1. A cavity in the primary nectocalyx of certain Siphonophora.
2. A cavity in pneumatophore of Siphonophora containing air or an oil droplet.

A nectocalyx, the swimming bell or umbrella of a jellyfish of medusa or any free swimming acaleph (so called from the stinging power they possess; as such, they are sometimes called sea nettles).

Siphonophora, an order of pelagic (open water) Hydrozoa (water animals) including species which form complex free-swimming communities composed of numerous zooids (animal forms) of various kinds, some of which act as floats or as swimming organs, others as feeding or nutritive zooids, and others as reproductive zooids.

Pneumatophore, a gas-filled sac that acts as a float in coelenterates (hydras, jellyfish, sea anemones, and corals, characterized by a single internal cavity serving for digestion, excretion, etc.); such as, the Portuguese man-of-war.