THE CCELENTERATA 



153 



conveying a message into the cell. The nematocyst is formed within 

 the cnidoblast as a metaplasmic product, and takes the shape of a 

 refringent oval capsule with a tough, probably double wall. At its 

 outermost point the capsular wall is pushed in or introverted to 

 form a narrow tubular ingrowth which extends nearly the length of 

 the nematocyst. It then quickly narrows down and is continued 

 as a very fine thread spirally coiled around the tubular ingrowth. 

 The remainder of the inside of the capsule is filled with a semi- 

 gelatinous fluid which renders the whole tense and turgid. Under 

 suitable conditions of hunger, etc., if a small water animal touches 

 the cnidocil it causes the nematocyst thread to be shot out with 

 explosive violence. The semi-gelatinous fluid is apparently very 



Cn. 



A. B. C. 



FIG. 50. Cnidoblasts of Hydra. 



A., large variety undischarged ; B., large variety discharged ; C., small variety discharged. 

 Note small size, absence of barbs and thickness of thread. 



Ba., barbs; C.B., cnidoblast cell; Cn., cnidocil or trigger process; N., nematocyst; Nu., 

 nucleus of cnidoblast ; N.T., nematocyst thread. 



hygroscopic, and the slight disturbance of the cnidocil in some way 

 or other allows it to absorb water, with the result that a pressure is 

 produced practically instantaneously and the thread is ejected, being 

 turned inside out in the process. When discharged the nematocyst 

 appears as an empty capsule on one side of which is a projection 

 like the short handle of a whip, often bearing several barbs large 

 and small, continued on. as a delicate thread-like lash many times 

 longer than the cell itself. These serve as offensive and defensive 

 weapons, and, as we have seen, there is force enough in the thread 

 when discharged for it to penetrate into the body of the water-flea. 

 The consequent paralysis of the animal is presumably brought about 

 by the thread carrying with it a small quantity of an irritant poison. 

 The presence of this poison has not been shown in Hydra, but some 

 of its allies, certain sea-anemones and jelly-fish, when they sting a 



