230 



Proceedings of the Royal Physical Society. 



confining his attention at first to the capsule, can readily ascertain the 

 precise stage in the process at which the fluid secretion becomes ejected 

 to the outside. For at this moment the capsule, hitherto stationary, 

 suddenly jerks forward and begins to shrink in size. The characteristic 

 bright appearance soon fades away, and finally the capsule, spent and 

 shrunken, shows faintly-outlined and dull. In the case of the filament, 

 also, the ejection of liquid is accompanied by a loss of brightness and a 

 diminution in size. More interesting, however, is the sudden and vigorous 

 recoil which occurs in the terminal region, and which frequently flicks the 

 extremity out of sight. In fact, the filament behaves in this respect precisely 

 like a piece of rubber tubing which, having been clamped at one end and 

 distended with water, suddenly, on being freed from restraint, contracts to 

 its normal size. When, on the other hand, eversion is only partial, the fluid 

 contents are not expelled, and in consequence the filament does not recoil, 



Fig. 1. 

 Before discharge. 



Fig. 2. 

 After discharge. 



nor does it alter in size or optical appearance. It differs, moreover, from 

 one which is completely everted in being as a rule spirally coiled. 



In Hydra, which is generally taken as a type for elementary study, 

 the nematocysts expel their contents with such rapidity that the process 

 of eversion cannot be followed. For teaching purposes, therefore, it has 

 been found useful to illustrate, by means of a model, the relation of the 

 filament to the capsule and the mode of discharge. 



A rough model, adapted for use under water, consists (Figs. 1 and 2) 

 of a glass receiver fitted with a rubber bulb — the two together representing 

 the capsule — and of a flexible tube which is open at both ends. The 

 receiver is of 250 cc. capacity, and is provided with a pair of opposite 

 tulmlures. To one of these is attached the rubber bulb, which is of about 

 the same capacity as the receiver itself; and over the other is fitted the 

 flexible tube which, in default of anything better, consists of a piece of 

 fresh gut. The place of the fluid secretion is taken by a coloured liquid, 

 which is introduced into the receiver through a third orifice (not shown in 

 the figs.). When ready for use the model is submerged under water, care 



