Hydrozoa

Organisms that are in Class (biology) Hydrozoa come from the Phylum Cnidaria. Many of these organisms are usually found to be marine and colonial. Their life cycle includes both the asexual polyp and the sexual medusa stages. The freshwater hydras do not have a medusa stage. 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 solitary polyps and they live under aqautic leaves and lily pads. They have pedal discs composed of gland cells that help them to attach to substrates and also allows them secrete gas bubble for floatation. Freshwater hydrozoans use their stinging tentacles and stun their prey with poison. The tentacles then lead the prey to the opening mouth. Hydras like to eat on small crustaceans, insect larvae, and annelid worms. When feeding, they use nematocysts that help surrender its prey. During asexual reproduction, buds leave the body wall and develop into young hydras. In sexual reproduction, eggs are mature one at a time and are fertilized by sperm into the water.

Colonial hydrozoans are those that have 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 eat prey like tiny crustaceans, worms, and larvae. In reproduction, new polyps can either be feeding polyps or reproductive polyps known as gonangia. When gonangia buds, a medusae is produced. These medusae will then mature and produce gametes. Zygote results from a free-swimming planula larva that finds rest in a substrate to develop into a hydroid colony by asexual reproduction.

Some examples of hydrozoans are Hydra, Obelia, Physalia, Tubularia.