An adapter for attaching a device to a distal end of an endoscopic instrument, the adapter comprising: a distal stop (120, 141) to limit penetration of the endoscopic instrument into the adapter; at least one flange projecting axially from the distal stop and comprising: a non-closed perimeter, the flange configured for insertion of the endoscopic instrument therein, the flange having at least one longitudinal opening; characterised in that the adapter further comprises a securing means, releasably attachable to the flange, to compress the flange against the endoscopic instrument to secure the adapter to the endoscopic instrument to prevent motion of the adapter with respect to the endoscopic instrument, the securing means, when attached to the flange, at least partially wrapping around the flange, and configured to span the opening of the flange and to at least partially wrap around the endoscopic instrument; and wherein the flange includes an expandable flange portion defining the longitudinal opening and a highly expandable, generally optically clear, sealable membrane portion with sufficient folded-back or serpentine geometry to accommodate the insertion of an endoscopic device having a diameter greater than that of the distal stop, the membrane portion covering the opening, and the securing means configured to span the membrane portion and the opening. a distal stop (120, 141) to limit penetration of the endoscopic instrument into the adapter; at least one flange projecting axially from the distal stop and comprising: a non-closed perimeter, the flange configured for insertion of the endoscopic instrument therein, the flange having at least one longitudinal opening; characterised in that the adapter further comprises a securing means, releasably attachable to the flange, to compress the flange against the endoscopic instrument to secure the adapter to the endoscopic instrument to prevent motion of the adapter with respect to the endoscopic instrument, the securing means, when attached to the flange, at least partially wrapping around the flange, and configured to span the opening of the flange and to at least partially wrap around the endoscopic instrument; and wherein the flange includes an expandable flange portion defining the longitudinal opening and a highly expandable, generally optically clear, sealable membrane portion with sufficient folded-back or serpentine geometry to accommodate the insertion of an endoscopic device having a diameter greater than that of the distal stop, the membrane portion covering the opening, and the securing means configured to span the membrane portion and the opening. a non-closed perimeter, the flange configured for insertion of the endoscopic instrument therein, the flange having at least one longitudinal opening; characterised in that the adapter further comprises a securing means, releasably attachable to the flange, to compress the flange against the endoscopic instrument to secure the adapter to the endoscopic instrument to prevent motion of the adapter with respect to the endoscopic instrument, the securing means, when attached to the flange, at least partially wrapping around the flange, and configured to span the opening of the flange and to at least partially wrap around the endoscopic instrument; and wherein the flange includes an expandable flange portion defining the longitudinal opening and a highly expandable, generally optically clear, sealable membrane portion with sufficient folded-back or serpentine geometry to accommodate the insertion of an endoscopic device having a diameter greater than that of the distal stop, the membrane portion covering the opening, and the securing means configured to span the membrane portion and the opening.