Which phyla are pseudocoelomates?

• A. Nematoda and Rotifera
• B. Nematoda and Annelida
• C. Rotifera and Mollusca
• D. Platyhelminthes and Nematoda

Answer: (A)

Pseudocoelomates have a body cavity that originates from the blastocoel and is not completely lined by mesoderm, leaving a hollow space that functions as a hydrostatic skeleton but isn’t a true coelom. The classic examples of this arrangement are nematodes and rotifers. In these groups, the gut is surrounded by a fluid-filled cavity that is not fully enclosed by mesodermal tissue, so the cavity isn’t a true coelom. This contrasts with true coelomates like annelids and mollusks, where the body cavity (the coelom) is completely lined by mesoderm, and with acoelomates like flatworms, which lack any true body cavity altogether. So the phyla that are pseudocoelomates are nematodes and rotifers.