MOVLIWENTS, ETO., OF FRESH-WATER PLANAKIANS. 671 



either a resting or a moving' specimen, it will very promptly 

 move out from under it. Furtlier, if crevices are arranged of 

 this form (^^r"^) by supporting cover-glasses at two corners, 

 and letting the two opposite corners rest on the bottom of the 

 dish, specimens will not go into them. The moment the 

 dorsal surface touches the cover-glass above, the worm 

 begins to react violently, changing its direction of movement, 

 and goes out from under the cover. 



With the existence of an apparent negative thigmotaxis of 

 the dorsal surface established, however, there still arises the 

 question as to whether this is the sole cause which induces the 

 inverted animal to right itself. The following experiment 

 throws light on this point : — A specimen is placed ventral 

 side up on a dry spatula in the air, and then the spatula is 

 placed just beneath the surface of the Avater in a tall jar or 

 large test-tube and quickly pulled out from under the worm, 

 so that the latter starts falling through the water in an in- 

 verted position. Another Avay in which the worm may be 

 started falling ventral side up is by holding it on a scalpel 

 point above the water, and then dropping it beneath the 

 surface in the desired position. Before the worm has 

 dropped any great distance it will give the characteristic 

 righting reaction, and turn itself over so as to bring the 

 ventral side down again. This is done in precisely the same 

 way as when the animal is inverted on the bottom (to be 

 described later). After the falling animal has thus righted 

 itself it may again give the same reaction, and thus turn 

 itself over so that the dorsal side is down again. In a few 

 cases I have seen a worm after righting itself the first time 

 keep right side up during the remainder of the fall. The 

 most usual behaviour is for the animal to keep giving the 

 righting reaction all the time that it is falling, although this 

 does not, of course, keep it all the time with the same side 

 uppermost. I have performed a large number of these 

 dropping experiments in which the animals were started in 

 both upright and inverted positions, and in all cases they 

 gave the righting reacting one or more (usually more) times 



