MOVEMENTS, ETC., OE EUESH-WATER PLANA RIANS. 661 



body parallel to the Hues of action of the stimu- 

 lus. This movement of the auimal iu a constant relation 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 wo find^ as has been shown above 

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

 reaction to a chemical unilaterally applied as it does to a 

 single weak niechauical 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. Ou the contrary, it 

 seems apparent that we are dealing here Avitli 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 organism 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 cu.t 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 parafRn 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 liltered tap 

 water and a number of planariaus placed in it, TUon into 

 the tube D was introduced a certain amount of the solution 

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



