A PRELIMINARY NOTE ON THE ABSORPTION OF 

 THE HYDRANTHS OF HYDROID POLYPS. 



H. F. THACHER. 



Loeb has found that when pieces of Campanularia are placed 

 in dishes of sea water, the polyps in contact with the glass 

 undergo a transformation and disappear completely into the stem. 

 This process, he states, is " due to contact, and is accomplished 

 by the liquefaction and subsequent withdrawal of the protoplas- 

 mic mass." In taking up this subject, at the suggestion of Pro- 

 fessor Morgan, my wish was to see whether in this case a study 

 of the histology would support Loeb's theory of the liquefaction of 

 the protoplasm as a result of contact : and also whether the proc- 

 ess in Campanularia resembles that in other hydroids in which ab- 

 sorption occurs, but is due merely to the change from natural to 

 laboratory conditions. For the latter point, in addition to Cam- 

 panularia, I examined Eudendrium and a few cases of Pennaria, 

 both of which forms also readily absorb their hydranths. 



To see if by chance Campanularia would also absorb its 

 hydranths when not in contact, I made a set of experiments, 

 placing the splinters of wood on which the hydroids were grow- 

 ing in dishes, so that, as far as possible, the animals would be in 

 a normal position. Under these conditions I found that the polyps 

 were absorbed as rapidly as when touching the glass. These 

 results show at least that contact is not essential to the produc- 

 tion of this phenomenon, and suggest the likelihood that the 

 absorption is due to the same cause in all cases. 



The beginning of the degenerative changes are first shown by 

 the appearance of large numbers of spherical granules in the 

 digestive current, and by an increase in its rapidity. Shortly 

 after, the polyp, which is to be absorbed, contracts into its cup, 

 and the tentacles fold closely over it. Gradually the polyp be- 

 comes shorter and shorter, and the tentacles pass from the length 

 found during ordinary contraction to a knob-like stage, and later 

 are completely absorbed. Towards the close of this period the 

 hypostome also disappears. At this time the digestive current 



96 



