Hydrozoa
Hydrozoa, the most diverse group, includes siphonophores, hydroids, fire corals, and many medusae. Hydrozoans come in two different bodily forms. One is the medusa, a jelly-like, umbrella-shaped, free-swimming form with a mouth and tentacles that face down. The other form is a colony of polyps, tube-shaped sacs that have a mouth and tentacles that face up. The polyp form is fixed to the material on which it lives. Most hydroids have both a medusa and a polyp stage. Medusae have an easily recognizable body shape. The shape of polyps ranges from giant coral-like colonies through feathers and flowers to microscopic balls of tissue. The medusae and polyps of most hydroids are clear. Among colored species, the color often comes from the food the hydroids eat. The most common color is red from crustaceans, which are water-dwelling animals, such as shrimp, that have jointed legs and a hard shell but no backbone.