96 THE SMALLEST LIVING THINGS 



Another type is illustrated by the ciliate Didinium nasutum. 

 It is a small but powerful organism which darts here and there 

 with an erratic movement, rotating on its axis at the same time. 

 During such sudden darts it may strike a Paramecium, or other 

 ciliate. A complex "seizing organ" bearing a dose of poison 

 is buried in the Paramecium, which is instantly paralyzed. The 

 seizing organ with prey attached is then drawn into the body of 

 the captor and the Paramecium is swallowed whole (Fig. 56). 

 This is quite a gymnastic feat, for the captured Paramecium is 

 frequently larger than its captor, and I have often seen a Didin- 

 ium swallow two conjugating individuals at one time ! 



Digestion of Food 



It is a significant fact that food substances engulfed by these 

 ciliates undergo pretty much the same treatment as though they 

 had been taken into the digestive tract of some higher type of 

 animal. Proteins and carbohydrates must be made soluble. In 

 all cases this is accomplished by hydrolysis (splitting of the com- 

 plex molecules of the solid food through the addition of water), 

 and is brought about through the agency of digestive ferments — 

 amylolytic ferments for transforming solid carbohydrates into 

 soluble sugars and proteolytic ferments for hydrolyzing pro- 

 teins. In the higher animals there are several kinds of proteo- 

 lytic ferments, some of which, like the pepsin of the stomach, are 

 active only in an acid medium, while others, like the trypsin of 

 the smaller intestine, can act only in an alkaline medium. 



Distribution and Assimilation of Food 



In ciliates, particularly in the case of Paramecium, the form 

 on which most of the observations have been made, analogous 

 digestive processes occur. Bacteria taken in as food are killed by 

 an acid secreted in the gastric vacuole. The acid reaction per- 

 sists for a short time (10 to 15 minutes), after which the fluid 

 of the vacuole gives an alkaline reaction. Disintegration of the 

 bacteria occurs in the alkaline medium, and the former bacteria, 

 now changed to the form of minute granules, are distributed 

 throughout the protoplasm. 



The acid and alkaline reactions of fluids in gastric vacuoles 

 vary widely in different types of protozoa and even in the same 



