operculum vs tegument

operculum

noun
  • A structure which serves as a cover or lid. 

  • A covering flap in animals, such as a gill cover. 

  • A gum flap covering (part of) a partially erupted tooth, usually a wisdom tooth. 

  • The lidlike portion of a moss sporangium or of a fruit that detaches to allow the dispersal of spores or seeds. 

  • One of several flaps of cerebral cortex covering the insula. 

tegument

noun
  • Something which covers; a covering or coating. 

How often have the words operculum and tegument occurred in a corpus of books? (source: Google Ngram Viewer )