THE ACTIVITIES OF CORYMORPHA 321 



I have confirmed these observations and have extended them 

 by making measurements of the specific gravity of the several 

 parts. If a strong solution of ordinary cane-sugar in sea-water 

 is added to sea-water in which a clean Corymorpha is floating, 

 is is comparatively easy to arrive at that concentration at 

 which the Corymorpha is just suspended. The specific gravity 

 of such a solution can then be determined by a hydrometer 

 and thus that of the Corymorpha ascertained. By this method 

 I found that the specific gravity of an expanded Corymorpha 

 was 1.0292, of a contracted one, due probably to the loss of 

 water, was 1.0320, of the stalk was 1.0268 and of the hydranth 

 was 1.0530. The sea- water in which these animals were living 

 had a specific gravity of 1.0250. It is clear from these records 

 that the negative geotropism of Corymorpha is in no sense the 

 direct result of the specific gravity of the whole animal, of the 

 stem, or of the hydranth. It must, therefore, be an indirect 

 response of some kind. 



If a Corymorpha is attached to a movable base, its hydranth 

 cut off, and the base so set that the stalk is in a horizontal 

 position, the stalk soon turns and assumes a vertical posture. 

 The rates of turning are about the same in the headless polyp 

 and in the whole one. Thus in one set of experiments five 

 stalks that had turned from horizontal to vertical in the average 

 time of one hour and forty minutes made the same turn after 

 the loss of their hydranths in one hour and fifteen minutes. In 

 a second set the normal individuals became vertical in an aver- 

 age period of one hour and ten minutes and after the loss of 

 hydranths in one hour and thirty minutes. Thus there seems 

 to be very little difference between the geotropic response of a 

 normal polyp and of a stalk without a hydranth. The geotropic 

 response must, therefore, be a feature of the stalk. 



If anchored stalks are placed horizontally in sea- water con- 

 taining enough sugar in solution to make it of the same specific 

 gravity as the stalk, the stalk soon takes on a vertical posture. 

 Evidently the stimulus to geotropism is not due to the weight 

 of the stalk pressing upon its support but must depend. upon 



