MUVEMENTS, ETC., OF FRESH-WATEU Pl.ANAKlANS. 533 



resting, i. e. the animal seems to be quickly fatig-ued by its 

 own movements. This is indicated by the fact that if one 

 stirs up a speciraeu, and sets it into activity ngain just as 

 soon as it comes to rest, the periods of spontaneous activity 

 will become progressively shorter, until finally the worm will 

 only move a very short distance before coming to rest again. 

 The periods of activity are longer and more frequent in 

 P. dorotocephala than in any of the other species I have 

 studied. 



d. Formation of Collections. — There is a well-marked 

 tendency for specimens of planarians to form well-defined 

 groups or collections when they come to rest on an open 

 surface like the bottom or sides of a glass dish, or on the 

 under side of rocks, under natural conditions. Of course, 



c3 



Tig. 1. — Diagram bliowiiig the appearance of a collect ion ot rebling 



planarians. 



this is in part a result of their reaction to light, as has been 

 noted by Loeb (^04). Besides this there seems to be some 

 other factor at work, for in the same dish one frequently 

 finds several localised collections from one to two inches in 

 diameter in different parts of the dish. In these collections 

 the specimens may be closely packed together, and with 

 some specimens ovei'lapping and lying partly over others, yet 

 in the species I have studied a looser arrangement of the 

 character shown in fig. 1 is the more usual one. On the 

 under surface of stones such groups are frequently seen ; 

 two or three may be found on the same medium-sized stone. 

 In this case light as a factor cannot be present, since the 

 conditions of all with reference to this stimulus are equal. 



