Cnidarians (Cnidaria) are a group of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. The basic form of the cnidarian body is quite simple and consists of a gastrovascular cavity with a single opening through which food is ingested and waste is released. Cnidarians are radially symmetrical and they have tentacles that encircle their mouth.
The body wall of cnidarians consists of an outer layer or epidermis, an inner layer or gastrodermis, and a middle layer or mesoglea. Cnidarians lack organs and posses a primitive nervous system known as a neural net.
Cnidarians exhibit two basic structural forms, a medusa and a polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity.
The polyp is a sessile form which attaches to the sea floor and often forms large colonies. The polyp structure consists of a basal disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth opening located on the top of the polyp, and numerous tentacles which radiate out from around the edge of the mouth opening.
Most cnidarians are carnivorous and feed on small crustacans. Prey becomes entagled in the tentacles and stinging structures embedded within the tentacles fire barbs and threads into the prey which secrete venom and paralyze the victim. The tentacles then bring the prey into through the mouth into the gastrovascular cavity where it is digested.
Cnidarians are divided into the following basic groups:
- Sea wasps or box jellyfish (Cubozoa)
- Cup animals and jellyfishes (Scyphozoa)
- Anemones and corals (Anthozoa)
- Hydrozoans (Hydrozoa)