Cnidaria are some of the simplest and most beautiful "animals." These creatures are flower-like and resemble plants in many respects. However, they have a mouth and a simple digestive system at the center of their tentacles. Because of these two features, these creatures are considered animals and not plants. There are 9000 species of Cnidaria.
The Cnidarians include the hydroids, jellyfish, anemones, and corals. All Cnidarians use tentacles which have stinging cells which are used to subdue and capture food. The word "Cnidarian" means "stinging creature" in Latin (an old language that is still used in identifying animals). A stinger can be set off when a tentacle touches something. The stinger contains a toxin which disables prey. Most Cnidarians stingers are not powerful enough to do any harm to people. However, some jellyfish can deliver very painful, and sometimes fatal, stings to people.
Cnidarians are simple, radially symmetrical, animals. Radial symmetry means that the body is a hub, like a bicycle wheel, and tentacles are spokes coming out of it.
Almost all cnidarians are carnivorous. Many eat small creatures or trap detritus, plankton and other microscopic titbits. But many also capture and eat large prey. Many cnidarians, however, more supplement their meals with food provided by photosynthetic zooxanthellae (symbiotic single-celled algae) that they harbour in their tentacles and bodies. These algae produce food from sunlight. In exchange, the cnidrian provides protection and simple minerals.