620 RAYMOND PEARL. 



Organism wliicli the Reactions to Mechanical 

 8tiiuuli explain. — That much of the behaviour of pla- 

 narians in their natural surroundings is the result of the re- 

 actions above described is very evident to any one watching 

 them. Among specific features of this sort in which these 

 reactions play a part may be mentioned the escape from 

 enemies or harmful surroundings, the getting of food (to be 

 discussed in detail later), the localities chosen for coming to 

 rest, the behaviour on meeting solid obstacles in the path of 

 movement^ the passing on to the surface film, etc. All 

 of these need not be discussed specifically, as their relations 

 will be evident enough on a moment's thought, but the last 

 two deserve special mention. 



The behaviour of plauarians on meeting solid bodies in 

 their path in the course of movement is entirely made up of 

 reactions to mechanical stimuli. The behaviour in detail is 

 as follows : — If a gliding specimen meets squarely head-on an 

 obstruction of considerable size, so that it cannot glide over 

 it without changing to some extent the position of its long 

 axis, it will stop an instant, raise the head, let it drop down 

 till it touches the obstruction again, and then glide directly 

 up on to and over the solid body. This behaviour is invari- 

 able, so far as my observations go, if the worm meets the 

 obstruction squarely. It is at once seen to be merely a 

 special case of the usual reaction to a weiik mechanical 

 stimulus, characterised by the raising of the head. The 

 behaviour is evidently purposeful in the long run, because it 

 will take the organism up on to food material just as well as 

 indifferent bodies. If the gliding worm meets the obstruc- 

 tion obliquely the behaviour depends in large part on the 

 physical nature of the object. If it is food material, or some- 

 thing else of a rather soft and yielding texture — as, for 

 example, another planarian, — the worm will immediately raise 

 the head, turn it towards the object, and crawl up over it. 

 'J'his behaviour is evidently the typical positive reaction to a 

 weak mechanical stimulus. A special and i-ather curious 

 case of this positive reaction, which i have twice observed. 



