In most, but not all protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes.
Early development differences between protostomes versus deuterostomes. In protostomes blastula divisions occur as spiral cleavage because division planes are oriented obliquely to the polar major axis. In deuterostomes, the cleavage is radial because planes are parallel or perpendicular to the major polar axis. During gastrulation, protostome embryos' mouth is given first by the blastopore while the anus is formed later, and vice versa for the deuterostomes
In animals at least as complex as earthworms, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. It is now known that the fate of the blastopore in protostomes is extremely variable. While the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name 'protostome' (in Greek: "first-mouth") is disputable.
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage.
Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm. Yet two of the most basal and ancient protostome phyla, the chaetognatha and priapulida, have deuterostome-like development.
The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species.
^Peters, Kenneth E.; Walters, Clifford C.; Moldowan, J. Michael (2005). The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history. 2. Cambridge University Press. p. 717. ISBN978-0-521-83762-0.
^Safra, Jacob E. (2003). The New Encyclopædia Britannica, Volume 1; Volume 3. Encyclopædia Britannica. p. 767. ISBN978-0-85229-961-6.