The cnidarians (Cnidaria) are a tribe of aquatic animals. The tribe includes the disc jellyfish, the box jellyfish, the hydroid polyps and the flower animals. Cnidarians live exclusively in water, most of them in the sea, but there are also groups that live in fresh water. Previously, the word cave animal (Coelenterata) was used instead of cnidarians to indicate this tribe. However, this is not recommended, because this name is not only used for the cnidarians, but also for the comb jellyfish and in the past also for sponge animals.
Cnidarians are characterized by their stinging cells; specialized cells with poison darts that they use to paralyze prey or deter predators. The body of a cnidarian is simple in construction and consists mainly of mesoglea. The life cycle of cnidarians falls into two stages: the polyp that is attached to the bottom, and the meduse that swims freely. Both the polyp (eg a coral or sea anemone) and the meduse (eg an adult jellyfish) have flexible tentacles around their mouth opening.
Cnidarians have been on Earth for a long time: they split off from the other animals (the Bilateria) very early in evolution. Since they don't make much of a hard skeleton, fossil evidence of their existence is limited to the stony corals and a few accidentally well-preserved prints of jellyfish and gnats. The earliest known complex multicellular life on Earth, the Ediacara fauna, dating to 542-630 million years ago, already contains clear fossils of cnidarians.
The basic plan of the cnidarians is a pouch with a single opening. This opening is a mouth, but also the anus of the animal. Around the opening are tentacles or arms that contain specialized cells that can shoot harpoons. These are the stinging cells to which these animals owe their name. The venom released from these harpoons paralyzes prey, which are then carried to the mouth by the tentacles. In contrast to sponge animals, for example, cnidarians have distinctly different organs: they have an epidermis, a gastrovascular cavity for digestion and a simple (diffuse) nervous system.
The life cycle of the cnidarian basically contains two stages: a polyp stage and a jellyfish stage. In the polyp stage, the animal is usually attached to the substrate with mouth and arms pointing upwards. In the jellyfish stage, the animal floats with the current in the sea with catch arms and mouth downwards. In corals and sea anemones, the polyp stages are the most important and the jellyfish stage is the larval stage. With jellyfish this is the other way around. Cnidarians have both sexual and asexual reproduction.
The Cnidaria are subdivided into several rootstocks and classes:
Class Flower Animals (Anthozoa) - inc. corals
Subgenus: Hexacorallia—Zoantharia, Ceriantharia (felt sleeve anemones)
Rootstock Medusozoa—Jellyfish:Class Cube Jellyfish (Cubozoa)—Australian Sea Wasp
Class Hydroid polyps (Hydrozoa)—hydra-like animals
Class Disc Jellyfish (Scyphozoa)
Class Staurozoa—stalked jellyfish
Class MyxosporeaCorals, jellyfish and sea anemones all belong to the tribe Cnidaria.
The most important class for the coral reefs is the class of Anthozoa. This is divided into two subgenera: the Hexacorallia with the order Zoantharia and the Octocorallia, also called Alcyonaria. The prefixes Hexa- and Octo- refer to the number of polyps the animal has: six or a multiple of six in the Hexacorallia, eight in the Octocorallia.
Various Flower Animals (Anthozoa)