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 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.