REGULATION OF REACTIONS 293 



learned to react to the shadow, the sign of the tactile 

 stimulus that regularly followed it. 



Hydroides becomes acclimated to a given stimulus with 

 surprising rapidity. Mrs. Yerkes (1906, p. 442) found that 

 in sixteen tests out of twenty-seven with different spec- 

 imens the animals responded to shadows passed over 

 them at regular intervals only from one to three times, 

 after which the decrease of intensity did not appear to 

 affect them at all. 



Hargitt records similar results in a paper published a 

 few months earlier than that of Mrs. Yerkes, and again in 

 a later paper. He observed (1909, p. 179) that specimens 

 taken at a depth of from eight to fifteen fathoms react to 

 shadows only in an indefinite way, and that many do not 

 respond at all, indicating clearly that the response depends 

 upon past experience as well as upon present conditions. 

 Jennings (1906) and others have observed similar effects 

 of other stimuli on numerous different species. 



One of the most interesting features in the behavior of 

 Hydroides, and one that has been most accurately recorded, 

 is the variation in the time that these animals remain in 

 the tubes after responding to a given reduction in light 

 intensity. In a series of ten trials Mrs. Yerkes (1906) 

 found the time to vary from 15 to 240 seconds, and in 

 another series of sixty trials from 10 to 710 seconds. 

 There is no apparent regularity in this variation. The 

 author says, referring to the last series mentioned above 

 (p. 447) : " The period of retraction is short the first three 

 times — 19 to 34'' — but the fourth time it is nearly four 

 minutes. For the next thirteen times it ranges from 

 eighteen to ninety-three seconds; then comes another 

 period of nearly four minutes followed by nineteen con- 

 tractions which last from twelve to eighty-five seconds 

 each and then a contraction of nearly twelve minutes' 

 duration. Thus after the fourth, eighteenth, thirty- 

 eighth and sixtieth trials the animal remained contracted 

 for a relatively long period, varying from four to twelve 



