Protostomia and Deuterostomia

Protostomia and Deuterostomia

Animals with a true coelom (a body cavity fully lined by mesoderm) split, early in their embryonic development, into two great evolutionary lines depending on what happens to the blastopore — the first opening formed during gastrulation.

• Protostomia ("mouth first") — the blastopore develops into the mouth, and the anus forms later as a second opening. Protostomes typically show spiral, determinate cleavage of the early embryo (the fate of each cell is fixed very early) and schizocoelous coelom formation, where the coelom arises from a splitting of the mesodermal mass. Protostomian phyla include Platyhelminthes, Nematoda, Annelida, Arthropoda and Mollusca (any four of these are commonly cited as protostomian phyla).
• Deuterostomia ("mouth second") — the blastopore develops into the anus, and the mouth forms secondarily at the opposite end. Deuterostomes typically show radial, indeterminate cleavage and enterocoelous coelom formation, where the coelom arises as outpocketings of the archenteron (embryonic gut). Deuterostomian phyla include Echinodermata, Hemichordata and Chordata.

Because echinoderms form their coelom by this enterocoelous, gut-pouching method, they are described as enterocoelomates — their body cavity is literally derived from pouches of the primitive gut wall, which is the hallmark developmental feature uniting them with the other deuterostomes, in contrast to the schizocoelous method used by protostomes such as annelids and arthropods. …