In Primnoidae, eight triangular scales covering the withdrawn tentacles.
In thecate hydroids, one or more flaps closing the end of a hydrotheca or gonotheca. In many hydroid species the end-cap of a developing hydrotheca, and of most gonothecae, is cast away when the hydranth (or embryos or medusae) emerges and this deciduous structure is also often termed an operculum: but a 'true' operculum remains part of the hydrotheca during its functional ('adult') life.