MOVEMENTS, ETC., OP FRESH-WATER PLANARTANS. 595 



conditions it is very difficult to get any certain and trust- 

 worthy results from the stimulation of the ventral surface. 

 My results have been as follows : — strong stimulation of the 

 anterior end on one side of the middle line causes the 

 negative reaction just as when the stimulus is applied at a 

 corresponding point on the dorsal surface. For mechanical 

 reasons the response is not as extensive as when the animal 

 is on a solid, but there seems no doubt of its character. The 

 positive reaction to weak stimuli I have not been able to 

 produce in any certainly recognisable form in response to 

 stimulation of the ventral surface, but I think this negative 

 result is due probably to the external conditions, and not to a 

 real failure of the organism to react. Strong stimulation of 

 the posterior end of the body causes the gliding to change 

 to the crawling just as under other conditions. Very 

 strong mechanical stinnilation of the ventral surface of the 

 body causes the animal to let go its hold and pass down 

 to the bottom. 



5. Reactions of Resting Specimens to Median ical 

 Stimuli. — A resting specimen gives no response whatever 

 to weak stimuli which are still strong enough to produce a 

 definite reaction when the worm is in the active condition. 

 The stimulus is simply below the threshold of the resting 

 animal's sensitiveness. To stronger stimuli the reactions 

 correspond in form with those given by the active animal, 

 but are less pronounced. For example, rather strong stimu- 

 lation at the anterior end induces a weak negative reaction ; 

 similar stimulation of the posterior end sets the animal off 

 into the crawling motion. Strong stimulation of any part of 

 the body besides producing the characteristic reaction for 

 that region (that is the negative reaction) will also in most 

 cases start the animal into movement. This will always be 

 the case it" the stimulus is of sufficient strength, or is several 

 times repeated. As would be expected from the low sensi- 

 tiveness of the resting flat- worm, it is impossible to call 

 forth from it any positive reaction. 

 ,6. Reactions to Stimuli given by Operative Fro- 



