A class of coelenterates that includes hydras and Portuguese men-of-war. Many of them are colonial, and some kinds have both polypoid and medusoid phases.
- The groups Hydrozoa, Scyphozoa and Cubozoa all are generally characterized as having polyps and medusae (although minor lineages have lost medusae and in some cases polyps).
- Another feature that is quite common in Hydrozoa but not typical of Scyphozoa is colonial organization.
- However, Hydra is not typical of the Hydrozoa as a whole.
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