192 THE PROTOZOA 



hand, the endoplasm shows a constant rotating movement, known 

 as " cyclosis." In Paramecium the vacuoles are carried round by 

 the current of the cyclosis, and each vacuole may either do a short 

 course or a long course ; the snort course is simply round the 

 nucleus, keeping close to it, while the long course travels the whole 

 length of the body, up one side and down the other. As a rule a 

 vacuole goes a short course two or three times, and then does a 

 long course (Nirenstein, 181). The path of the vacuole varies, 

 according to the nature of the contents ; but the tendency is to 

 keep them in the region posterior to the nucleus, where the contents 

 are either cast out through the anal pore, or the vacuole circulates 

 again in the cyclosis. In Carchesium the food- vacuoles, when 

 formed at the base of the oesophagus, pass down to one end of 

 the horseshoe-shaped nucleus, and then glide close along its concave 

 margin, passing round and up to the opposite end of the horseshoe 

 into the region near the upper end of the vestibule, from whence the 

 vacuole is finally emptied through an anal pore into the vestibule 

 itself (Greenwood, 162). 



The process of digestion within the food- vacuole has been studied 

 by a number of investigators, amongst whom Le Dantec, Greenwood 

 (162), Metschnikoff (180), Metalnikoff (179), Nirenstein (181), and 

 Khainsky (170'5), must be specially mentioned. Their results are 

 not always in agreement, indicating that the process of digestion 

 is not always the same in different cases, even in the food- vacuoles 

 of one and the same species. According to Nirenstein (181), the 

 food-vacuoles of Infusoria exhibit changes which can be divided 

 nto two periods : in the first the vacuole shows an acid reaction, 

 and the ingested organisms are killed ; in the second the vacuole 

 has an alkaline reaction, and the albumens are digested. According 

 to Khainsky (170'5), however, the reaction of the food-vacuoles of 

 Paramecium is acid during the entire period of the proteolytic 

 process, and only becomes neutral and finally alkaline when the 

 solution of the food-substance is at an end. 



In the first or acid period, according to Nirenstein (181), the 

 ingested food-particles e.g., bacteria after being rendered im- 

 mobile, are clumped together, enveloped in a turbid substance 

 which makes their outlines indistinct. The reaction of the vacuole 

 is strongly acid, due to the presence of mineral acid in the vacuole. 

 During this period, which lasts from four to six minutes, the vacuole 

 diminishes in size, till it is not more than one-third of its original size. 

 When the vacuole was first formed, its wall was surrounded by a 

 number of granules which stain very distinctly with neutral-red ; 

 these granules pass suddenly into the interior of the vacuole after 

 it has become diminished considerably in size. Nirenstein regards 

 the red -staining granules as bearers of a tryptic ferment. 



