PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 79 



with the secreting cells. In the protozoa the solid food is taken directly 

 into the living cell, and the processes of digestion are all within the 

 living matter. Such a method is known as intracellular digestion, as 

 contrasted with intercellular digestion of the higher animals (Fig. 25). 



When a rotifer or other small animal is enwrapped by the pseudo- 

 podia of an ameba, or swallowed by an actinobolus or other preda- 

 tory form, a certain amount of water is taken in with it so that the 

 victim moves freely within the body of its captor and in its normal 

 water environment. The water, with victim, forms a gastric vacuole 

 or an "improvised stomach," and is surrounded on all sides by a wall 

 of living protoplasm, and this soon begins to pour a secretion into the 

 vacuole. With the first changes in chemical nature of the surrounding 

 water the prey begins to strviggle, and ceases its efforts to escape only 

 when killed by the secretion. This, according to the researches of 

 Fabre-Domergue, Meissner, le Dantec, and others, is acid in nature, 

 but, beyond the fact that it is some mineral acid probably hydrochloric 

 as in other animals, nothing is known as to the exact chemical nature 

 of this digestive fluid. Whatever it is, its manufacture is intimately 

 connected with the chromatin material of the nucleus, for Hofer and 

 Verworn have shown that digestion does not take place when the 

 nucleus is absent. This was determined by cutting an ameba into 

 two parts, one of which contained the nucleus, the other, a recently 

 ingested animal. The enucleated protoplasm retained its vitality for 

 from nine to fourteen days without any change in the gastric vacuole ; 

 the nucleated fragment, on the other hand, soon recovered from the 

 operation and began to digest as usual. It is probable that the minute 

 particles of nucleoproteids that are constantly arising in the neighbor- 

 hood of the nucleus contain digestive ferments which stimulate the 

 formation of the mineral acid in the vicinity of the gastric vacuole. 



In those protozoa in which the mouth is continually open, as in 

 Paramecium, vorticella, dileptus, bursaria, etc., the food is usually 

 minute forms of unicellular algte, or, most often, bacteria. These are 

 collected in water in the protoplasm at the base of the vestibular 

 opening until a great number are massed together, or until the vacuole 

 has assumed a certain size. It is then caught up in the flow of proto- 

 plasm on the interior of the organism, and dragged away from the 

 mouth, while a new vacuole begins to form. The process of digestion 

 in one of the bacteria-eating vorticellids, carchesium, has been studied 

 by Greenwood, who found that the aggregate of bacteria passes into a 

 region of protoplasm in the immediate vicinity of the horseshoe-shaped 

 nucleus, where the water disappears, leaving the bacteria in close con- 

 tact with the protoplasm. This state of "storage" lasts for from one 

 to twenty hours, and during the time the many separate or individual 

 bacteria are massed together into a compact ball of food. This mass 

 is then again surrounded by fluid, this time having a decidedly acid 



