Hydrozoa

Hydrozoa is a class of cnidarians whose members include hydras, siphonophores, and many jellyfish. Their life cycles may include polyp stages, medusa stages, or both.

Cnidaria: A phylum of aquatic animals characterized by stinging cells called cnidocytes and radial body organization. Hydrozoa is one of the major classes within this phylum.
Hydroid: A polyp or polyp colony belonging to a hydrozoan, often attached to a submerged surface. Hydroids are the polyp form found in many hydrozoan life cycles.
Hydra (genus): A genus of small freshwater hydrozoans with solitary polyps and no medusa stage. Hydra shows that a hydrozoan life cycle can remain polyp-only.
Scyphozoa: A cnidarian class containing the true jellyfish, typically with a prominent medusa stage. Scyphozoans are often contrasted with hydrozoans because their medusae usually dominate the life cycle.
Cnidocyte: A specialized stinging cell that captures prey or deters threats using a rapidly discharged capsule. These cells give hydrozoans their characteristic prey-capture and defense system.
Budding: A form of asexual reproduction in which a new organism develops as an outgrowth of its parent. Hydrozoan polyps commonly produce new polyps or medusae by budding.
Siphonophorae: An order of colonial hydrozoans whose integrated colonies consist of specialized zooids. Siphonophores display elaborate division of labor within a hydrozoan colony.
Cubozoa: A cnidarian class of box jellyfish, known for box-shaped bells and complex sensory structures. Box jellyfish belong to a separate class despite their medusa resemblance to some hydrozoans.
Polyp (zoology): A typically cylindrical cnidarian body form attached to a surface, with its mouth and tentacles facing outward. Many hydrozoans live as polyps, sometimes forming colonies.
Gonophore: A reproductive structure in some hydrozoans that produces gametes or develops into a medusa. Gonophores connect reproductive function to the polyp colony.

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