254 



THE HYDRA 



and Kerona which often infest the outer surface of hydras. 

 On the other hand, suitable chemical stimuli are very effective, 

 as when dilute acetic acid is used to discharge the nematocysts in 

 laboratory study. It seems, therefore, that under natural condi- 

 tions the nematocysts become discharged by the stimulation of 

 chemical substances given off by the water-fleas and other small 

 organisms that are the common food of hydra. In contrast to this, 

 the muscular responses of the tentacles, in the capture of prey and 

 otherwise, appear to be controlled by the nervous system. 



The mechanism by which the eversion of the nematocyst thread 

 is brought about, following the chemical stimulation of the cnidocil, 



Fig. 124.— Cnidoblasts of hydra. 



A, B, and C, stage? in formation of the stalk (sO of the cnidoblast after attachment 

 to the supporting lamella (.si). The exposure of the nematocyst at its outer end is not 

 normal, but due to shrinkage of the cnidoblast in the preparation. D, E, and F, cnidoblaste 

 and their nematocyst? as seen in macerated specimens, en, cnidoblast; n, nematocyst; 

 7iu, nucleus of cnidoblast; si, supporting lamella; st, stalk. (After Hadzi, loc. cit.) 



is of interest. Experiments and observations indicate that the 

 threads are everted by an increase of pressure inside the capsule. 

 Before discharge, the cavity within the capsule and surrounding 

 the thread contains a fluid (Fig. 123) having the property of 

 swelling rapidly when it can imbibe water. Apparently, a chem- 

 ical stimulation of the cnidocil brings about changes in the cnido- 

 blast and its contained nematocyst such that water suddenly 

 enters the cavity of the latter and causes a rapid swelHng of the 

 enclosed fluid. Thus the mechanical effect is similar to that 

 which might be produced by blowing into the wrist of a rubber 

 glove and causing the eversion of an inturned finger. 



From the examination of animals paralyzed by a hydra and 

 removed from its clutches before being swallowed, it appears that 



