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.