velum

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velum From the Latin velum, ‘ship's sail’. 1. In Mollusca, the ciliated lobe possessed by the veliger larva.2. In most hydroid medusae (Cnidaria), a ridge or narrow shelf inside the border of the sub-umbrellar (See UMBRELLAR) cavity which reduces the size of the subumbrellar aperture and so increases the force of the water jet by which the medusa swims vertically.

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