This light- and electron-microscopic study has investigated the structure, the morphodynamics of discharge, and the impact of the stenotele cyst of Hydra attenuata (Hydrozoa, Cnidaria) on the prey's integument. The triggered capsule, which is ejected from the cell, discharges its tubular content (shaft, stylets and tubule) by a process of evagination. In doing so the three joined stylets punch a hole into the cuticle of the prey through which the long evaginating tubule penetrates into the interior of the target. The behaviour of the tubule is described in detail and the functional significances of the various parts of the capsule are discussed.