Organisms that are in Class Hydrozoa come from the phylum Cnidaria. Most species of Hydrozoa are marine and colonial and their life cycle includes both the asexual polyp and the sexual medusa stages. Medusas have a well-developed muscular velum that helps them move through water. Their exoskeleton is made of chitin or sometimes of calcium carbonate.

The freshwater hydrozoans are called Hydras and are solitary and do not have a medusa stage nor do they have any form of exo-skeleton. Hydras live in still and slow moving waters attached to leaves and stems. They have pedal discs composed of gland cells that help them to attach to substrates. Hydras use nematocysts in their tentacles to stun their prey with venom. The tentacles then lead the prey to the opening mouth. Hydras eat small crustaceans, insect larvae, and annelid worms. During asexual reproduction, buds leave the body wall and develop into young hydras. In sexual reproduction, eggs mature one at a time and are fertilized by sperm in the water.

Colonial hydrozoans have both a medusa stage and a polyp stage in their life cycle. They have a base, a stalk, and one or more polyps. Most polyps are feeding polyps called hydranths. They prey on tiny crustaceans, worms, and larvae. In reproduction, new polyps can either be feeding polyps or reproductive polyps known as gonangia. When a gonangia buds, a medusae is produced. These medusae will then mature and produce gametes. Zygotes become free-swimming planula larvae that settle on a suitable substrate, allowing them to develop into a hydroid colony by asexual reproduction.

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