ABSTRACT Nematocyst development was watched in living cells of a siphonophore, Rosacea cymbiformis. The secretion differentiates in the formative cell into an ovoid capsule, continuous with a long apical tube. The entry of this ‘external’ tube into the capsule creates the coiled internal’ thread’ of the mature nematocyst. Electron microscopy shows that the tip of the external tube inverts and moves down the tube towards the capsule. Complex pleats form during inversion. Thus formation of the internal thread is the geometrical converse of discharge. Extensive repacking of the triply pleated, internal thread occurs, and subsequently the barbs form.