1. The appearance of one type of nematocyst from Corynactis before, during, and after discharge is described. Discharge is a progressive eversion of the intracapsular filament.
2. Weill's observation that the speed at which dried nematocysts can be made to discharge depends on the quantity of water available is confirmed.
3. In water, dry isolated fragments of undischarged filament undergo changes in size which normally accompany discharge, while remaining uneverted. This reaction is an anisometric swelling of the shaft wall due to water absorption, and is largely responsible for the normal progression of discharge in nematocysts.
4. Electron microscopy has been used to investigate the morphology of various nematocysts.