GEORGE B. CHAPMAN 133 



(HD). Distal to the head, may be seen a narrowed extension 

 of the tubule ending in a hook (HK). Just proximal to the 

 head, the longitudinal section reveals a rather fluted contortion sug- 

 gesting why the cross sections of tubules often appear as three- 

 bladed propellers. The enlarged head of the tubule is of particular 

 interest for it is the only non-structural portion of the capsular con- 

 tent which is distinct in its texture from the matrix. One could 

 speculate that this small packet of material might possibly rep- 

 resent the location of the venom of Hydra. It might further be 

 speculated that the minute (approximately 150 A in diameter) 

 dense granules in the tubule head represent the 5-hydroxy tryp- 

 tamine which Welsh and Moorhead ( 15 ) have suggested may be 

 a constituent of coelenterate venom. Such a location for the 

 venom would be most effective for the presence of the hook at the 

 termination of the tubule suggests that the head portion of the 

 tu]3ule may be attached to the base of the invaginated capsular wall 

 and thus be drawn out of the discharging nematocysts in advance 

 of the rest of the tul)ule, thus causing the head to encounter the 

 Hydras prey at the earliest possible moment, thereby facilitating 

 the utilization of the venom. If the enlarged head of the tubule 

 does not contain the \'enom, it is difficult to imagine where the 

 venom could reside save in the capsular matrix material. The lat- 

 ter possibility seems less satisfactory from an operational stand- 

 point. (The head of the tubule may be projected rapidly to a dis- 

 tance of many microns from the body of Hydra, while the matrix 

 material would have to depend largely on diffusion to reach the 

 prey. ) 



The above observations thus tend to reopen the question of the 

 nature of the tubule discharge mechanism. If eversion occurs, the 

 contents of the tubule head could encounter the prey only when 

 the tubule is fully everted. Prey located close to the surface of 

 Hydra might be by-passed by the head of the tubule and any 

 possible function of the contents wasted. If the head is discharged 

 somewhat like a fishing lure is cast, it could quickly encounter 

 prey located anywhere within its maximum range. The latter meth- 

 od of discharge, of course, raises the questions of how the head of 

 the tubule could get outside of the invaginated capsular wall and 



