Flatworms do not have a coelom even though thay are triploblastic.
Roundworms are pseudocoelomates
The coelom is a fluid-filled body cavity that forms in the mesoderm.
Coelom. It is partitioned by septa, which is a divider between segments of the worm.
A coelom is defined as a body cavity or area around the gut, so therefore no. Cnidaria such as jellyfish, hydra, coral, and sea anemones do not have a coelom or body cavity