[Radiolaria Polycystina] In Nassellaria: a porous plate obliterating the mouth (A. denticulata group? intro).

[Hydromedusae] Horizontal fold projecting inwards from the umbrellar margin leaving a central, circular aperture, the velar opening. It consist of two layers of ectoderm separated by a mesogleal lamella, the inner ectoderm, of subumbrellar origin, with striated muscles. The velum serves in propulsion and orientation of the medusa, acting similar to a photographic diaphragm, the diameter of its aperture is adjustable during swimming, from as wide as the umbrella to almost closed.

[Gastropoda] Membraneous locomotory structure lined with cilia present in most pelagic molluscs before the development of wings or fins.