An Aid in the Study of Nematocysts. 237 



being taken to exclude all air. The bulb is fully expanded and the tube, 

 turned inside out, is retracted into the receiver (Fig. 1). 



When, in order to illustrate the process of discharge, the rubber bulb 

 is compressed, the water which is thereby displaced tends to escape through 

 the inturned tube. Owing, however, to the fact that its walls are flexible 

 and collapse under the pressure, this outlet closes and the water is unable 

 to get through. As a result the tube becomes everted to the outside (Fig. 2), 

 the process beginning at the place where the part which is fitted over the 

 tubulure doubles back into the receiver. 



This experiment indicates that in a nematocyst, previous to discharge, 

 the pressure of the fluid contents of the capsule squeezes the filament into 

 a "thread"; and, further, that subsequently during discharge the same 

 pressure, acting, however, in the opposite direction, distends the thread, as 

 it becomes everted to the outside, into a hollow thin-walled " tube." The 

 mechanical effect of this arrangement is, no doubt, to facilitate the process 

 of eversion by providing a clear space for the passage of the thread as it 

 pays out from the capsule. 1 



Besides illustrating the process of eversion, the experiment seems also 

 to throw light upon the condition of the nematocyst itself. Assuming that 

 the wall of the capsule is elastic — which is probably true — we may infer 

 that a nematocyst, before it is discharged, is distended beyond its natural 

 dimensions. For supposing that, instead of being distended in this manner, 

 the capsule possesses its natural size, then it is plain that discharge can only 

 be effected by external pressure contracting the capsule and driving out 

 the contents, and that, when the external pressure is removed, the capsule 

 in returning to its original shape will tend to draw the filament back 

 into its former position. That this takes place, when a capsule with 

 elastic walls is compressed, is readily seen on referring to the model. So 

 long as the rubber bulb is kept under pressure, the tube remains extended. 

 When, however, the pressure is released, the elastic recoil immediately 

 comes into play and sucks the tube back again into the receiver (Fig. 1). 

 In a nematocyst, on the other hand, the filament, after eversion has been 

 accomplished, is not withdrawn but remains permanently extended, 2 and 



1 The curious movements of the filament, which are observable during eversion, both in 

 the " skein " (which spins round as it unwinds) and in the " thread " (which appears to 

 vibrate as it runs out through the "tube") cannot be reproduced in this form of model — a 

 much more exact copy of a nematocyst being required for the purpose. 



2 An exception is said to occur in the case of Millepora, in which the filament is stated 

 to be retractile (Willey, Zool. Results, pt. ii., 1899, p. 127 ; Hickson, Camb. Nat. Hist., vol. 

 i. p. 248). In this form, moreover, the filament when in the expanded condition is, if one 

 may judge from the published figures, closed at its free end (Willey, he. tit., pi. xvi., 

 fig. 1 ; cf. also Moseley, Challenger Reports, vol. ii., pi. xiii.-fHydrocorallinae] fig. 1). 



