Final answer:
The primary difference between protostomes and deuterostomes lies in the sequence of mouth and anus development from the embryonic blastopore.
Step-by-step explanation:
The main difference between protostomes and deuterostomes is the timing of the development of the mouth and anus during embryonic development. Protostomes, which include groups such as arthropods, mollusks, and annelids, are characterized by the mouth forming first from the embryonic opening called the blastopore. In contrast, deuterostomes, which include chordates and echinoderms, have the anus forming first from the blastopore, with the mouth developing later at a different site.
During the process of gastrulation, the protostomes exhibit spiral cleavage and the coelom forms through a process called schizocoely, where cavities within the mesodermal mass merge to form the coelom. Deuterostomes, on the other hand, display radial cleavage with a more organized and indeterminate cleavage pattern, and their coelom forms via enterocoely, where the coelom develops as pouches pinched off from the endoderm.
The main difference is the developmental sequence of mouth and anus from the blastopore: mouth first in protostomes, anus first in deuterostomes.