integument vs plastron

integument

noun
  • An outer protective covering such as the feathers or skin of an animal, a rind or shell. 

  • The outer layer of an ovule, which develops into the seed coat. 

  • A shell or other outer protective layer. 

plastron

noun
  • The nearly flat part of the shell structure of a tortoise or other animal, similar in composition to the carapace. 

  • A breastplate. 

  • A half-jacket worn under the jacket for padding or for safety. 

  • A film of air trapped by specialized hairs against the body of an aquatic insect, and which acts as an external gill. 

  • An ornamental front panel on a woman's bodice. 

  • A man's shirt-bosom. 

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