Complex sentence: The most common corals in the subclass "Hexacorallia" have a hexameric body plan; their polyps have sixfold internal symmetry and the number of their tentacles is a multiple of bclass "Octocorallia" have polyps with eight tentacles and octameric radial symmetry.
Split into simpler sentences: The most common corals in the subclass "Hexacorallia" have a hexameric body plan; their polyps have sixfold internal symmetry and the number of their tentacles is a multiple of six. Corals belonging to the subclass "Octocorallia" have polyps with eight tentacles and octameric radial symmetry.