[Hydromedusae] Papillae in some medusae situated either between the marginal tentacles, at the base of some marginal structures (tentacular bulbs, non-tentacular or rudimentary bulbs, or marginal warts), or on the radial canals. They provide an opening or pore in contact with the cavity of the bulbs, or of the gastro-vascular system. Regarded as having an excretory function.