Protostomes and deuterostomes make up the two divisions of the triploblastics groups.
These organisms are characterised during embryonic development by the formation of the orifice (blastopore) in one of the first stages of the embryo (gastrulation): this orifice forms the mouth, while the anus is formed by a secondary orifice.
The mouth of these organisms therefore appears first, hence their name: proto-, first, -stome, mouth.
Protostomes constitute most of the triblastic phyla.
The earthworm, as an annelid in the subclass Oligochaeta, is a protostome, much like many other triblastics. © Michael Linnenbach, Wikimedia CC by-sa 3.0