58 COLLEGE ZOOLOGY 



orient itself with the forward end pointing upward, so that if a 

 number are equally distributed in a test tube of water, they will 

 gradually find their way to the top. In running water, Para- 

 mecia swim upstream, probably because the current would inter- 

 fere with the beating of the cilia if any other direction were 

 taken. The electric current also affects the beating of the cilia 

 and causes certain definite movements. 



Frequently Paramecium may be stimulated in more than one 

 way at the same time. For example, a specimen which is in 

 contact with a solid is acted upon by gravity, and may be acted 

 upon by chemicals, heat, currents of water, and other stimuli. 

 It has been found that gravity always gives way to other stimuli, 

 and that if more than one other factor is at work the one first 

 in the field exerts the greater influence. 



Both the spontaneous activities, such as swimming, and re- 

 actions due to external stimuli, are due to changes in the internal 

 condition of the animal. The physiological condition of Parame- 

 cium, therefore, determines the character of its response. This 

 physiological state is a dynamic condition, changing continually 

 with the processes of metabolism going on within the living 

 substance of the animal. Thus one physiological state resolves 

 itself into another; this " becomes easier and more rapid after 

 it has taken place a number of times," giving us grounds for the 

 belief that stimuli and reactions have a distinct effect upon 

 succeeding responses. 



" We may sum up the external factors that produce or deter- 

 mine reactions as follows: (i) The organism may react to a 

 change, even though neither beneficial nor injurious. (2) Any- 

 thing that tends to interfere with the normal current of life 

 activities produces reactions of a certain sort (' negative ') 



(3) Any change that tends to restore or favor the normal life 

 processes may produce reactions of a different sort (' positive ') 



(4) Changes that in themselves neither interfere with nor assist 

 the normal stream of life processes may produce negative or 

 positive reactions, according as they are usually followed by 



