∞ generated and posted on 2016.09.01 ∞

Animals characterized first and foremost by the secondary embryonic formation of their anus versus their mouth.

Protostome literally means first mouth, that is, a mouth that forms from an embryoís blastopore. See by contrast deuterostome.

Protostome animals additionally possess early cleavage characteristics that are both spiral cleavage relative to the embryo and what is known as determinate cleavage, meaning that every cellís fate is determined from the outset of development (contrasting cleavage that instead occurs radially and is indeterminate). These animalís body cavities (coelom) further form in the course of a splitting apart of mesodermal tissue.

Included among the protostomes are phyla Annelida, Arthropoda, Mollusca, Nematoda, Platyhelminthes, and Rotifera.