All of the following are coelomates except which?

Explanation

Coelomates are animals that have a true coelom - a fluid-filled body cavity completely lined with mesoderm. Platyhelminthes (flatworms) are acoelomates, meaning they lack a true coelom. The others - deuterostomes, hemichordates, and protostomes - are all groups that include coelomate animals: Deuterostomes include chordates and echinoderms (true coelomates). Hemichordates are marine animals with a true coelom. Protostomes include arthropods, mollusks, and annelids, all of which are coelomates.