[Cnidaria] In most hydromedusae, the velum is a thin sheet of tissue stretched across the perimeter of the bell like the skin of a drum but having a hole in the centre, the velar aperture, opening or pore. The aperture may be small to nearly as wide as the bell. In a few species the velum may be absent altogether.

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