444 ' Journal of Comparative Neurology aj^d Psychology. 



respond immediately and invariably except in certain instances to 

 be mentioned later. 



In these experiments three or more individuals in the same dish, 

 usually growing on the same piece of shell or stone, were used 

 simultaneously. At the beginning all were given ten trials with 

 shadow alone with the usual result. Then the shadow was given 

 and if the animals did not respond to this the mechanical stimu- 

 lation caused by touching the filaments gently w4th a glass rod 

 follow^ed immediately. The period of retraction was noted and 

 when the animals w^ere again expanded the experiment was 

 repeated. The trials were recorded in groups of ten each and 

 from one to six of these groups were given daily. A number of 

 specimens were tested thus for several days each. Some of these, 

 having been in the laboratory for some time, then became inactive 

 and refused to come out of the tube after being placed in the experi- 

 menting dish! Tw^o animals, however, Nos. 40 and 41, obtained 

 from fresh material were tried for ten days and the records of these 

 two, which show a number of interesting points, will be discussed 

 in detail. 



In the trial experiments in which shadow alone was given at 

 intervals of 5 to 10 seconds No. 40 responded hot at all and No. 41 

 only the first time. After a short interval the shadow and touch 

 series was given. In this No. 40 reacted to shadow alone three 

 times during the first ten trials, No. 41 only once and neither of 

 them again throughout the forty trials given that morning. The 

 fact that when responses to shadow alone occurred they almost 

 always came within the first twenty trials soon became evident and 

 the records will, therefore, be given for these twenty alone. The 

 results for the ten days can best be represented by the following 

 curves in which the number of responses to shadow alone for each 

 of the successive days, as indicated on the base line, is marked on 

 the ordinates. 



In the case of No. 40 there is a marked increase in the number of 

 responses to shadow from the first to the eighth day and those of 

 the later days are much higher than at first. For No. 41 the 

 increase is not so great, though the rise in the curve from the 

 second to the fifth day is pronounced. Even at the beginning, the 

 responses of this individual were not as numerous as for No. 40 

 and after the tenth day it became so inactive, sulking at the mouth 

 of the tube or remaining retracted into it so long, that it became 



