MOVEMENTS, ETC, OF FRESH-WATER PLANAETANS. 569 



The coming to rest in the collection seems to be due 

 simply to the direct effect of the chemical on the organism. 

 There is no evidence that the animals are held in the group 

 as a result of a negative reaction to the surrounding water, 

 as is the case in the collections formed b}- the infusoria 

 (cf. Jennings, '99, Ij). The method of formation of collections 

 in chemicals in the case of the infusoria is as follows: — 

 Specimens swimming about at random come to the edges of 

 drops of chemicals purely by chance. If, for example, the 

 chemical happens to be a weak acid, the specimens will pass 

 into the drop without giving any reaction. When, however, 

 they reach the opposite edge of the drop and attempt to pass 

 from the chemical back into the water they are stimulated, 

 and give their usual motor reaction. This turns them back 

 into the drop, in which they are, as it were, " caught in a 

 trap." Asa consequence of this method of reaction a very 

 dense collection will be formed in a short time. AV^ith the 

 flat-worm the case is very different in that an active in- 

 dividual frequently passes into and out of one of these 

 collections without showing the faintest trace of a reaction 

 on either side. The only way in which any stopping in the 

 region is brought about by a chemical is by a chemokinetic 

 reaction. The fundamental difference in the reactions of the 

 two groups of organisms on which this difference in the matter 

 of forming collections is based will be brought out in the 

 section on the reactions to chemicals. 



To sum up, the formation of collections of individuals 

 seems to be due, in the first instance, to the tendency of the 

 organisms to come to rest in areas of a certain degree of 

 intensity of light, and in a lesser degree to a tendency to 

 turn towards and come to rest in areas containing some 

 substance secreted or excreted by the worms themselves. A 

 prerequisite in the formation of collection, as in the coming 

 to rest under any circumstances, is a proper physiological 

 condition of reduced tonus. 



There does not appear to be any special biological sig- 

 nificance to this tendency of the animals to collect in groups. 



