From ZooTerms (Dictionary of Invertebrate Zoology)
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velum (noun; plural vela; Latin velum, curtain): 1. A thin membranous covering.

2. (Annelida: Hirudinoidea) A membrane separating the buccal cavity from the cavity of the oral opening.

3. (Arthropoda: Crustacea) In Ostracoda, a ventral ridge, flange, or frill that may extend around part or all of the anterior and posterior ends.

4. (Arthropoda: Insecta) The membrane forming part of the apical and marginal areas of a paramere.

5. (Cnidaria) A shelf of tissue extending inward near the margin of the bell of medusae.

6. (Mollusca) a. In Bivalvia, the large, ciliated swimming disc of larval oysters. b. In Gastropoda, the swimming membrane consisting of two large semicircular folds bearing cilia.

7. (NEMATA) The ventral membranous winglike extensions on the spicule of some male nematodes.

8. (Platyhelminthes: Cestoda) The membranous posterior margin of a proglottid overlapping the anterior of the following one.