What is a cniderian?

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.