- Name Meaning: Stinging nettle
- English Common Name: Coelenterates, cnidarians, sea anemones, jellies, hydra
- Major distinguishing characteristics: Cnidocytes (stinging cells)
- Approximate number of species described: about 11,000
All species are aquatic, most are marine. Most are predators, even if not energetic ones. They rely primarily on currents and chance to bring food to them, though some jellies are surprisingly active on their own.
Most species have an early planktonic phase, then drop to a substrate of some kind. They may attach to rocks, plants, or whatever else is on the bottom. The body-form of this phase is called a polyp. Sea anemones are cnidarian polyps.
In the jellies (familiarly if incorrectly called "jellyfish"), as it grows the crown of tentacles of the polyp releases to swim freely. This body form is a medusa. It is essentially a very short, swimming polyp, with the tentacles generally oriented downward.
The Phylum Cnidaria includes one subphylum and seven classes. The Anthozoa do not have a designated subphylum.
- Class Anthozoa (sea anemones, corals)
- Subphylum Medusozoa (jellies and hydrozoans)
- Class Cubozoa (box jellies, sea wasps)
- Class Hydrozoa (hydroids, hydra-like animals)
- Class Polypodiozoa (parasites)
- Class Scyphozoa (true jellies)
- Class Staurozoa (stalked jellies)
- Class Myxozoa (parasites)
The bulk of Cnidarian bodies consist of mesoglea, a jelly-like substance, between two layers of living tissue that is usually only one cell thick. The mesoglea is non-living.
There are two basic body forms: a medusa (which are free-swimming, like sea jellies) and polyps (which attached to a substrate of some kind).
The unattached end of the polyp, and the "lower" end of a medusa, is a ring of tentacles with specialized cells called cnidocytes. The mouth is central to the ring. The cnidocytes capture prey either by stinging with harpoon-like "darts" or by being sticky. The darts remain attached by a tether to the tentacle, allowing the cnidarian to draw in the prey. The stinging cnidocytes are called nematocysts.
Cnidarians maintain their body structure by holding in water, forming a hydrostatic skeleton.
The Fossil Record
The earliest generally recognized fossil animal is a Cnidarian. It dates from around 580 million years ago.
Being soft-bodied, cnidarians other than corals fossilize very poorly.