X TRICIIOCYSTS 113 



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 contractions 

 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 medullary protoplasm, 

 and only the non-nutritious parts cast out at the anal spot. 

 It has been found by experiment that this infusor can 

 digest not only proteids but also starch and perhaps fats. 

 The starch is probably converted into dextrin, a carbo- 

 hydrate having the same formula (CgHioOj) but soluble 

 and diffusible. Oils or fats seem to be partly converted 

 into fatty acids and glycerine. The nutrition of Paramoecium 

 is therefore characteristically holozoic. 



It was mentioned above (p. no) that the cortex is ra- 

 dially striated in optical section. Careful examination with 

 a very high power shows that this appearance is due to the 

 presence in the cortex of minute spindle-shaped bodies (b 

 and c, trch) closely arranged in a single layer and perpen- 

 dicular to the surface. These are called trichocysts. 



When a Paramoecium is killed, either by the addition of 

 osmic acid or some other poisonous reagent or by simple 

 pressure of the cover glass, it frequently assumes a remark- 

 able appearance. 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 

 times their original length. But these filaments have really 

 nothing to do with the cilia ; they are contained under or- 

 dinary circumstances in the trichocysts, probably coiled up ; 



I 



