Merriam-Webster Collegiate Dictionary - velum

 noun  (plural vela)  Etymology: New Latin, from Latin, curtain  Date: 1753  1. a membrane or membranous part resembling a veil or curtain: as  a. soft palate  b. an annular membrane projecting inward from the margin of the umbrella in some jellyfishes (as the hydromedusae)  2. a swimming organ that is especially well developed in the later larval stages of many marine gastropods