gastropod vs mother-of-pearl

gastropod

noun
  • Any member of a class of mollusks (Gastropoda) that includes snails and slugs; univalve mollusk. 

mother-of-pearl

noun
  • The hard pearly inner layer of certain mollusk shells; nacre. 

  • A butterfly of the genus Salamis. 

adj
  • Made from or looking like mother-of-pearl; iridescent or pearly. 

How often have the words gastropod and mother-of-pearl occurred in a corpus of books? (source: Google Ngram Viewer )