Cnidaria are aquatic invertebrates such as sea anemones, medusae, corals, box jellyfish and true jellyfish. Most of them live in the ocean, but a few, like the hydra, live in freshwater. They are symmetrical, which means if you cut them in half each half will be a mirror image of the other. They have neither head nor brain, but a mouth, which is the single body opening. Usually the mouth is surrounded by tentacles that contain stinging cells called nematocysts.
There are four recognized classes of cnidarian so far: hydrozoa, cubozoa, scyphozoa and anthozoa, but scientists aren't completely sure of the evolutionary relationships between them. Cnidarians are made mostly of water and so fossilize poorly. Hydrozoa are tiny predatory animals which live in the ocean and can be found singularly or in colonies. Scyphozoa are jellyfish and anthozoa are sea anemones and corals. Cubozoa are box jellyfish, which differ from true jellyfish in that they have a primitive nervous system and eyes. Box jellyfish are also among the most venomous animals on land or sea. Their sting can be fatal to humans, especially children.
Cnidarians' bodies and tentacles have two cell layers, the endoderm, and the ectoderm, which is analogous to the epidermis. The mesoglea exists between the two cell layers. Mesoglea might be little more than a glue or make up most of the animal, as in the case of the jellyfish. Cnidarians all have hydrostatic "skeletons" even if they're made out of mostly water. The muscles of the body wall help a medusa swim, and the tentacles of anemones and coral are also extended through hydrostatic action.
Many cnidarians have an asexual stage, usually in the form of a polyp which produces a medusa asexually. The medusa, which is often free swimming, carries gametes. The medusa releases eggs and sperm into the water, the zygote develops into a larva that settles on a substrate and becomes a polyp that asexually produces more polyps, which eventually becomes a colony. At one point one of the polyps produces a medusa and the cycle begins again. But there are some types of cnidarians, like sea anemones and corals, that lack the medusa stage. They simply release eggs and sperm into the water. The reproductive cycle of true jellyfish can also vary.
Cnidarians are carnivorous and their stinging tentacles help them capture prey. Because most of them either can't move or, like jellyfish, have limited power to move through the water, their prey, like small fish or crustaceans, comes to cnidarians through misadventure. Some cnidarians seem to be able to absorb dissolved organic material directly from the water but it's not known how common this ability is.