Definition of protostome in US English:

protostome

noun

Zoology
  • A multicellular organism whose mouth develops from a primary embryonic opening, such as an annelid, mollusk, or arthropod.

    • ‘Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies.’
    • ‘If the ancestor of protostomes displayed a segmentation comparable to annelids and arthropods, this necessarily means that segmentation was lost independently in different protostome lineages.’
    • ‘Once this had appeared, the protostomes could develop.’
    • ‘In the complementary category, the protostomes, which includes annelids and arthropods, the blastopore develops into the mouth.’
    • ‘These animals are all protostomes - the mouth develops before the anus in the young embryo - and they have long been recognized as belonging together as a group.’

Origin

1950s: from proto- ‘primitive’ + Greek stoma ‘mouth’.

Pronunciation

protostome

/ˈprōdəˌstōm/