626 RAYMOND PEARL. 



vioiisly "gripped" (Fig. 26,(1). Tlieii tlie pharynx is ex- 

 truded and feeding begins (Fig. 26, e). After a time the 

 worm voluntarily leaves the food and glides off over the 

 bottom. 



Having described the typical case of a food reaction, we 

 may take up some of the more important variations from the 

 type, and describe the various phases in the reaction in 

 greater detail. 



Starting with the very beginning of the reaction, it may 

 be said that the distance from the food at which any effect 

 on the planarian is produced varies greatly, as is to be 

 expected. This distance, of course, depends on the extent 

 which the juices or chemicals of the food have diffused from 

 it. When a piece of meat is first put into the water 

 specimens will pass very close to it without being stimulated. 

 In fact, if a specimen finds a piece of food within three or 

 four minutes after it is put into the dish, it wiH usually have 

 done so as a result of accidentally coming in contact with it. 

 As has been brought out above, when a gliding worm 

 touches anything of a rather jaelding texture, like food, it 

 immediately gives the positive reaction and passes up over 

 it. This plays an important part in the getting of food, 

 because, as I have found in experiments, unless the food is 

 crushed and pressed with forceps the juices diffuse rather 

 slowly, and for some time specimens will not give the 

 positive reaction unless they actually touch the food. On 

 the other hand, after the food has been in the water for 

 some time, so that diffusion has taken place, the distance at 

 which specimens may be affected becomes quite considerable. 

 I have seen specimens gliding by a small piece of meat at 

 a distance of Hem. from it give the positive reaction and 

 turn towards it. At greater distances than this food is not 

 effective, according to my observations. The distance from 

 food at which a given specimen will give the positive 

 reaction and go towards it depends also on the jihysiological 

 condition of the individual. Specimens in a state of general 

 excitation will, as I have frequently observed, go closely by 



