The class Hydrozoa is one of quite a few of classes of marine organisms. One of the best-known species of the class hydrozoa is the hydra that never goes through a medusa stage and stays as a polyp. This, however, is not like most other organisms in the class hydrozoa. Most of them go through both polyp and medusa stages. The class hydrozoa has about 2700 species in it. The organisms in this class were created on the 5th day of creation with other sea creatures according to the book of Genesis in the Bible.
This class is probably most known for its changing from the polyp stage to the medusa stage and then starting over again as a polyp. Some related organisms of the hydrozoans are zooanthids, corals, anemones, and jellyfish. 
The anatomy of the medusa stage differs greatly from that of the polyp stage. The polyp stage has a base that is usually attached to a rock or some hard substrate, and grows from there. Usually half of the organism is the feeding half and the other is a reproductive polyp that produces medusa. They also reproduce asexually by budding. The plantlike stem is called hydrocaulus, which branches off into what is called hydrotheca with many tentacles forming out of it. In the center of the tentacles is a mouth that leads to the gastrovascular cavity leading down to the hydrocaulus. The stem of the reproductive polyp is called the hydrorhiza. These branch off and form buds on their ends. The buds grow and become developing medusa in what is called the gonotheca. 
The medusa stage instead is a free-floating organism. The outer layer of the dorsal portion of the organism is called the exumbrella. The layer under that is called the outer mesoglea. A thin layer beneath the outer mesoglea is the endodermal lamella, which covers the gastrovascular cavity. This leads to the manubrium, which goes to the mouth. Also leading from the gastrovascular cavity is the radial canal, which leads to the ring shaped ring canal that circles the bottom of the organism. Below the gastrovascular cavity is the inner mesoglea, which is above the subumbrella. Below the subumbrella is the velum, which leads down to the long, hanging tentacles under the umbrella shape of the organism. 
For the hydrozoans that have both a polyp and a medusa stage, reproduction occurs as follows. A female hydrozoan in the medusa stage collects the sperm from a male medusa which fertilizes eggs in the female. Then the zygote develops into a planula larva that settles on the sea floor and becomes the first polyp. This polyp buds (asexual reproduction) to form a colony of polyps. Some of the poylps in the colony feed on small crustaceans, insect larvae, and annelid worms. The others make medusae asexually by budding. This colony continues to grow by budding. Then each of the medusa released by the polyps will release sperm or produce eggs, sexually reproducing. The female medusa's eggs become fertilized, starting the process over again.
Some hydrozoans do not have a medusa stage. These polyps reproduce asexually by budding where a polyp starts to form in the body wall of the adult polyp, and breaks off when it becomes mature. In harsh conditions like cold winters, sexual reproduction occurs, producing eggs that are unfertilized until sperm fertilizes them. As the adult dies, the eggs await better conditions to hatch and become an adult.
One species of hydrozoa, Turritopsis nutricula, can change from the polyp to the medusa stage and back. This is achieved by being able to switch the state of the cell into another type of cell which is called transdifferentiation. 
Most Hydrozoans usually don't react very much with other organisms and the environment. This is probably due to the fact that many hydrozoans form colonies that look similar to seaweed. These colonies usually attach to a rock and form there, feeding on small organisms, and reproducing by budding. The genus Hydractinia, forms thick crusts on the shells of hermit crabs, and helps defend the crab from its predators. They are usually carnivores or detritivores. Most polyp hydrozoans feed on tiny crustaceans, worms, and larvae. Many of the species of Hydrozoa have both a polyp and medusa stage in their life cycle. The medusa stage usually has a shorter life span and usually dies after sexual reproduction occurs. The medusa stage feeds on the same type of things as the polyp stage does (tiny crustaceans, worms, and larvae), but the medusa stage uses the tentacles to trap its prey and bring them up into the mouth and gastrovascular cavity. Some of the organisms that might eat hydrozoans are many kinds of fish, crustaceans, and mollusks like sea slugs. Some of these sea slugs eat the hydrozoans and steal defenses of the hydrozoans. 
- Hydrozoa From Wikipedia.
- Animal Diversity Web The Animal Diversity Web, University of Michigan Museum of Zoology
- UCMP University of California Museum of Paleontology
- Hydrozoa by Wikispecies. Accessed November 3, 2010.