CHAPTER V 



MOTOR RESPONSE IN UNICELLULAR ANIMALS 



S. O. Mast 



Responses consist of changes in structure, composition, form, or move- 

 ment in organisms, which, in turn, are correlated with changes in the 

 constituents of the environment or the organisms. Responses are found 

 in all living systems and are among the most fundamental distinguish- 

 ing characteristics of life. Motor responses consist of changes in rate 

 or direction of movement of organisms or their constituents. They 

 facilitate the control of the environment by the organisms involved and 

 are consequently of great importance to them. Knowledge concerning 

 these responses and their relation to the factors correlated with them 

 makes it possible to control the activities of organisms, it throws light 

 on the distribution of pain and pleasure (consciousness) which pro- 

 foundly aifects the attitude of man toward his fellow creatures, and it 

 illuminates the processes involved in instincts and in learning. Such 

 knowledge is therefore very valuable. 



The unicellular organisms are in many respects extraordinarily favor- 

 able for the study of the more fundamental characteristics of these re- 

 sponses. They are relatively simple in structure. Many of them can be 

 readily procured and maintained in great numbers, and the factors in 

 their environment can be accurately controlled or changed as desired; 

 and, in addition, details concerning the responses can be readily seen. 

 Moreover, the motor responses in these organisms are very favorable 

 for the study of adaptation, as pointed out long ago by Jennings (1906) . 



In the following pages are presented the more important facts in hand 

 concerning the motor responses of the rhizopods, flagellates, ciliates, and 

 colonial forms to light, electricity, and chemicals. These are presented 

 with the view of encouraging the use of these organisms in further work 

 on various biological problems. Important results have also been ob- 

 tained on responses to contact, temperature, and gravity, but limitation in 

 space prevents the consideration of these. 



