ABSORPTION OF THE HYDRANTH IN HYDROID POLYPS. 299 



These results show that contact cannot in any case be con- 

 sidered the only factor to which the absorption of campanularian 

 polyps is due, and that the process closely resembles that in 

 other polyps in which under similar conditions we find either 

 absorption or direct shedding of the hydranths with subsequent 

 regeneration. 



The material for study was obtained fresh each day, so that 

 the animals should be in thoroughly good condition. Pieces of 

 Campanularia were then cut and laid in watch crystals in contact 

 with the glass in the way described by Loeb. The stages in the 

 absorption of Eudendrium and Pennaria, which I used for com- 

 parison, being more difficult to obtain, were taken whether in 

 contact or not, according to where they presented themselves. 

 All the material was killed in cold corrosive acetic, and stained 

 with Delafield's haematoxylin and congo red. 



Within a few minutes after the removal of a piece or stalk, the 

 cut end closes over, and the digestive current begins to flow slowly 

 from one end of the hydroid to the other. It passes forward, and 

 then is driven backward mainly by the contraction of the circular 

 muscles of the polyps in the region just below the tentacles, but 

 not involving a contraction of the whole animal ; a slight pause 

 occurs between each change in direction. The irregularity in 

 the contraction of the polyps sometimes complicates the course 

 of the current. At first the polyps remain expanded, and the 

 only change noticeable is in the digestive fluid which becomes 

 more and more laden with spherical granules of all sizes. The 

 current is sometimes driven with such force that the contents 

 break their way through a newly formed stolon or through the 

 mouth of the polyp. The animal has up to this time been fully 

 expanded except for the rhythmic contractions which decrease 

 only the diameter of the body, but now it gradually contracts 

 into its cup, and the body becomes shorter and broader, the latter 

 change being largely due to the thickening of the ectoderm as 

 can be seen even in the living animals. The tentacles undergo 

 excessive contraction, becoming a crown of mere stubs, and then 

 disappear altogether ; their cells passing into the cavity of the 

 polyp. At the same time, the hypostome absorbs. 



These changes take some time and normally occupy at least 



