plastron

noun

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

   “The great tortoise that in myth is said to support the world and is thus an embodiment of the galaxy, without whose swirling order we would be a lonely wanderer in space, is supposed to have revealed in ancient times the Universal Rule, since lost, by which one might always be sure of acting rightly. Its carapace represented the bowl of heaven, its plastron the plains of all the worlds.”

2. A half-jacket worn under the jacket for padding or for safety.
3. A man's shirt-bosom.
4. An ornamental front panel on a woman's bodice.

   “I bought here a wedding dress perhaps twenty or thirty years old [...] a sequin plastron to be worn over the womb as a feminine equivalent to a cod-piece, and a gauze veil embroidered in purple and gold.”

5. A breastplate.

   “"Why, compared to what thou wast, Hugo, thou art as a deerskin coat to a steel plastron.-- Art thou not in love, man? Answer me!" "Something like, I fear me, beau sire,” replied the squire.”

   “He therefore persuaded the Casa de Antillas to supply him with a hundred steel plastrons or corselets, with armlets and shoulder plates, and a hundred morions, or pikemen's helmets. This heavy armor was suitable […]”

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

   “The plastron of a diving beetle is not directly a source of oxygen, but acts as a gill, acquiring oxygen from the surrounding water.”

   “Total independence of atmospheric air is possible only if insects have a permanent gas store or incompressible gas gill, called a plastron. Unlike compressible gas stores, the volume of a plastron remains constant and it is incompressible.”

   “2013, Jon F. Harrison, Lutz T. Wasserthal (revisions & updates), 17: Gaseous Exchange, R. F. Chapman, Stephen J. Simpson (editor), Angela E. Douglas (editor), The Insects: Structure and Function, 5th Edition, page 535, The plastrons of other insects are generally less efficient than that of Aphelocheirus as they have a less dense hair pile from which the air is more readily displaced.”