These especially small masks were specifically designed for use on small exotics such as pocket pets and birds. The small circuit hooks up the same way as a non-rebreathing circuit does to an anesthesia machine. The masks fit tight around the nose and mouth. Soft conforming cone can be trimmed to custom fit animal?s nose. The oxygen/gas tube position can be adjusted inside mask. Also circuit can be taped to the table to help keep it positioned on the animal?s nose or muzzle. 12.5mm opening, 10" tubing.