PARAMECIUM: EFFECT OF STIMULI rj 



by external stimuli. The movements of Paramecium arc much 

 more active and definite than those of Amceba, and it is corre- 

 spondingly easier to observe the effect of various stimuH upon 

 the animal. These effects are of two kinds, of which the first is 

 merely an alteration in rate of movement. Many acids, alkalis, 

 salts, and other substances in dilute solutions cause an increase 

 in the rate of motion owing to a more rapid working of the cilia. 

 Increase of temperature up to about 35° C. has the same effect. 

 On the other hand, dilute solutions of narcotics, such as alcohol, 

 ether, or chloroform, cause the cilia to work more slowly. All 

 these reactions may be merely the products of the direct effects 

 which such changes in the environment are known to have upon 

 protoplasm. Many of the above also have another effect, although 

 this may only be produced at a critical point. They cause the 

 animal to give a particular response known as the avoiding reaction 

 or phobotaxis ; the creature stops, may swim a short distance 

 backwards, and then moves forwards again at an angle to its 

 previous path. By these means it can avoid a solid obstacle, hot 

 water, or an acid, and if it is placed in a situation where there is a 

 continuous gradient, e.g. of temperature, it may appear to be 

 attracted, for example, to the cool end ; in reality, as can easily 

 be seen with a low-powered microscope, it is repelled from the hot, 

 (Figs. 25-26). In the same way it may become trapped in a drop 

 of acid. Paramecium has two other types of response ; under 

 some circumstances contact with a solid body, especially on two 

 sides at once, as when it swims into a comer, causes protrusion 

 of the trichocysts, and this can also be induced by chemicals such 

 as tannic acid ; lastly, when two Paramecia meet they ma}' 

 adhere in conjugation (see below). These two responses suppress 

 the avoiding reaction which is normally produced by the stimuhis 

 of contact. 



REPRODUCTION 



Paramecium reproduces by binary transverse fission. The 

 meganucleus divides amitotically, that is, without division 

 of the constituent chromosomes (p. 688) or separation of their 

 pairs, the micronucleus by a mitosis in which, as in that of 

 Amceba, the nuclear membrane does not break up, and the place 

 of centrosomes is taken by pole plates. Meanwhile a groove appears 

 round the middle of the body and deepens till the cytoplasm 



