Biological Studies on Corymorpha, 



405 



portion is two-thirds, one-third, or even one-eighth the original 

 length of the stem. Evidently the stem is generally responsive to 

 the geotropic stimulus. The only difference lies in the time con- 

 sumed in reaching the vertical position. The longer the stem the 

 shorter the period. 



The result is also in the end the same whether the hydroid is 

 hung vertically upside down, by the proximal extremity, or right 

 side up, by the "neck," just below the hydranth, or by the middle. 

 And it matters not whether the hydranth and foot are both or 

 either one present or absent. 



Fig. 1. 

 Three vertical stems cut at different levels, which were parallel with S one hour 

 before. 



. Numerous experiments justify this summary. Typical cases 

 will be described. To exclude the possible influence of light and 

 oxygen on the direction of orientation, the hydroids were com- 

 pelled to orient themselves in sealed jars quite full of water, which 

 were placed in dark closets. Check experiments in the light and 

 in open aquaria gave identical results. Further, about a dozen 

 individuals were subjected for three hours to light coming from but 

 one direction, without any observable result on their orientation. 



