GENERAL PHYSIOLOGY 193 



bacteria, while Endamoeba dysenteriae, Dientamoeba fragilis, etc., 

 engulf, with other food substances, red blood corpuscles and digest 

 them. According to Haughwout (1919), the flagellate Pentatricho- 

 monas sp. likewise ingests red blood corpuscles. In the majority of 

 protozoan parasites, however, the organisms do not digest the food 

 necessary for the growth of their own protoplasm. They practically 

 live in a huge gastric vacuole and are surrounded by food already 

 digested or partly digested, which is absorbed by osmosis through 

 their body walls. Doflein thinks that such food substances, if not 

 appropriate for the up-building of protoplasm of the parasite, may 

 be made suitable by the secretion from the parasite of special diges- 

 tive substances and is ready for absorption after the action of such 

 secretions. He further suggests that the cytolytic action upon cells 

 and tissues of the host may be due to such secretions (for example 

 Endamoeba dysenteriae) and that other toxins of pathogenic Pro- 

 tozoa, probably enzymatic in their activity, may be similar digestive 

 secretions from the parasites (see p. 362). 



Secretions and Digestive Fluids.— Products of metabolic activity 

 in the form of secretions and precipitations play most important roles 

 in structure and activities of all kinds of Protozoa. Skeletons, shells 

 and tests, gelatinous mantles, stalks, cyst and spore membranes, 

 and the like are all evidences of the secretory activity of the proto- 

 zoan protoplasm (see Chapter IV). There is evidence that these 

 activities, like secretory activity of the gland cells in Metazoa, are 

 dependent upon the general function of irritability and that specific 

 secretory response follows a specific stimulus. Thus Bresslau (1921) 

 finds that gelatinous mantles or tubes about Colpidium colpoda may 

 be called forth at will by the use of certain chemicals (iodine, fatty 

 acids). If fatty acids are used, the individuals, as in artificial 

 parthenogenesis, must be replaced in a suitable medium before the 

 membranes are formed. Enriques (1919) gives evidence to show 

 that the secretion of stalk material in Anthophysa vegetans depends 

 upon the quantity of food available. Stimulation, through the 

 agency of foreign proteins, is without much doubt responsible for 

 the secretion of digestive fluids and ferments in holozoic nutrition, 

 and considerable advance has been made in our knowledge of intra- 

 cellular digestion. This advance has been due mainly to the appli- 

 cation of the method first devised by Gleichen (1778) of introducing 

 into the body with food substances inorganic, usually colored par- 

 ticles which clearly outline the limits of the digestive cavities. These 

 cavities, early termed gastric vacuoles, were recognized as digesting 

 centers of the organisms, and Gleichen's method, employed by 

 Ehrenberg (1833-1838) led to his elaborate and at first widely 

 accepted, but erroneous, conception of the Polygastrica. Modern 

 applications of this method consist in the introduction with the 

 food of delicate chemical substances, or indicators, which change 

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