The phylum’s name is derived from the Greek root word cnid- meaning nettle, a stinging plant. Cnidarians are found in many aquatic environments. Sea anemones are widely distributed, from cold arctic waters to the equator, from shallow tide pools to the bottom of the deep ocean. Jellyfish float near the surface of the open oceans and in some tropical freshwater lakes. Corals are found primarily in shallow tropical waters, but a few grow in deep cold ocean waters. Small anemone-like cnidarians like Hydra sp. are also found in freshwater lakes and streams.
n the phylum Porifera we saw a body formed of aggregated cells with no organization into tissue layers or organs. Cnidarians have a slightly more organized body plan, and have tissues, but no organs. Most cnidarians have two tissue layers. The outer layer, the ectoderm, has cells that aid in capturing food and cells that secrete mucus. The inner layer, the endoderm, has cells that produce digestive enzymes and break up food particles. The jellylike material between the two layers is called the mesoglea. All of these body layers surround a central cavity called the gastrovascular cavity, which extends into the hollow tentacles.
The body plans cnidarians generally have radial symmetry.
Many cnidarians take two main structural forms during their life cycles, a polyp form and a medusa form.
