The Cnidarians compromise a very diverse group of organisms. All Cnidarians (anemone like organisms) share a similar body plan. They can be simply described as a sac with in a sac. These critters posses no distinct head, digestive or structural organs, and all possess Cnidae (specially modified stinging cells). Cnidarians can exist as either a polyp (an anemone like organism), or a medusa (a jellyfish like organism ). It is the degree to which these three groups of Cnidarians exists as either a polyp or medusa that help taxonomists identify them. 

The Hydrozoans are the first group we will discuss. There about 2700 species of hyrozoans, and most of the are marine. A Hydra, fire coral or the imfamous Man-O-War are perhaps some of the more familiar examples of the hydrozoa. All hydrozoans alternate between a polyp, and a medusa phase of living. They may occur as microscopic attached forms, or be fifty feet long and live in the open ocean. Despite their prevalence, they are very rarely seen in tropical fish stores. 

Scyphozoans are the next group of Cnidarians, and they are the familiar jellyfish. The 200 plus species of scyphozoans are all marine free swimming medusa. Jellyfish require good water quality, and specific tank, called a Kreisel, to swim in. Exhibits of jellyfish are a big draw at public aquariums. They can range from 1 cm. clear blobs to barrel sized multicolored organisms that look positively bizarre. Only the curious bottom dwelling Cassiopeia is found in fish stores. This tropical jelly fish lives up-sidedown on the bottom of shallow tropical lagoons, only moving when disturbed. They survive by harvesting the algae that they culture within their tissues. A well lit aquarium with a tranquil patch of sand is their prefered environment. 

Cubozoans contain 15 different species, such as the Box Jellyfish. Some are as large as 12 inches, but most are 1 to 2 inches. These jellyfish are extremely dangerously. Their sting can be lethal. 

Anthozoans are the familiar anemones and corals. There about 6000 species of anthozoans, and they all exist exclusively as a polyp. There are two general groups of anthozoa that will interest the hobbyist. The first is the anemones and true corals. These animals usually can be readily identified as a Cniderian due to their large conspicuous polyp like structure. The other group are the gorgonian like animals. These animals are a bit more difficult to characterize. Soft corals, sea pens and gorgonians all are included in this group. Usually the polyp is quite small, and will bear eight tentacles. An fairly crude yet accurate way to tell the difference is that many octocorals do not resemble anemones at first glance. The specimen may appear tree like, feather like, and or encrusting and the tentacles will bear smaller tentcles (pinnate). 

Midwater Systems has developed a custom acrylic jellyfish tank, called the Jelliquarium, and is working on holding systems used to keep these midwater collections and gelatinous organisms in suspension.

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