Skeletal System
Hydrozoans have two body forms. One is the medusa , a jelly-like, umbrella-shaped, freely swimming form with a mouth and tentacles that face down. The other is a colony of polyps or tube-shaped sacs that have a mouth and tentacles that face up. The other form is the polyp form and is fixed to the material on which it lives. Most hydroids have a medusa as well as a polyp stage. Medusae have a typical and 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. However some species are colored and 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.