<P> Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites . Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians . Many scleractinian corals--which form the structural foundation for coral reefs--possess polyps that are filled with symbiotic photo - synthetic zooxanthellae . While reef - forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater . </P> <P> Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians . Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later . However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago, almost 200 million years before the Cambrian period as well as any fossils . </P> <P> Cnidarians form an animal phylum that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals . However, both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet - like basement membranes; muscles; nervous systems; and some have sensory organs . Cnidarians are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey . In some species, cnidocytes can also be used as anchors . </P> <P> Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly - like material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no intermediate jelly - like layer . Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges . However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer . As a result, some recent text books classify ctenophores as triploblastic, and it has been suggested that cnidarians evolved from triploblastic ancestors . </P>

How do you know something is a cnidarian