α. 1800s– metastoma.
β. 1800s– metastome Brit. 
/ˈmɛtəstəʊm/, U.S. /ˈmɛdəˌstoʊm/.
metastoman.
/mᵻˈtastəmə/, /ˌmɛtəˈstəʊmə/, U.S. /məˈtæstəmə/, /ˌmɛdəˈstoʊmə/ Zoology.† 2. Haeckel's name for: a mouth formed from a secondary embryonic opening, not from the primary blastopore. Contrasted with protostoma n. at proto- comb. form 2a. Cf. deuterostoma n. at deutero- comb. form . Obsolete. rare.
1. The labium of a crustacean.
ΚΠ
1858 T. H. Huxley in Trans. Zool. Soc. 22 232 The labrum..in the Crustacea..appears to be developed from the sternum of the antennary or third somite, the metastoma (or so-called labium or lingua) of Crustacea.
1876 D. Page Adv. Text-bk. Geol. (ed. 6) xii. 210 A broad heart-shaped metastome or mouthpiece.
1900 Jrnl. Anat. & Physiol. July 471 The metastoma represented the fused last pair of prosomatic appendages, and so formed a ventral lip to a prosomatic or oral chamber.
1961 J. Stubblefield Davies's Introd. Palaeontol. (ed. 3) v. 148 Behind the mouth is a large median plate (metastoma).
1994 Jrnl. Marine Biol. Assoc. 74 787 The membranous lobe of Thenus orientalis (Lund) is a broad fleshy protuberance... Its position, morphology and structure are described and its association with the metastome of other crustaceans discussed.
ΚΠ
1879 tr. E. Haeckel Evol. Man II. 469 My distinction of the primitive intestine and primitive mouth (protogaster and protostoma) from the later, permanent intestine and mouth (metagaster and metastoma) has been variously attacked.
This entry has been updated (OED Third Edition, December 2001; most recently modified version published online March 2022).
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n.1858