Hydroids: Hydrozoa

Hydroids (HIGH-droyds) have two body forms. One is the medusa (mi-DOO-suh), a jelly-like, umbrella-shaped, freely swimming form with a mouth and tentacles that face down. The other is a colony of polyps (PAH-luhps), or 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 (mi-DOO-see, the plural of medusa) 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 umbrella of medusae can be one–sixty-fourth of an inch to more than 6 feet (500 micrometers to 2 meters) in diameter. The medusae and polyps of most hydroids are clear and filmy. Among colored species, the color often comes from the food the hydroids eat. The most common color is red from crustaceans (krus-TAY-shuns), which are water-dwelling animals, such as shrimp, that have jointed legs and a hard shell but no backbone. Other colors are green, white, orange, yellow, blue, and purple.

Hydroid colonies are made up of different types of polyps that work together to serve the entire colony. One type catches prey and defends the colony's territory. Another type takes the prey from the catcher-defenders and swallows and digests it. A third type of polyp is in charge of reproduction.