Hydrozoa
Scientific classification
Kingdom: Animalia Subkingdom: Metazoa Phylum: Cnidaria Class: Hydrozoa
Orders
Actinulida Capitata Chondrophora Filifera Hydroida Siphonophora Trachylina

Organisms that are in Class 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, which they live under aquatic leaves and lily pads. They have pedal discs composed of gland cells that help them to attach to substrates and also allows them to secrete gas bubbles 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 subdue its prey by inflicting stings. 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 a gonangia buds, a medusae is produced. These medusae will then mature and produce gametes. Zygotes result from a free-swimming planula larva that find rest in a substrate, allowing them develop into a hydroid colony by asexual reproduction.

Some examples of hydrozoans are: Hydra, Obelia, Portuguese man o' war, Chondrophores, Physalia, and Tubularia.

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