What does the name of the phylum Cnidaria mean?

The name of the phylum "Cnidaria" means "the nettle-like", from Greek "knidi", nettle. All cnidarians contain venomous cnidocyst cells, which they use on preying their food and defending themselves.

The cnidarians is a very old phylum, encountered already in the Ediacaran period 600 million years ago. It is divided into three subphyla: Anthozoa (classes Octocorallia, Hexacorallia and Ceriantharia (sea anemones)), Medusozoa (classes Cubozoa (sea wasps), Schyphozoa (true jellyfish), Hydrozoa (polyps), Polypodiozoa (parasite polyps) and Staurozoa (stalked jellies)) and Myxozoa, which all are parasites.

All cnidarians are radially symmetric jelly-like invertebrates. Some corals and sea pens grow themselves a hard bone-like calcite exoskeleton. All cnidarians are predators, and they eat mainly plankton and small invertebrates, but some large species can prey on small crustaceans and small fish. They themselves are eaten by other cnidarians, crustaceans, mollusks, fish and seabirds. Cnidarians are also distinguished by the fact that they have only one opening in their body for ingestion and excretion i.e. they don't have a separate mouth and anus. All live in water.

There are some 11,000 known cnidarian species. The most evolved subphylum are the cubozoans, which have rudimentary eyes, muscles and a neural network.