Which group includes only coelomates from the set?

• A. Annelida, Mollusca, Arthropoda, Echinodermata, Chordata
• B. Porifera
• C. Cnidaria, Platyhelminthes
• D. Nematoda, Rotifera

Answer: (A)

A true coelom is a body cavity completely lined by mesodermal tissue. The phyla in this group all have such a true coelom: Annelida (segmented worms), Mollusca (snails, clams, etc.), Arthropoda (insects, spiders, crustaceans), Echinodermata (starfish, sea urchins), and Chordata (fish, amphibians, reptiles, birds, mammals). Porifera lack true tissues and cavities, so they don’t have a coelom. Cnidaria and Platyhelminthes are acoelomates or lack a true body cavity, and Nematoda and Rotifera have a pseudocoelom—a body cavity not completely lined by mesoderm. Since every group in this set has a true coelom, this option is the correct one.