MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. G19 



stimuli. As has been sLowu above^ removal of tlio anterior 

 end of the body containing the brain causes the disappearance 

 of this positive reaction, and this result is probably due rather 

 to the lowering of tonus than to the removal of any special 

 centre having the causation of this reaction as its function. 

 Additional evidence on this view that lowering of the tonus 

 is the chief cause of the disappearance of the reaction is found 

 in the fact that other injuries to the head, such as longitudinal 

 splitting, which produce a lowering of the general tonus, also 

 cause the disappearance of the positive reaction. 



This very close dependence of the reaction on the general 

 tonic conditions of the organism makes its analysis difficult, 

 but it seems most probable that its mechanism is as follows : — 

 a light stimulus, when the organism is in a certain definite 

 tonic condition, sets off a reaction involving (1) an e(|ual 

 bilateral contraction of the circular musculature, producing 

 the extension of the body ; (2) a contraction of the longi- 

 tudinal musculature of the side stimulated, })roducing the 

 turning towards the stimulus (this the definitive part of the 

 reaction); and (3) contraction of the dorsal longitudinal 

 musculature, producing the raising of the anterior end. In 

 this reaction the sides do not act independently, but there is 

 a delicately balanced and finely co-ordinated reaction of the 

 organism as a whole, depending for its existence on an 

 entirely normal physiological condition. It is to be noted, 

 however, that the definitive part of the reaction, namely, the 

 turning, is a response of the side of the body stimulated. 

 This point is one of fundamental importance for the general 

 theory of the reactions. 



The mechanism of the other reactions to mechanical 

 stimuli are evidently very simple. The crawling movement, 

 which must be considered as the specific reaction to mechanical 

 stimulation of the posterior region of the body, is due to 

 rhythmical contraction of the longitudinal musculature. The 

 only other reactions to mechanical stimulation are local con- 

 tractions, whose mechanism is evident. 



e. Features in the General Uehaviour of the 



