Abstract: Apparatus for forming an aperture, upon the occurrence of a predetermined event, in a resilient membrane such as a breathing hose, which is attached to a breathing mask worn by a person and which is partially filled with water due to immersion in the water followed emergence of a portion of the hose adjacent the breathing mask from the water, to enable the person to breathe ambient air through the aperture. The apparatus includes a blade, a primer for impelling the blade under explosive force into the resilient member, and a circuit responsive to the occurrence of the predetermined event for firing the primer.