MOVEMENTS, ETC., 01-' FKESlI-WATEll I'LANAKIANS. G61 



body pai'iiUol to the lines of action of the stimu- 

 lus. This movement of the animal in a constant rehition to 

 a constantly acting stimulus is^ as I understand it, the funda- 

 mental criterion of an orientation according to the theory 

 above mentioned. Now if we lind, as has been shown above 

 to be the case, that the organism gives precisely the same 

 reaction to a chemical unilaterally applied as it docs to a 

 single weak mechanical stimulus similarly applied, it seems 

 doubtful whether we can consider that there is such an 

 orientation in the case of the chemical, even though the head 

 is directed very precisely towards it. On the contrary, it 

 seems apparent that we are dealing here with a well co- 

 ordinated motor reflex only — such as, for example, the reflex 

 of a frog's hind leg, which brings its foot very exactly to the 

 point stimulated on the side of the body. 



A crucial test of this point may be obtained by submitting 

 the animals to the action of some chemical to which they are 

 known to give the positive reaction when it is applied locally, 

 only arranging the experiment so that it is diffusing over a 

 large area. Under these conditions, if the oi'ganism shows 

 positive orientation, it ought to move along the lines of diffu- 

 sion straight up to the source of diffusion. To test this matter 

 I constructed a trough of the form shown in Fig. 33, I. 

 On a plate of glass A was fastened the trough B, which was 

 cut from a block of paraffin. The internal dimensions of this 

 trough were 50 mm. x 50 mm. x 5 mm. Only the sides were 

 of paraffin, the glass plate serving as the bottom. A hollow 

 was cut in one end of the trough, and a glass tube D, about 

 4 cm. long, was fastened into it in an upright position. Then 

 from the point x on the inside of the trough a fine needle was 

 thrust through the paraffin till it came out into the hollow 

 previously cut in the wall. A sectional view of this part of 

 the device is shown in Fig. 33, II. When it was desired to 

 use the apparatus the trough was filled with filtered tap 

 water and a number of planarians placed in it. Then into 

 the tube D was introduced a certain amount of the solution 

 whose effects were to be tested. By varying the amount of 



