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(1) An organ of locomotion and seizing food in medusae, siphonophores, and the larvae of many marine mollusks (veligers). In medusae and in the floating bells of the siphonophores, the velum is an overhanging border edging of the umbrella. Because of contraction of the circularly placed epithelial-muscle (in medusae) or muscle (in siphonophores) cells, the velum is pressed against the interior surface of the umbrella and, by expelling water from under it, causes locomotion of the animal. In veligers the velum, located at the front end of the body, resembles a fringed disk, often of complex form and covered with cilia.
(2) A fold of the mucous membrane in the lancelet that divides the preoral hood from the pharyngeal cavity.