SUSCEPTIBILITY IX IIVDROIDS. IO V S 



The length of time between exposure to the agent and the 

 beginning of disintegration and the rate of its progress may of 

 course be varied widely by varying the concentration. The 

 concentrations most used are those in which disintegration be- 

 gins in one half to one hour after exposure and is completed in 

 three to six hours. 



The susceptibility relations in the well developed hydranth of 

 Tnbiilaria crocea and in Bougainvillea, except that in the latter 

 there is only one series of tentacles, are similar to those in Pen- 

 naria. The campanularian hydranths examined also show essen- 

 tially the same relations in fresh material. 



THE DEATH GRADIENTS IN STEMS AND STOLONS. 



Data on susceptibility of stems to chemical agents, as measured 

 by disintegrative changes, are of course open, in all hydroids 

 possessing a perisarc, to the possible objection that differences in 

 permeability and thickness of the perisarc at different levels 

 may constitute complicating, or perhaps the chief factors in 

 determining the time of death. The thickness of the perisarc 

 is least in the most recently developed, i. e., in any particular 



X 



axis, the most apical portions of the stem and its thickness 

 increases in general basipetally. 



As a matter of fact, however, the perisarc is apparently pene- 

 trated rather readily by many agents. It does not obstruct or 

 retard greatly the entrance of vital dyes and permanganate pene- 

 trates it almost at once, even where it is thick. While the 

 possible complicating effect of the perisarc must of course be 

 admitted, various facts cited below indicate clearly enough that 

 it is not responsible for all the differences in susceptibility ob- 

 served in stems. 



In general a stem is less susceptible than the hydranth it bears, 

 and the susceptibility decreases basipetally in the stem, at least 

 near the hydranth, in all agents used in lethal concentration. 

 In concentrations of KNC, neutral red, etc., which do not kill 

 within a few hours, the more apical portions of stems usually 

 undergo resorption and retraction within the perisarc after more 

 or less complete disintegration or resorption of the hydrant h>. 

 These processes of resorption and retraction of the more apical 



