MOVEMENTS, ETC., OF FRESH-WATEE PLANAETANS. 601 



reflexes, but are complexes of several simple reflex acts. It 

 may be well to present in tabular form the different com- 

 ponents in each of these reactions, indicating- by the position 

 in the table the relations of the parts. 



Component Phases of the Reactions to Mechanical 

 Stimuli, with special reference to the Head 

 Region. 



rosiTivE. Negative. 



A. Momentary stopping of pre- A. Same as in positive. 



vious movement. Referred 

 to as " pause " or " hesita- 

 tion " in description. 



B. Longitudinal extension of the 1?. Longitudinal contraction of ante- 



anterior end to greater or rior end of greater or less 



less extent. Amount de- intensity. Tends to make A 



peuds on previous extension. appear more pronounced and 



Usually distinctly noticeable. longer in duration. 



C. Turning towards one side, viz. C. Turning towards one side, viz. 

 that stimulated. This side that not stimulated. Defined 

 is deGned by the position of as in positive. No sharp 

 the source of the stimu- " orientation." 

 lus, not structurally. Sharp 

 "orientation." 



C. Raising of anterior end. This 

 takes place at the same time 

 asC. 



D. .Movement towards stiujulus. D. Movement away from stimulus. 



Direction determined by Direction determined as in 



position taken i)y anterior positive, 



end at termination of C. 



Time relations arc indicated by vertical position in the table. Components 

 occurring at the same time are included in braces. 



Each of the components before D may be considered as a 

 single reflex, and thus there are in one case four and in the 

 other case three simple reflexes which go to make up the 

 whole reaction. That these reactions are composites of the 

 distinct parts is evidenced, lirst, by direct observation of the 

 reactions themselves ; and second, by the fact that it is 



