SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 283 



at least, symbiotic forms arise through the inability of the animal pro- 

 toplasm to digest the plant ; either can live equally well without the 

 other, although it is probable that there is some mutually beneficial 

 action between the two when together. Beyond the fact of the indi- 

 gestible cellulose coating, there is nothing to show why the plant-cell 

 is not acted upon by the dissolving fluids of the body, as is the case 

 when the same plant-cells are taken in by other Protozoa, and the 

 subject approaches very near to the time-honored but yet unsolved 

 problem why the stomach does not digest itself. 



Considerable interest attaches to the nature of the fluid which 

 causes the disruption and digestion of the proteids in living proto- 

 plasm of plants and animals serving as food. It has been asserted 

 (Maupas, '83) that the protoplasm of the prey becomes a part of 

 the protoplasm of the captor without further change, the implica- 

 tion being that there is no especial fluid created by the carnivorous 

 organism, as in higher animals, to digest the food. A consider- 

 able body of evidence has grown up, however, showing that this is 

 not true, and that a definite acid is formed, by means of which the 

 solid food particles are disintegrated and dissolved. The method of 

 procedure in determining this point is based upon the same principle 

 of differential staining as that employed by cytologists in working out 

 the chemical nature of the various parts of the cell in higher animals. 

 The application, however, is very much more limited, for only those 

 chemical substances can be used which by intra-vitam application 

 have no deleterious effects upon the organisms. As far back as 1879, 

 Engelmann, observing that blue litmus granules turn red and remain 

 so in the protoplasm of some rhizopods and ciliates (Amoeba proteus, 

 Paramecium aitrelia, Stylonycliia mytilus, and 5. pustulata), cred- 

 ited the color-change to an acid in the cytoplasm. Subsequent 

 work by numerous physiologists has fully substantiated the results 

 obtained by Engelmann, and similar but more refined methods in the 

 hands of Meissner, Fabre-Dumergue, Metschnikoff, Greenwood, and 

 Le Dantec have given a firm basis for the belief that digestion of 

 proteids in Protozoa is similar to that in the Metazoa, but beyond the 

 fact that the dissolving fluid is a mineral acid, nothing further con- 

 cerning it is known. 



The bare statement of Engelmann's view of an acid cytoplasm is 

 subject to misinterpretation. While it is undoubtedly true that an 

 acid is present in the cytoplasm, it by no means follows that the cyto- 

 plasm itself is acid. On the contrary, it was soon demonstrated that 

 in some forms at least (ciliates, Metschnikoff, '88, and Meissner, 

 '88, as well as Amoeba and Actinosphcerium, Meissner), the cytoplasm 

 has a decidedly alkaline reaction while the acid is confined to the fluid 

 in the gastric vacuoles. Dujardin ('41) long before had noted that a 



