SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 285 



parts of the protoplasm, to be called out, perhaps, by the stimulus of 

 the ingested substances, and the conclusion is obvious that the 

 vacuole or improvised stomach in the various forms is not absolutely 

 essential. 



Greenwood ('94) has given a very careful description of the gastric 

 vacuoles in the ciliate Carchesium polypinum, one of the Vorticellidse. 

 Here the gastric vacuoles, after leaving the mouth opening, pass 

 downward into an area 

 bounded by the horse- 

 shoe - shaped macronu- 

 cleus (Fig. 147), where 

 they pass into a state of 

 "storage" (characterized 

 by the loss of the water 

 taken in with the food), 

 and the food particles are 

 thus left, for the time 

 being, in. direct contact 

 with the protoplasm. 

 This stage, which may 

 last from one to twenty 

 hours, is eventually ended 

 by the formation of a 

 vacuole again, about the 

 ingesta. 1 Preceding the 

 period of vacuole forma- 

 tion there is a sudden 

 concentration of the pe- 

 ripheral food particles of 

 the ingesta into a central, 

 solid ball. This condi- 

 tion — Greenwood calls it 

 the " aggregation " stage, 

 — is brought about, she 

 believes, as in clotting, 

 by the accumulation and 

 concentration of some re- 

 tractile substance. The fluid freshly secreted to form the reappearing 

 vacuole has a decidedly acid reaction, and a powerful solvent action 



Fig. 147. — Digestion in Carchesium. [GREENWOOD.] 

 The path which the food takes is represented by dots : 

 a (circular round marks) represents the position of stor- 

 age ; b (crosses) represents the position of rest ; c (dots), the 

 region of the later changes. 



*Le Dantec ('95) does not believe that the vacuole disappears during the so-called quies- 

 cent phase, but is invisible merely because at this time it has the same index of refraction as 

 the surrounding plasm, becoming visible again after diffusion of the digested parts. The 

 strength of Le Dantec's criticism is taken away by his own observations upon Gromia fluvi- 

 atilis, where the ingesta is digested without the formation of a vacuole. 



