shellfish vs snail

shellfish

noun
  • A fisheries and colloquial term for an aquatic invertebrate having an inner or outer shell, such as a mollusc or crustacean, especially when edible. 

  • A culinary and nutritional term for several groups of non-piscine, non-tetrapod, aquatic animals that are used as a food source. The term often exclusively refers to edible aquatic crustaceans, bivalve mollusks and cephalopod mollusks; but sometimes echinoderms may be included as well. 

snail

noun
  • Any of very many animals (either hermaphroditic or nonhermaphroditic), of the class Gastropoda, having a coiled shell. 

  • A spiral cam, or a flat piece of metal of spirally curved outline, used for giving motion to, or changing the position of, another part, as the hammer tail of a striking clock. 

  • A tortoise or testudo; a movable roof or shed to protect besiegers. 

  • A slow person; a sluggard. 

  • A locomotive with a prime mover but no traction motors, used to provide extra electrical power to another locomotive. 

  • The pod of the snail clover. 

verb
  • To move or travel very slowly. 

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