MOVEMh^NTS, ETC., OF FRKSH-WATER PLANARIANS. 673 



the reaction is chiefly due. While the negative thiguiotaxis 

 of the dorsal snrface plays some part in the reaction, it is, as 

 the experiments described above show, a comparatively un- 

 important factor. The specific relation of these two factors 

 to the definite righting reaction will be brought out in the 

 next section, in which the form and mechanism of this 

 reaction will be set forth. 



b. The Righting Reaction. — The righting reaction is 

 a very characteristic piece of behaviour, and can best be 

 described in a single phrase by saying that when the animal 

 is placed on its back it throws itself into a spiral in such a 

 way that tlie ventral surface of the head comes into contact 

 with the bottom. This ventral surface then attaches itself to 

 the bottom by means of the mucous secretion, and starts 

 gliding ahead. As it goes forward it unwinds the remainder 

 of the spiral, as each successive posterior part of the ventral 



Fig. 36. — Showing the form taken by Plaiiaria in the righting reaction. 



surface comes into full contact with the bottom. The form 

 of this spiral just after the ventral surface of the head has 

 come into contact with the bottom is shown in Fig. 36. The 

 spiral is thrown very quickly after the dorsal surface touches 

 the bottom, and usually includes the whole length of the 

 body at once. However, by observing a specimen in which 

 it takes place a little more slowly than usual, it can be seen 

 that the movement is started at the anterior end. Beginning 

 with, for example, the right side of the head, this is turned 

 under, while at the same time the left side is raised. This, 

 of course, brings the ventral surface of the head region down, 

 and at the same time makes a twist in the body, just back of 

 the head. In some cases this is the only twist that is made, 

 while in others another similar twist, is thrown in the body 

 farther back. As the anterior end alter it is righted glides 



