'" NUTRITION 267 



toplasm also takes place in a definite direction, and thus a 

 circulation of food-vacuoles is produced, as indicated in 

 Fig. 70 B by arrows. 



After circulating in this way for some time the water of 

 the food-vacuoles is gradually absorbed, being ultimately 

 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 halfway 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 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 

 nutrition of Paramoecium is therefore characteristically 

 holozoic. 



It was mentioned above (p. 264) 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 (a 

 and B, /rc/i) 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 

 some poisonous reagent or by simple pressure of the cover 



