MOVEMENTS, ETC., OF FRESn-\YATER PLANARTANS. 665 



amounts to a negative reaction to the snrronnding water. 

 Tlie question^ then, is, can we get any such formation of 

 collections b^^ the retention of those specimens wliich have 

 entered an area by chance in the case of PI an aria ? 



This pi'oblem was attacked in a nnmber of different ways, 

 but the clearest results could be obtained by the " two-drop " 

 method of Massart. Two drops of fluid of equal size are 

 placed near each other on a slide, and a narrow connecting- 

 band is made between the two by drawing some of the fluid 

 across with a needle. One of them was usually of culture 

 water, while the other was of the solution to be tested. Now 

 evidently, if the animals form collections by the "motor 

 reflex " method, they ought to pass into the drop of solution 

 without any reaction, but when they attempt to pass back 

 into the water drop they should be stimulated to a negative 

 reaction and thus turned back. 



An experiment with a solution to which the animal gives a 

 sharp positive reaction may first be reported. One of the 

 drops was tap water, and the other was 1 per cent, sugar 

 solution, to which the specimens gave a strong positive re- 

 action. Several small planarians were put into the water 

 drop. They glided rapidly about this drop, and soon one 

 came up to the bridge connecting the water with the sugar. 

 It was headed straight for the sugar drop, and passed over 

 into it without any reaction whatever. Up to this point the 

 behaviour is like that of the Infusoria towards the acid drop. 

 This specimen circled about in the sugar drop, and after a 

 time became directed towards the connection between the 

 sugar and water, and passed back into the water drop with- 

 out giving the faintest trace of a reaction of any sort. All 

 the specimens passed back and forth between the two drops 

 without giving any reaction, except in some cases a weak 

 positive one. The conditions under which a positive reaction 

 is given are that a specimen should come more or less trans- 

 versely across one end of the connecting bridge, as shown in 

 Fig. 34. It then usually gives a weak positive reaction and 

 tui-ns slightly towards the other drop. It ma}^ do this on 



