MOVEMENTS, ETC., OF FRESH-WATER PliANARIANS. 707 



factor in the reaction is the asjniimetry of the body (cf. 

 •Jennings, :00). Now Jennings has further founcP that 

 certain rotifers, which live freely in the water and move 

 about by the activity of cilia in a similar way, and further- 

 more are asymmetrical in fundamentally the same way that 

 the Infusoria are, react in essentially the same manner as do 

 the Infusoria. Similarly, I believe that the general reactions 

 method of the planarians may be found to be in the main the 

 method by which all organisms presenting the same general 

 structural relations and mode of life react. Only one 

 example on which this conviction is based may be given 

 here. In the case of such fresh-water molluscs as Physa it 

 is apparent that the actual locomotor and sensory organisa- 

 tion is symmetrical in form, and furthermore these forms live 

 in fresh water on the surface of solid bodies just as do 

 planarians. Now I have found, in a series of observations 

 not yet published, that in the case of several of these molluscs 

 the fundamental scheme of reaction is like that in the 

 planarian. They react in the same way with reference to 

 the location and intensity of the stimulus, and these are the 

 fundamental things. In fact, the general behaviour is 

 strikingly alike in the two widely separated groups. 



13. Purposive Character of Reflexes. — A fact which 

 is strongly impressed on one working on the behaviour of an 

 organism whose activities are largely reflex is the purposive 

 character of these reflexes. They are so adjusted that in the 

 long run they keep the animal out of danger, and get it into 

 favourable conditions. In the flat-worm these two things are 

 very well done in general by the negative and positive 

 reactions. Of these two reactions it is easy to see that the 

 positive is the more highly developed, in particular in the 

 fact that it is much more precisely localised with reference to 

 the position of the stimulus. We can see a reason for this in 

 the fact that under the conditions of the ])]anarian's life the 



' Complete observations not yet published. Eor preliminary account see 

 ' Science,' N. S., vol. xv, pp. 524 and 525 ; and Jennings, :01, in bibliography 

 at the end of this paper. 



