Protostomes

The group of animals to which the molluscs, arthropods and annelid worms belong is called the Protostomes.

Protostomia means "first mouth", so called because the mouth develops from the first opening into the gut (blastopore). The body cavity or coelom forms from a split in the embryonic middle tissue (mesoderm). The larvae of Protostomia are called trochophores.

Contrast this with the other major group of animals, the Deuterostomes.