The Cnidarians compromise a very diverse group of organisms. They can be simply described as a sac with in a sac. 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.
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.