PARAMECIUM CAUDATUM 63 



quantity of poison contained in the thread; in Paramecium, 

 however, there is no evidence to show that the trichocysts are 

 poisoned. The extrusion of trichocysts may be seen by adding 

 a small quantity of dilute acetic acid to the medium. 



Nutrition. Paramecium lives primarily on bacteria which 

 are always present in infusions. A constant stream of water 

 passes into the mouth and down the gullet, and bacteria carried 

 by the stream are constantly taken into the endoplasm. A 

 gastric vacuole forms at the bottom of the gullet which gradu- 

 ally fills with water and bacteria. The vacuole is then carried 

 away from the mouth by the streaming protoplasm (cyclosis) 

 and the process of digestion begins, as in Amoeba, by the secre- 

 tion into it of a mineral acid. The bacteria are killed by this 

 acid and begin to swell preparatory to dissolution. After from 

 10 to 15 minutes the vacuoles show an alkaline reaction, and 

 the further processes of digestion, requiring several hours, are 

 completed in this alkaline medium. In well-fed Paramecia the 

 cell becomes loaded with these vacuoles containing bacteria in 

 various stages of digestion. As in Amoeba again, this later diges- 

 tion is brought about by a proteolytic enzyme which acts like 

 trypsin. Finally the liquid of the vacuole disappears, as the 

 digested food becomes intimately mixed with the protoplasm, 

 and assimilation, presumably as in Amoeba proteus, takes place. 



An instructive picture of the protoplasm of Paramecium can 

 be obtained by systematically over-feeding it for a long 

 period, e.g., for some months on a rich hay infusion diet. The 

 protoplasm becomes filled with dark granules, the vacuoles of 

 the protoplasm become indistinct or lost, the contractile vacu- 

 oles lose their rhythmic action, and movements are slow and 

 irregular. At such a time the organism is said to be in a state 

 of depression. It is loaded down with reserves of food partly 

 digested, and seems to be unable to assimilate (Fig. 27). If a 

 small quantity of salt be added (potassium phosphate or potas- 

 sium chloride) , the dense structure slowly disappears, first in the 

 region around the nucleus; in a few days the protoplasm 

 becomes as clear and vigorous as ever. Such experiments show 

 either that some of the oxidative ferments are exhausted so that 



