Yerkes, Behavior of Hydroides. 



443 



rium, it is impossible to say. The former seems probable, since 

 one animal was tested which responded every one of the ten 

 times, although the conditions were apparently exactly similar to 

 those of another worm which was tried simultaneously and 

 reacted only four times out often. 



After some experiments of a different kind, to be described later, 

 had been given, some of the specimens used in them and a few new 

 ones were tested again for the effect of shadow alone. The 

 shadows this time were given at regular intervals of one or two 

 minutes. The number of responses in these cases is larger than in 

 the preceding, for out of twenty series of ten trials each, /. ^., a 

 possible two hundred reactions, there were ninety-five responses to 

 the shadow. The longer interval between shadows seems, there- 

 fore, to interfere with the process of getting accustomed to the 

 stimulus which occurs when the intervals are only a few seconds in 

 length. In Table I are shown the records of two series with one 

 specimen. In the first series the intervals between shadows were 

 fifteen seconds. In the second series, given four days later but 

 without any intervening training, the intervals are two minutes. 



TABLE I. 



No. 

 I 



2 



3 

 4 

 5 

 6 



7 



Series i. 



Shadow. 



Time. 



In all of the tables o means no reaction, and 



A series of experiments was now started in which two different 

 stimuli were given in rapid succession. The response to light, it 

 has been shown, is variable, since the animals sometimes respond 

 and sometimes do not. The response to mechanical stimulation 

 caused by jarring the animals or by touching the branchial fila- 

 ments with a glass rod is much more regular. In fact, the worms 



