medusa (biology)

In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the gastrovascular cavity and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles.
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