Only those Protozoa which take in solid food have been satisfacto- 

 rily studied in this connection. The processes of digestion and assimi- 

 lation in parasitic, holophytic, and saprophytic forms are in the main 

 unknown. Among the carnivorous forms, however, some advance 

 has been made, although it cannot be said that the digestive processes 

 even here are fully understood. The classic objects for research in 

 this direction have been the large predatory ciliates (Stylonychia, 

 Prorodon, Climacostomum, Cyrtostomum, Stentor, etc.), the Heliozoa 

 (Actinophrys, Actinosph&rium\ and the rhizopods (Amoeba, Poly- 

 stomella, and Pelomyxa}. The first observations upon intra-cellular 

 digestion in these forms were made in the last century (Corti, 1774; 

 Goeze, 1777), when it was seen that living ciliates or flagellates, when 

 taken into the cell-body of another protozoon, soon cease their strug- 

 gling and die. The details of the process and the causes of death 

 have been made out in the last quarter-century, and it is now safely 

 determined that an acid secretion plays the most important part in 

 the killing and subsequent digestion of the captives. 



The length of time which an ingested protozoon can live in a gastric 

 vacuole of another form varies,' according to the organisms, from five 

 or ten minutes to several hours. After the prey has become quiet, 

 the digestive processes, indicated by the disruption of the body of 

 the captive and gradual absorption of its digestible parts, go on more 

 or less rapidly. The indigestible parts, in the form of a granular 

 residue, are voided to the outside. 



It has been determined upon pretty safe evidence that the chief 

 and probably the main source of nutriment of the Protozoa consists 

 of the proteid substances of the organisms taken in as food, while, 

 with a few exceptions, carbohydrates and fats are not assimilated. 

 Carbohydrates, in the form of ordinary starch, appear to be 

 untouched by the body fluids of many Rhizopoda (Greenwood, '86 ; 

 Meissner, '88; Fabre-Dumergue, '88); although the recent observations 

 of Stole ('oo) indicate that in one form, at least (Pelomyxa palustris 

 Greeff), starch grains are easily corroded and at least partially 

 digested. Certain kinds of starch are more easily- digested than 

 others ; rice starch is much more soluble than potato starch in 

 the digestive fluids of Pelomyxa. The Infusoria seem to have a much 

 more developed power of starch dissolution, for the grains of potato 

 starch are all more or less disfigured (Fig. 145), resembling in this 

 respect the partial digestion of the starch grains in the higher animals 

 (Meissner, Fabre-Dumergue, Greenwood). The starch after solution 

 forms a dextrin (Meissner) or an erythrodextrin (Fabre-Dumergue), 

 but the transformation of these into glucose has not been made out. 1 



1 " In man, according to recent investigations, starch is said to be broken up by diastase into 

 five successive hydrolytic cleavage products, as follows: (i) Amylodextrin (C22H 2 oOio)54, 



