Cnidarians are incredibly diverse in form. Yet, these diverse animals are all armed with stinging cells called nematocysts. Thousands of cnidarian species live in the world's oceans A smaller number of species are found in rivers and fresh water lakes.

Anthozoa, which includes true corals, anemones, and sea pens. Cubozoa, the amazing box jellies with complex eyes and potent toxins. Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae. Scyphozoa, the true jellyfish.

Anthozoans are probably the most famous cnidarians They include the corals that build great reefs in tropical waters, as well as sea anemones, sea fans, and sea pens. They have a long and diverse fossil record, extending back at least 550 million years. The oldest anthozoans are probably some of the polyp-like and sea pen-like fossils from the late Precambrian.

They look like your basic jellyfish They can swim pretty fast, maneuver around things, and see fairly well despite not having a brain. They also have four evenly spaced out tentacles or bunches of tentacles There are 20 known species found in tropical and semitropical waters. Are known to eat fish, worms, and crustacean arthropods. Box jellies are very rarely found on beaches because they are unusually strong swimmers.

Hydrozoans Most species of hydrozoans show alternation of generations. One generation consists of polyps and is almost always colonial The colony is known as a hydroid because of the resemblance of its individual polyps to the noncolonial hydra, the simplest of all hydrozoans. The other generation, members of which are known as medusae, consists of free-swimmers that produce either eggs or sperm.

Medusafire coral

Scyphozoans include most of the jellyfish familiar to beach-goers. True jellyfish are graceful, and sometimes deadly creatures. Jellyfish range in size from a mere twelve millimeters to more than two meters across. Despite their often enormous size, jellyfish have no head, no skeleton, and no special organs for respiration or excretion. Their life cycle involves an alternation between sesslie polyp phase and a free-swimming medusa stage, though the medusa stage, usually predominates.