MOVKMKN'I'S, E'I'O., OF FRKSll- WATKi; I'l.ANAK lANS. 533 



resting, i. e. the aniiua-l seems to ])e (luickly fatig-ued by its 

 own movements. Tin's is indicated by the fact that it' one 

 stirs up a specimen, and sets it into activity again just as 

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

 will become progressively shorter, until finally the wcn-m 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. — Tlu'rt; 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 oC rocks, under natural conditions. Of course. 



C.^ 



I'lG. 1. — Diagram bliowiiig I he apjjcaraiice of a culled ion of ifbUiig 



jilaiiariaiis. 



this is in part a result of tlieir reaction to ligld, as has been 

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

 other factor at work, for in the same dish one fre([uently 

 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 t(.)gether, and with 

 some specimens overla]iping 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 arc frequently seen ; 

 two or three nniy bi^ ft)uud on the same medium-sized stone. 

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

 couditions of all with reference to this stimulus are equal. 



