OF MICR O- OR GANISMS. 63 



when the animalcule perceives certain kinds of sub- 

 stances and particularly those substances which serve 

 it as customary food, it invariably goes through the 

 same movement, which consists of an act of prehension; 

 when the substance touched, seen, or collided with, 

 as the case may be, is of another kind, the Micro-or- 

 ganism does not go through this act. Such is the 

 phenomenon; as to the explanation of the same, we 

 are unable to give one. 



According to M. E. Maupas, if certain Infusoria 

 feed exclusively upon a certain species, it is because 

 their buccal apparatus, or organ of prehension, makes 

 it impossible for them to feed upon different species 

 which possess different tegumentary envelopes. The 

 question is to ascertain whether this explanation is 

 applicable only in certain cases, as appears very prob- 

 able to us, or whether, on the other hand, it is of com- 

 plete and universal applicability. We confess that 

 the hypothesis of M. Maupas does not explain to us 

 why a hunter Infusory that throws trichocysts, like 

 the Didinium, attacks the Param&cium aurelia and not 

 the Paramcecium bursaria. 



It is possible that certain species attract the or- 

 ganisms which feed upon them, by means of a phys- 

 ical or chemical excitation. 



The researches of Prof. Pfeffer, of the Tubingen 

 Botanical Institute, lend a certain confirmation to this 

 hypothesis. 



3. Calculation of the position occupied by the exter- 

 nal body. It is a universal fact that Micro-organisms 

 not only perceive external bodies, but that they also 

 indicate, by their movements, an exact knowledge of 

 the position occupied by these bodies. It might be 

 said that they invariably possess a sense of position in 



