OBSERVATIONS OF THYONE BRIAREUS. 283 



ments and performed the righting reaction more rapidly than the 

 anterior halves. The only response in which the anterior half 

 approached the posterior in the number of individuals which re- 

 sponded was in the righting reaction. This was probably due, 

 in part at least, to the structure of the tube-feet which have more 

 efficient sucking discs on the ventral than on the dorsal surface, 

 and those on the ventral side would hence be more Hkely to be- 

 come attached if all of them were active. Two of the individuals 

 were kept in a dish without changing the water for three days 

 after they had been bisected. Both the posterior halves remained 

 in good 'condition and gave characteristic shadow responses at 

 the end of the third day, but both the anterior portions threw 

 out the viscera during the second day and were dead on the third 



day. 



These experiments show that the presence of the circum-oral 

 nerve ring is not essential for the performance of correlated reactions 

 and that the posterior half of the body is apparently able to carry 

 on movements better than the anterior. This greater efficiency 

 shown by the posterior end of the body is perhaps what might 

 be expected from the fact that the whole anterior portion is often 

 cast off and dies while the posterior end lives and regenerates the 

 lost organs. These conclusions do not agree with those reached 

 by Henri (103^) from his work with Stichopns regalis. He be- 

 lieved that the oral nerve ring was necessary for general muscular 



reflexes. 



IX. Variability of Reactions. 



Few of the reactions which have been described in this paper 

 could always be induced by a repetition of a stimulus which had 

 previously brought them about. The response which was perhaps 

 the most unfailing was the contraction of the body which resulted 

 from gentle mechanical stimulation with some pointed object, but 

 even this response varied with the strength of the stimulus, the 

 condition of the individual and other factors. Not only did char- 

 acteristic responses often fail to take place after a stimulus but 

 they were sometimes modified so that they did not take place in 

 the usual manner. Such differences in reactions may be due to 

 internal causes which have to do with the structure or the past 

 experience of the individual, or they may be caused by various 



