A study of the change in arrangement of the barbs on the thread of the large holotrichous glutinant nematocysts of Corynactis viridis during discharge shows that the wall of the tubular thread undergoes anisometric dilatation. Together with observations on the dilatation of the thread in capsules ruptured in an aqueous medium, this strongly suggests that discharge of the thread is due to an intrinsic swelling mechanism: the tubular thread itself swells anisometrically. Though the intrinsic mechanism of discharge operates at the tip only of the discharging thread, swelling of the capsular fluid and elasticity of the capsule-wall must also play a part in normal discharge. While it has not as yet been possible to determine the optical characteristics of the wall of the undischarged thread (because of the strong birefringence of the barbs), the observed birefringence of the discharged thread indicates an anisotropy of structure such as anisometric swelling would lead us to expect.

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