Any of various invertebrate animals of the phylum Cnidaria, characterized by a radially symmetrical body with a saclike internal cavity, and including the jellyfishes, hydras, sea anemones, and corals, are called cniderians.
There are about 9000 species of cniderians and they are mostly marine aquatic invertebrates. They possess specialized stinging cells known as cnidocytes which are borne on their tentacles. Cniderians range in size from microscopic to more than 100 feet (30 m). Cniderians occur in polyp and medusa forms.
Cnidarians include jellyfish, anemones, hydras, and corals. They have a stomach and a tentacled mouth and are radially symmetrical.
Insects, spiders, crustaceans, and anything with an exoskeleton.
The phylum Cnidaria encompasses over nine thousand aquatic species. All of these species share common characteristics such as being invertebrates and having no major systems. For example, instead of having a complex nervous system, cnidarians have something called a nerve net which is a simple nervous system. They lack a brain and cephalisation which means the concentration of sensory organs in the head region. However, a nerve net allows the organism to have a response to its environment. In the middle of the nerve net is a nerve ring which allows the cnidarian to swim. An example of an organism with a nerve net is a Hydra.