They are usually less than 1 cm tall and prey on smaller invertebrates among the filaments and leaves of freshwater algae and plants. Hydra have no medusa stage. Class Scyphozoa Scuphozoans are commonly called jellyfish because the gelatinous medusa dominates their life cycle. This group also includes one of the largest invertebrates in the world, Cyanea capillota . Cyanea lives in the North Sea and can exceed 2 m in diameter.
Anthozoans have no medusa stage and are often colonial.
Corals Coral polyps are structurally similar to anemones, but corals are usually colonial and much smaller. Most corals secrete a hard skeleton of calcium carbonate with many small cups surrounding the polyps. The small, fragile polyps are probably absent. The tropical organ pipe coral, Tubipora , is organized differently. Long parallel polyps are encased in calcareous tubes connected at intervals by transverse plates.