MODIFIABILITY OF BEHAVIOR IN HYDROIDES 

 DIANTHUS V. 



ADA WATTERSON YERKES. 



The sensitiveness of the tubicolous annelid Hydroides dianthus 

 v., manifested by its prompt withdrawal into its tube when jarred 

 or when a shadow falls upon it, suggested this worm as a good 

 object for a study in modifiability of behavior. Such a study was 

 carried on for a few weeks at the Marine Biological Laboratory at 

 Woods Hole during the past summer. Unfortunately the time 

 was too short for extensive experiments but the results obtained 

 thus far seem to be of sufficient value to warrant presentation. 



The reactions of Hydroides to light have been studied by 

 Hargitt,^ who found that ''under varying degrees of light intensity 

 * * * the results of all experiments involving increased intensity of 

 light were uniformly negative. On the other hand, experiments 

 involving a sudden decrease of light intensity gave results as uni- 

 formly positive" (p. 319). This confirms a statement made by 

 LoEB^ (p. 103) concerning H. uncinata to the effect that only a 

 decrease in the intensity of light acts as a stimulus upon the ani- 

 mals. In the present experiments this stimulus was, therefore, 

 one of those selected for use and the decrease in intensity of the 

 light was obtained by bringing down rapidly a black cardboard 

 screen between the window and the dish which contained the 

 worms. After one or two seconds the screen was removed. For 

 convenience this decrease in light intensity will be referred to as a 

 shadow. Though the window faced south, the sunlight did not 

 fall directly upon the worms during the experiments. The 

 response to the stimulus was usually a quick contraction of the 

 animal into its tube. Occasionally this contraction occurred 



'Hargitt, C. W. Experiments on the Behavior of Tubicolous Annelids. Journal of Experimental 

 Zaology, Vol. 3, pp. 295-320, July. 1906. 



^LoEB, J. Ueber kiinstliche Umwandlung positiv heliotropischer Thiere in negativ heliotropische und 

 utngekehrt. Pf tiger's Archiv. Vol. 54, pp. 81-108. 1893. 



