PHYSIOLOGY 101 



toward the prey and the whole Euglena is engulfed in 2 to 15 min- 

 utes (Fig. 35, b). 



In the ciliates, there are many types of cytostome and associated 

 organelles, but the food-capturing seems to be in general of two 

 kinds. When the cytostome is permanently open, the organism in- 

 gests continuously food particles that are small enough to pass the 

 cytostome and cytopharynx, as in the case of Paramecium. The 

 other type is carried on by organisms bearing cytostome which is 

 ordinarily closed such as seen in Coleps, Didinium, Perispira (Dewey 

 and Kidder, 1940), but which expands to often an extraordinary size 

 when the ingestion of prey takes place. Cannibalism in Protozoa 

 (Dawson, 1919; Lapage, 1922; Gelei, 1925a; Tanabe and Komada, 

 1932; Giese and Alden, 1938; Chen, 1950). 



The ingested food particles are usually surrounded by a film of 

 fluid which envelops the organism and the whole is known as the 

 food vacuole (p. 88). The quantity of fluid taken in with the food 

 varies greatly and, generally speaking, it seems to be inversely pro- 

 portional to the size, but proportional to the activity, of the food 

 organisms. Food vacuoles composed entirely of surrounding liquid 

 medium have occasionally been observed. Edwards (1925) noticed 

 ingestion of fluid medium by an amoeba by forming food-cups under 

 changed chemical composition. Brug (1928) reports seeing Ent- 

 amoeba histolytica engulf liquid culture medium by formation of lip- 

 like elevation of the ectoplasm and Kirby (1932) figures ingestion 

 of the brine containing no visible organisms by the cytostome of 

 Rhopalophrya salina (Fig. 36). Mast and Doyle (1934) state that if 

 Amoeba proteus, A. dubia, A. dofleini, or A. radiosa is placed in an 

 albumin solution, a hypertonic balanced salt solution, or a hyper- 

 tonic solution of calcium gluconate it rapidly decreases in volume, 

 and forms numerous tubes filled with fluid, which disintegrate sooner 

 or later and release their fluid content in the cytoplasm. At times 50 

 or more such tubes may be present, which indicates that the organism 

 ingests considerable quantities of fluid in this way. The two authors 

 consider that it is "a biological adaptation which serves to compen- 

 sate for the rapid loss of water." 



The food vacuoles finally reach the endoplasm and in forms such 

 as Amoebina the vacuoles are carried about by the moving endo- 

 plasm. In the ciliates, the fluid endoplasm shows often a definite 

 rotation movement. In Paramecium, the general direction is along 

 the aboral side to the anterior region and down the other side, with 

 a short cyclosis in the posterior half of the body. 



Some observers maintain that in ciliates there is a definite "diges- 



