Phylum Coelenterates

Marine coelenterates include sea anemones, coral polyps and jellyfish. A sea anemone is a solitary, attached polyp similar to the hydra but much larger and coloured (up to 0.5 m across); some live in symbiosis with hermit crabs – both benefit. Coral polyps form colonies; because their internal cavities are connected, caught food is shared, and most have a limestone skeleton. Polyps multiply by budding and the buds do not detach, so the colony grows. In shallow tropical seas colonies build coral reefs and islands; reefs are home to thousands of fishes and other animals and shield coasts from waves, so they are protected. A jellyfish is a free-swimming predator with an umbrella-shaped, transparent body; it contracts the umbrella to push water out (jet propulsion). The Arctic jellyfish and the rhizostome jellyfish can be dangerous to swimmers, while aurelia is considered harmless to people. Coelenterates are among the oldest animals and are thought to descend from colonial flagellates.