Protostomia

Part of speech: noun

Definitions

1. A major grouping of animals in which the mouth develops from the first opening formed during embryonic growth, including arthropods and mollusks
2. The taxonomic clade of bilaterian animals whose blastopore becomes the mouth during early embryonic development, as opposed to the deuterostomes
3. Animals classified together because their embryonic mouth arises from the initial embryonic opening, a feature shared by many invertebrate groups

Etymology: Protostomia is a 19th-century scientific coinage, not an ancient word. Ernst Haeckel, the German zoologist and champion of Darwin's ideas, introduced the term in the 1870s for a major grouping of animals. It is built from Greek "prōtos" meaning "first" and "stoma" meaning "mouth", and the name refers to a developmental quirk: in these animals, the embryo's first opening (the blastopore) tends to become the mouth, rather than the anus. The label is the counterpart of "Deuterostomia", where the first opening instead becomes the anus and the mouth forms later. Protostomes include arthropods such as insects and crabs, molluscs such as snails and squid, and annelids such as earthworms. Humans, by contrast, are deuterostomes, which places us on the other side of this ancient split in the animal family tree.

Antonyms: deuterostomia