644 RAYMOND PEARL. 



slightly larger than that at the lower, which was used in 

 giving the stiniulas. The tube was filled with solution by 

 suction. The rate of diffusion can be regulated by changing 

 the sizes of the openings, and can be determined for each 

 tube from the rate at which the fluid sinks at the upper end 

 of the tube. Considerable experimenting is necessary in 

 order to get the best rate of diffusion for work on planarians. 

 Since the animal is moving rather rapidly while the stimulus 

 is being applied it is necessary to have reasonably rapid 

 diffusion or the worm will not react at all, or not for so long 

 a time after the stimulation has begun that one cannot be 

 certain of the results. It is easily possible to get the 

 capillary so fine that no results can be obtained. On the 

 other hand, when it is too large the solution affects too large 

 a portion of the body at one time, and furthermore, as will 

 be shown later, may cause a rheotactic reaction of the 

 organism. This, of course, introduces a possible source of 



Fig. 32.— Glass tube used in giving localised chemical stimuli. 



serious error. It can be avoided by frequent and proper 

 control experiments. 



It will be well to describe in advance the conduct of a 

 typical experiment and the precautions taken, so that it may 

 not be necessary to repeat these details in the account of 

 each experiment. Six to ten normal active planarians were 

 taken from the aquarium dish and put in a Petri dish of 

 about 10 cm. diameter, in freshly drawn, filtered tap water. 

 Enough water was put in the dish to give a depth of about 

 1 cm. Two or three of the capillary tubes with different 

 sized openings were filled with the tost solution. These 

 tubes were all tested before a final experimental series was 

 begun, and usually only one which had been found to allow 

 diffusion at the satisfactory rate was used. In some cases, 

 however, varying degrees of sensitiveness among the 

 different specimens made it necessary to use for some in- 



