676 RAYMOND PEARL. 



an active but large specimen (about 12 mm. long) of 

 P. dorotocepliala the average time taken to regain com- 

 pletely the normal position after being inverted was 8'68 

 seconds. With a small specimen (5'5mra. long) the average 

 time taken in righting in ten trials was 5*22 seconds. The 

 time taken in the reaction also depends, of course, on 

 the general physiological condition of the animal. Thus in 

 ten trials with a sluggish specimen, approximately 9 mm. 

 long (thus shorter than the first specimen mentioned), the 

 average time taken in regaining- the normal position was 

 10'90 seconds. 



The thigmotactic irritability may be modified or reduced 

 in several ways, and, as a consequence, the righting reaction 

 will disappear entirely or in part. One of these cases has 

 been mentioned above (p. 670) where it was shown that a 

 specimen placed on its back in a 10 per cent, solution of 

 NaCl makes no attempt to right itself. Similarly a specimen 

 put in an inverted position on a dry surface, care being taken 

 that no water surrounds the animal, will not give the righting 

 reaction. In both of these cases the specimens are able to 

 move. 



The Mechanism of the E-eaction. — It is a very 

 difiicult matter to determine exactly the muscular mechanism 

 of this righting reaction, since it is such a complicated move- 

 ment, and is ordinarily done in its most essential feature — 

 the formation of the spiral — so very quickly. Furthermore, 

 as will appear from the operation experiments to be described, 

 it is almost impossible to devise crucial experiments of a 

 character which will demonstrate what the mechanism is. 

 What I shall do, then, will be to present a tentative explana- 

 tion of the mechanism of the reaction, together with the 

 evidence for it which I have been able to obtain. I may say 

 that the view to be presented is the result of a long and 

 careful study of the phenomena both in normal and operated 

 worms, and I believe that it is a correct explanation. 



The mechanism of the righting reaction is probably as 

 follows : — The half of the body of an inverted specimen which 



