188 BIOLOGY OF THE PROTOZOA 



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

 it comes in contact with pseudopodia of many rhizopods and 

 HeHozoa, Ehrenberg (1833) for Actlnophrys sol; F. E. Schultze 

 (1875-1876) for AUogromia and Polystomella; Winter (1907) for 

 Peneroylis, etc., with tentacles of Acfinobolus radians Woody (1912) 

 Calkins (1901), or of Suctoria, or with the proboscis of Bidinium, 

 nasutum, Thon (1905), Mast (1923), Calkins (1915). In some 

 cases, at least, it is not improbable that this paralyzing killing sub- 

 stance is analogous to, if not the same as, the digestive fluids which 

 kill bacteria and other prey after they are taken into the body proto- 

 plasm. Thus bacteria become motionless in about thirty seconds 

 after the gastric vacuole is detached from the cytophar;yTix of 

 Parameciiim caudatum (Metalnikoft", 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, Metalnikoft" 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 Metalnikoft, 

 in the gastric vacuoles of Paramecium, but Khainsky concluded 

 that the acid reaction is maintained during the entire period of 

 digestion, becoming alkaline only after the dissolution of the protein 

 substances is at an end. In other cases, however, no acid reaction 

 at all can be demonstrated. Thus, Metalnikoft, also in the case of 

 Paramecium, found that some vacuoles never give an acid reaction; 

 others much more rarely show an acid reaction throughout, while 

 still others in the same organism are first acid and then alkaline. 

 Minchin (1912) suggests, in connection with this diverse history of 

 vacuoles in the same species, that dift'erent food substances incite 

 different responses on the part of the protoplasm much as different 

 antibodies are formed from cells of the Metazoa in response to 

 toxins from dift'erent types of pathogenic parasites. No acid has 

 been demonstrated in gastric vacuoles of ActinosphoBriiim eichhornii 

 or in Amoeba proteiis (Greenwood), nor could Metschnikoft' find it 

 in Nodihica miliaris or EuiAotes. This may be correlated with 

 the fact that in the rhizopods and Heliozoa at least the prey is 

 killed upon contact with the pseudopodia, or body protoplasm, 

 the killing agents in such cases perhaps corresponding with the 

 acid secretion of ciliates during the first stages of digestion. 



From the number of different ferments which have been isolated 

 from different types of Protozoa, it is quite probable that digestion 

 does not take the same course in all types. Pepsin-like ferments, 

 which dissolve albumins in an acid medium, were isolated by 



