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.