342 MOTOR RESPONSES 



C. CILIATES 



The most prominent response of the cihates to chemicals consists in 

 reversal in the direction of the effective stroke of the locomotor cilia, 

 and consequent backward swimming. This reversal may be so brief that 

 it results in scarcely perceptible backward movement, or it may continue 

 for several minutes. It has been investigated in some detail in Paramecium, 

 but not in any of the other ciliates. The rate of locomotion in all the 

 ciliates is doubtless correlated with the chemical composition of the 

 surrounding medium, but no measurements concerning this correlation 

 have been made in any of them. 



The reversal in the direction of the stroke of the cilia of paramecia 

 in response to chemicals extends to the entire surface of the body, ex- 

 cept the oral groove. It consequently results in backward swimming. It 

 is the same as the reversal induced by contact and by rapid changes in 

 temperature or osmotic concentration, and it is usually followed by turn- 

 ing toward the aboral surface and forward movement in a new direction, 

 i.e., it usually constitutes the first stage in the avoiding reaction. 



Jennings (1906) describes in detail how this response results in aggre- 

 gation of paramecia in regions which contain ineffective chemicals in re- 

 lation to those in adjoining regions, e.g., regions which are slightly 

 acid, surrounded by regions which are slightly alkaline. If this obtains, 

 he says, paramecia do not respond as they enter the acid region, but do 

 when they reach the edge of this region and are about to enter the alkaline 

 region. They consequently remain in the acid region. As more paramecia 

 enter, owing to random movements, an aggregation is formed. 



He maintains that these responses usually result in aggregations in re- 

 gions which are favorable for the organisms, but that there are exceptions. 

 That is, he holds that these responses are, in general, adaptive. However, 

 he has made no suggestions concerning the processes in the organism in- 

 volved in producing these responses. 



Merton (1923) found that sodium and potassium salts induce reversal 

 in Stentor and that calcium and magnesium do not, but he offers no 

 suggestions concerning the nature of the action of the former nor the 

 cause of inaction of the latter. 



Mast and Nadler (1926) ascertained the effect of fifty-six different 

 chemicals on the direction of the effective stroke of the cilia in Parame- 

 cium. They maintain that all of the monovalent cation salts and hydrates 



