PROTOSTOMIA

group of animalsincluding the arthropods (e.g., insects, crabs), mollusks (clams, snails), and annelid wormsclassified together on the basis of embryological development. The mouth of the Protostomia (proto, first; stoma, mouth) develops from the first opening into the embryonic gut (blastopore). The coelom (body cavity) forms from a split in the embryonic mesoderm (middle tissue). Larval (immature) forms, if present, are called trochophores. The Protostomia constitute one of two divisions of the coelomates (animals with a body cavity, or coelom). Compare Deuterostomia.