134 MANUAL OF ELEMENTARY ZOOLOGY 



observed, it is of course necessary that the stimulus should 

 fall unequally upon different sides of the animal. It will 

 then move to or from the direction in which the stimulus 

 is strongest. This can be arranged by placing with a fine 

 pipette a small drop of some solution in the vessel in 

 which the animals are confined, or by heating or lighting 

 one side only of the vessel. Paramecium will move 

 towards weak acids or moderate warmth and away from 

 alkalies, strong acids, warmth above 25° C, etc. Such 

 actions are known as tropisms. It was believed that they 

 could be explained in a simple way by the supposition 

 that the effect of the stimulus in each case was either 

 to slow or to quicken the working of the cilia on the side 

 nearest to it, so that the animal was driven mechanically 

 either towards or away from the stimulus by the unequal 

 working of its cilia. What really happens, however, is by 

 no means so simple. The effect of all stimuli to which 

 Paramecium reacts naturally is to repel it. The animal on 

 receiving a stimulus first withdraws, by a definite backward 

 movement due to a reversal of the working of its cilia, from 

 the stimulus. It then turns towards the dorsal side and 

 swings the front end of its body round in a circle with that 

 side outwards till it comes to point in some direction in 

 which the stimulus is not acting, and in that direction it 

 swims forwards. Thus its approach to conditions which 

 appear to attract it is in reality due to an avoidance of the 

 relatively less agreeable conditions which it meets in other 

 directions during automatic wanderings. It behaves as if 

 it were " trying " different directions of movement till one 

 is found from which it is not repelled. It is claimed that 

 in this procedure, known as the method of trial and error, 

 the lowest animals, from Amoeba upwards (see p. n 6), 

 show a rudiment of the intelligence of the higher. 



Paramecium reproduces by binary transverse fission. 

 The meganucleus divides amitotically, the 



Reproduction. ? , .... ... /' , 



micronucleus by a mitosis in which, as in that 

 of Amosba, the nuclear membrane does not break up, and 

 the place of centrosomes is taken by pole plates. Mean- 

 while a groove appears round the middle of the body and 

 deepens till the cytoplasm is sundered into two, each half 

 containing a daughter nucleus of each kind and one of 



