Polyp - A cylindrical form which attaches to something by the aboral end of the body and extends tentacles around the oral end.
Medusa - A flattened, oral opening down, bell-shaped form; moves freely in water, tentacles dangle from the oral surface pointing downward.
Cidocytes on tentacles
Gastrovascular Cavity -Central Digestive system with only one opening (mouth and anus)
3 Interesting Facts
1. The oldest cnidaria is arguably corals in a reef, which can live as a single sheet by more than 4,000 years. Some polyp types only live between 4–8 days. 2. Cnidarians such as jellyfish are likely to be tolerant of climate change, but corals (such Acropora spp) are listed as threatened by the ICNU wherever they are found, by ocean acidification and environmental damage. 3. Different cnidarian species may also be collected for trade for aquariums and jewelry.
How Many Species?
9,000
Habitat
Cnidarians live in aquatic environments and inhabit all depths, from the sandy substrate up to the surface. They can be found from the Great Barrier Reef rising off the coast of northeastern Australia to the continental shelves of bone-chilling arctic oceans, and just about every saltwater marine habitat in between.
Some jellyfish are even found in freshwater lakes, such as the freshwater jellyfish (Craspedacusta sowerbyi), which haunts several lakes in New Zealand.