m NUTRITION 267 



After circulating in this way for some time the water 

 of the food-vacuoles is gradually absorbed, being ulti- 

 mately excreted by the contractile vacuoles, so that the 

 contained particles come to lie in the medulla itself 

 (refer to figure). The circulation still continues, until 

 finally the particles are brought to a spot situated about 

 half-way between the mouth and the posterior end of 

 the body : here if carefully watched they are seen to 

 approach the surface and then to be suddenly ejected. 

 The spot in question is therefore to be looked upon as a 

 potential anus, or aperture for the egestion of faeces or 

 undigested food-matters. It is a potential and not an 

 actual anus, because it is not a true aperture, but only a 

 soft place in the cortex through which, by the con- 

 tractions of the medulla, solid particles are easily forced. 



Of course when Paramoecium ingests, as it usually 

 does, not carmine but minute living organisms, the 

 latter are digested as they circulate through the me- 

 dullary protoplasm, and only the non- nutritious parts 

 cast out at the anal spot. It has been found by experi- 

 ment that this infusor can digest not only proteids but 

 also starch and perhaps fats. The nutrition of Para- 

 moecium is therefore characteristically holozoic. 



It was mentioned above that the cortex is radially 

 striated in optical section. Careful examination with a 

 very high power shows that this appearance is due to 

 the presence in it of minute spindle-shaped bodies (A 

 and B) closely arranged in a single layer and perpendicu- 

 lar to the surface. These are called trichocysts. 



When a Paramcecium is killed, either by the addition 

 of some poisonous reagent or by simple pressure of the 

 cover-glass, it frequently assumes a remarkable appear- 

 ance. Long delicate threads suddenly appear, projecting 

 from its surface in all directions (C) and looking very 

 much as if the cilia had suddenly protruded to many 



