HYDEOIDS 193 



means simple. Its extent and ramifications can be made 

 out easily by the injection into it of india ink under slight 

 pressure. In this way it can be demonstrated that the 

 body of the hydranth, including the proboscis, contains a 

 considerable digestive cavity (May, 1903). Extensions 

 of this reach out into the peduncles carrying the medusa?, 

 but not into the tentacles. Proximally the digestive space 

 contracts into a central tube as it passes from the hy- 

 dranth into the stalk, at the distal neck of which it con- 

 nects with the dozen or more longitudinal canals which 

 extend in somewhat parallel courses proximally through 

 the more peripheral layer of the stalk well down to its 

 buried end. These tubes show not infrequent anastomoses, 

 especially in the proximal portions of their course. The 

 injection of the whole digestive system is best made by 

 cutting off the proximal tip of a polyp, inserting a canula 

 among the vacuolated cells, and after tying it in that posi- 

 tion, slowly injecting india ink under slight pressure. 

 The ink quickly makes its way into the canals and flows 

 forward freely till the neck between the stalk and the 

 hydranth is reached. Here commonly a slight block is 

 encountered, due probably in part to the contraction of 

 the sphincter in the circular muscle of the stalk and in 

 part, perhaps, to the fenestrated membrane (Torrey, 

 1907). By continued slight pressure the ink will sud- 

 denly spurt through this neck and quickly fill the cavity 

 of the hydranth, where again it may meet a block if the 

 mouth is closed. This obstruction, which seems also due 

 to a sphincter, may likewise be overcome, whereupon the 

 ink will flow in a fine stream through the mouth into the 

 outer water. 



The gathering of food into the digestive cavity is de- 

 pendent upon one of the best organized and most char- 



13 



