REACTION'S OF PLAXARIA TO WATER. 



119 



the experimental worms so that the conditions as regards ve- 

 locity of current were practically the same in the two dishes. 

 Progressive diminutions in velocity were practically the same 

 in the two dishes. Differences of reaction given by the worms 

 in the two dishes at the same time of observation must then be 

 traced to other factors than the velocity of the current. 



The following experiment will illustrate the method of the 

 controlled experiments and the characteristic reversal of reaction 

 induced by change from aquarium water to fresh water. Many 

 hundreds of worms of miscellaneous sizes were tested in two 

 large galvanized iron tanks. A current was produced by shaking 



TABLE III. 



EXPERIMENT SHOWING REVERSAL OF REACTION FROM POSITIVE TO NEGATIVE 

 INDUCED BY CHANGE FROM AQUARIUM TO FRESH WATER AT THE 



SAME TEMPERATURE. 



Trial. 



Experiment. 



Control. 



Water. 



Reactions. 



Reactions. 



Aquarium + good general response 



Fresh 



4 



5 



Aquarium 



Fresh 

 Aquarium 



+ on second stirring, even in 

 weak current 



even in fairly strong current. 



on second stirring, even in 



fairly strong current, a 

 few + in strongest cur- 

 rent 



slight; worms slow in dis- 



tributing on the bottom. 

 + on second stirring, even in 

 only moderately strong 

 current. 



even in fairly strong current. 



on second stirring, no posi- 



tive reaction. 



movement very slow, cur- 

 rent reduced before any 

 reaction was given. 

 + on second stirring only 

 moderately strong cur- 

 rent. 



+ in strong current 

 in weak current 



+ on second stirring, uni- 

 versal. 



+ even in very reduced cur- 

 rent. 



+ even in very weak current. 



+ even in very weak current. 



+ even in very weak current. 



the tanks in the hands, which swept the worms to the center. In 

 the experimental tank aquarium water was used in trials i, 

 3 and 5 ; and this was replaced by fresh water in the alternate 



