mesosome

from The Century Dictionary.

• noun In crustaceans, the pereion or thorax. Compare metasome and urosome.
• noun Same as mesosoma.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A folded invagination in the plasma membrane of bacteria, produced by the chemical fixation techniques used to prepare samples for electron microscopy, but previously thought to be part of the normal structure of the cell.