Most species belonging to the class of aquatic creatures known as hydrozoans may exist in either a medusa or polyp state. Some, however, such as the hydra, only exist as polyps.

Many hydrozoan species exist in colonies that are formed by the asexual budding of members. Generally, the buds in the colonies remain attached to the parent polyp, but some specialized reproductive polyps may produce free-swimming medusae. Hydrozoan medusae are bell-shaped and propel themselves forward by alternately constricting and relaxing muscles, which causes water to shoot out of a constricted opening in their bodies.

Pennaria is a genus of hydrozoans whose members feature tentacles that occur in two different forms: filiform and capitate. The filiform tentacles are long and tapering, similar to those of the hydra, with sets of nematocysts distributed over their entire surface. The full nematocyst array of capitate tentacles, however, is confined to a single knob located on the end of the structures. As medusa buds, Pennaria species lack tentacles entirely.