded by fluids 

 secreted by the protoplasm. 



With many types of Protozoa the process of digestion begins 

 before the living prey is taken into the protoplasm of the captor. 

 This is manifested in most cases by the paralysis of the victim when 

 it comes in contact with pseudopodia of many rhizopods and 

 Heliozoa, Ehrenberg (1833) for Actinophrys sol; F. E. Schultze 

 (1875-1876) for Allogromia and Polystomellina; Winter (1907) for 

 Peneroplis, etc. In some cases, at least, it is not improbable that 

 this paralyzing killing substance is analogous to, if not the same as, 

 the digestive fluids which kill bacteria and other prey after they 

 are taken into the body protoplasm. Thus bacteria become motion- 

 less in about thirty seconds after the gastric vacuole is detached 

 from the cytopharynx of Paramecium caudatum (Metalnikoff, 1903 

 and 1912). The color changes of chemical indicators, for example 

 alizarin sulphate, show that the killing agent is acid in nature; 

 this was early detected by Greenwood and Saunders (1894), who 

 interpreted it as a mineral acid without further specification. Later 

 observers have confirmed this suggestion, Nirenstein, Metalnikoff 

 and others showing that digestion in the vacuole is a process which 

 is divisible into two periods, in one of which the reaction of the 

 vacuole contents is acid, while in the other it is alkaline. The acid 

 reaction lasts for about fifteen minutes, according to Nirenstein 

 and Metalnikoff, in the gastric vacuoles of Paramecium, but Khain- 

 sky concluded that the acid reaction is maintained during the 

 entire period of digestion, becoming alkaline only after the dissolu- 

 tion of the protein substances is at an end. In other cases, however, 

 no acid reaction at all can be demonstrated. Thus, Metalnikoff, 

 also in the case of Paramecium, found that some vacuoles never give 

 an acid reaction; others much more rarely show an acid reaction 



