THE GENERAL PHYSIOLOGY OF THE PROTOZOA 191 



the body-substance of their prey, probably by the aid of secreted 

 ferments. Together with the food a certain amount of water is 

 ingested, forming a drop or food-vacuole in which the actual 

 digestion takes place. The quantity of water ingested with the 

 food varies considerably, and, speaking generally, is inversely pro- 

 portional to the size of the object that is devoured ; that is to say, 

 small food-particles, such as bacteria, lie as a rule in a very distinct 

 vacuole, but large bodies, such as diatoms, usually appear as if 

 imbedded in the cytoplasm, with no liquid vacuole visible around 

 them. Amoebae not infrequently devour organisms larger than 

 themselves, so that the cytoplasm of the amoeba appears like a 

 thin skin or envelope over the surface of the prey. According to 

 Greenwood (161), Amoeba proteus takes in but little fluid when it 

 ingests quiescent solid matter, such as starch-grains or yeast-cells, 

 but when actively- moving prey is dealt with an area of water not 

 inconsiderable surrounds it ; on the other hand, non-nutritious 

 particles are not surrounded by fluid when they lie in the endoplasm. 



In forms in which food is ingested through a cytostome, as in 

 Ciliata, the food-particles, usually of small size, are wafted down 

 the oesophagus and collect at its proximal blind end, where a depres- 

 sion arises in the endoplasm, which gradually deepens, and finally 

 closes over and separates from the oesophagus as a closed vacuole 

 containing the food. According to Nirenstein (181), the food- 

 vacuole is detached from the oesophagus by suction of the endoplasm, 

 like a process of swallowing (" Schlingvorgang "). The vacuole is 

 at first immured in a thin layer of less fluid protoplasm, doubtless 

 as the effect of contact with water (see p. 44) ; consequently the 

 vacuole is not at first circular, but often spindle-shaped in its 

 contours ; it soon, however, assumes a spherical form, indicating 

 that its protoplasmic envelope has become liquefied. 



In cases where actively- motile organisms are devoured as, for 

 example, flagellates by amoebae the prey can often be seen to per- 

 form violent movements within the vacuole ; but soon the move- 

 ments become feebler and cease entirely. Bacteria ingested by 

 Paramecium become immobile about thirty seconds after the 

 vacuole has become detached from the oesophagus. In many cases, 

 however, the prsy is killed when seized by the pseudopodia, and 

 before being ingested, as in Heliozoa and Foraminifera. After the 

 prey is killed it is slowly digested within the food-vacuole. 



During the process of digestion the food-vacuole may perform 

 definite migrations within the body of the animal. In amoebae the 

 vacuoles are carried about by the currents of the protoplasm, 

 without, however, pursuing any definite course, and they tend to 

 become aggregated in the hinder end of the body, when the animal 

 is moving in a definite direction. In the Infusoria, on the other 



