protostomePronunciation: /ˈprəʊtə(ʊ)stəʊm/Definition of protostome noun Zoology a multicellular organism whose mouth develops from a primary embryonic opening, such as an annelid, mollusc, or arthropod. Origin: 1950s: from proto- 'primitive' + Greek stoma 'mouth'protostome in other Oxford dictionaries Definition of protostome in the US English dictionary
