MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 681 



settling- back less after each trial. Successive stages of tliis 

 rising are shown in Fig. 39, h, c, d. Finally, it worked up till 

 it stood directly on the posterior end (e), and then the next 

 contraction caused it to fall over of its own weight and come 

 down right side up (/"). The sticky mucous secretion at the 

 posterior end was undoubtedly what held the piece up after 

 each successive trial. This behaviour, as described, was 

 uniform in all the trials. 



The behaviour of these pieces brings out several points of 

 importance. First, it is to be noticed that no trace of the 

 typical spiral righting reaction is to be seen ; yet, on the other 

 hand, we find the pieces bending strongly towards the cut 

 side when first inverted, which is just the effect which would 



ot 



.^fl 



f 



EiG. 39. — Diagram sliowiiig the nielliod of riirliiiiisf adopteil by one 

 of llie pieces shown in Fig. 38. 



be produced by the lengthening of the stimulated side in the 

 normal righting reaction, provided, as actually obtains in this 

 case, there was no opposite side to keep the long axis of 

 the piece straight. Thus we get precisely the result 

 which would be exjDected if the view given of the mechanism 

 of the reaction is the correct one. Another fact that is 

 brought out by this experiment is the apparent adaptation 

 shown. AVhen the animal is unnblo to give the usual reaction 

 for righting itself it very quickly reacts in an entirely 

 different way, but attains the same end result. 



A worm was cut so as to give a ])iece of the form shown at 

 A in Fig. 40. This piece Avas placed in an inverted position 



