gill cover February 15, 2023 The flat bony plates that cover the branchial cavity and gills, consisting of the preopercle, interopercle, subopercle, and opercle; see operculum. Related Articles: Glossary: branchiostegal membranesGlossary: head lengthGlossary: interopercleGlossary: preopercleGlossary: subopercle