The second dichotomous branch point of the animal phylogenetic tree distinguishes between the radiata and the bilateria. The radiata include organisms that have a radial morphology, including Phylum Cnidaria and and Phylum Ctenophora (in the past these two groups were placed in the Phylum Coelenterata, but we now know that they are separate lineages). Examples of cnidarians include jellies (often called jellyfish), corals, and sea anemones. Some cnidarians are bioluminescent (radiate light), and some (e.g., the Portuguese Man-of-War) can sting.
All cnidarians have true tissues and are members of the Eumetazoa. They have one of two characteristic body plans, the polyp and the medusa and Hox genes are found in their genome. The polyp form is often sessile, anchored to a substrate. The medusa form is mobile. Recall that the radiata have only two embryonic tissue layers. Specifically, they lack mesoderm (the tissue that gives rise to structures, including muscles, in triploblastic organisms). As a consequence, they lack the more sophisticated movement seen in triploblastic organisms, but they do have muscle-like cells at the base of the epithelial (covering) cells that enable them to move by contraction. They have a nerve net that allows them to react more quickly to their environment. Cnidarians have a gastrovascular cavity that is used for both digestion and excretion; the mouth and anus are a single structure. The mesoglea is a jelly-like substance between the two tissue layers that provides support.
Members of the Cnidaria include the corals, hydras, jellies and sea anemones. Most species are marine although some hydra are found in freshwater environments.
Cnidarians have cnidocytes (specialized cells that function in defense and the capture of prey); cnidocytes contain organelles called cnidae, which are able to evert. Cnidae that sting are organelles called nematocysts. These nematocysts can immobilize fish for capture, and they can also be used for defense.