metasome

Century Dictionary and Cyclopedia

1. n. The posterior part of the body of a cephalopod, which is enveloped in the mantle and contains the viscera. The name is also given to the posterior part of the body of bivalve mollusks, behind the mesosome and the foot, containing the posterior adductor muscle.
2. n. In some crustaceans, as the amphipods, the first three segments of the pleon or abdomen. Compare mesosome and urosome.

Wiktionary

1. n. a segments of the body of an animal, generally the rearmost one

GNU Webster's 1913

1. n. (Zoöl.) One of the component segments of the body of an animal.