In which group of animals, the coelom is filled with blood?

• The coelom is the characteristic feature of the metazoan. Coelom fluid-filled body cavity body between the body wall and the alimentary canal.
• In Arthropods, the coelom is filled with the haemocoel.
• In Arthropoda, the coelom is filled up with blood. The coelom is the space between the body wall and the gut wall lined by mesoderm on the two sides.
•  A genuine coelom known as schizocoelom is available in Arthropoda filled up with blood. The cavity or coelom forms from the breaking of the mesoderm.

Hence, option A is the correct answer.