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Phylum Cnidaria Jellyfish, Corals, and Sea Anemones General Characteristics • • • • • Tentacles Hydrostatic skeleton Nerve net Radial symmetry Saclike digestive system (only one opening for mouth/anus) • Two layers of cells with mesoglea (jelly-like material) in between. • Lack special organs for respiration, excretion, and have no blood • Nematocysts (stinging cells) – used for feeding, defense, and some contain toxins. Two Body Forms of Cnidarians • Medusa – free floating, motile, part of the plankton, tentacles and mouth point down • Polyp – sessile, part of the benthic community, tentacles and mouth point up. Classes of Cnidarians • Hydrozoans – Most are colonial – Polyp body form for most of its life cycle – Some do not have a medusa stage others do not have a polyp stage, but most have both stages in their life cycle – Examples: Portuguese Man-o-War, Hydra • Scyphozoans – Most are solitary – Medusa body form for most of its life cycle – Examples: true jellyfish like the Moon Jelly (Aurelia) • Cubozoa – Box Jellyfish (Sea Wasp) – The deadliest jellyfish in the world are a type of box jellyfish, with the typical cube body shape – Not all species are deadly, but can cause very painful stings • Anthozoans – Only found in the polyp body form – Reproduce both asexually (budding) and sexually (shed eggs and sperm into the water) – Three groups of anthozoans: • Anemones – soft fleshy polyps, usually solitary • Soft Corals – sea fan and sea whips • Hard Corals – which have a calcareous skeleton (usually) and build coral reefs, usually colonial Most anthozoans have a symbiotic relationship with a dinoflagellate known as zooxanthellae living in their tentacles that produce food for the coral in exchange for a place to live and nutrients.