gastropod vs univalve

gastropod

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

univalve

noun
  • A univalve mollusk or its shell. 

adj
  • having one valve; typically used to refer to mollusks, notably slugs and snails. 

  • consisting of a single valve or piece, used to refer to a shell. 

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