622 



l?AYiMOXD PKAHL. 



exact behaviour in any given case depends on several 

 different factors. These are the position of the animal with 

 reference to the obstruction, the physical nature of the 

 obstruction, and the physiological condition, -whether of 

 greater or less excitation. 



So, again, with reference to the habit of the animal of 

 moving about on the surface film, a problem is presented. 

 When a specimen, gliding up the side of a dish, touches its 

 anterior end to the surface film at the point where the latter 

 joins the glass, it immediately gives a characteristic positive 

 reaction, precisely like that in response to any other weak 

 mechanical stimulus. The head is raised and turned towards 

 the side from Avhich the stimulus came, and then dropped 



Fig. 25. — 1, 2, 3, 4, 5, and fi arc successive staples in ilie reactions of 

 Planaria on mcctinp; obliquely an obstacle in its path. Tlie hcav}' 

 straight line represents the obstacle. 



again. As a consequence of this reaction, the head end 

 comes to rest on the under side of the surface film at a point 

 some little distance out from the side of the dish. The 

 ventral surface of the anterior end of the body flattens out on 

 the surface film, and the animal glides out on to the film, 

 following the direction determined by the reaction of the ante- 

 rior end. Thus it is seen that the going on to the surface film is 

 only a special case of a response to a weak mechanical stimulus, 

 i. e. the positive reaction, the film itself acting as the stimulant. 

 The leaving of the surface film and passing down the side of 

 the dish is evidently also due to the same positive reaction. 



