MOVEMENTS, ETC., OF FRESH-WATER FLANAKIANS. 627 



a piece of food without turuing towards it, while other 

 specimens in a more normal condition will give the positive 

 reaction some distance from it. 



After the first specimen has began feeding on a piece of 

 material the zone of influence of that piece becomes almost 

 immediately widened appreciably. As the number of feed- 

 ing specimens increases the area in the surrounding water 

 which affects others becomes correspondingly greater. This 

 phenomenon is very striking in many cases, as an illustration 

 will indicate. Several pieces of crushed snail were put in a 

 dish with a number of planarians. In a short time a 

 specimen in gliding about the dish had come near to one 

 of these pieces, had given the positive reaction and begun 

 feeding. At almost the same time another of the pieces of 

 food had " attracted " another specimen. The other bits of 

 food were quite similar in every way to these two, and lay in 

 the dish no-t far from them. Yet at the end of fifteen 

 minutes the two j)ieces by which the first two worms had 

 been affected were completely covered with feeding 

 specimens, while the remaining pieces of food, with a single 

 exception,^ did not have a specimen on them. This increase 

 in the effectiveness of the food as a stimulus must be due 

 to the diffusion of more chemical substance from it. 

 Apparently the increase is due either to some secretion of 

 the feeding animals or to some change which they induce in 

 the food. It is probably due to a combination of these two 

 factors. That a digestive secretion is poured out through 

 the pharynx of the feeding worm is well known, and clearly 

 shown by the appearance of a piece of food on which a 

 specimen has been feeding. The surface of the meat is 

 turned white, and rendered very soft and almost flocculent. 

 It is probable that this digestive secretion acts as a positive 

 chemotactic stimulus to other worms, and that coupled with 

 this there is an increased diffusion of juices from the food 

 itself caused by the changes which it is undergoing. 



The reaction which is caused by this chemical stimulus 



' Uue piece iuitlicbl ieiiiovci.1 from the olliciii liad ;i single specimen on it. 



