Hydrozoans belong in the phylum Cnidaria. Most hydras alternate between an asexual polyp stage and a sexual medusa
stage, though the best-known Hydrozoan, Hydra, never becomes a medusa, spending its whole life as a polyp.
Found in still or slow-moving water, Hydras live a solitary lifestyle attached to leaves and stems. Nematocysts,
characteristic of all Cnidarians, are coiled thread-like structures that Hydras use to stun prey with poison. Once
their prey is helpless, Hydras use their tenacles to lead their meal to their mouth. Hydras eat small crustaceans,
insect larvae, and annelids. However, their poison is fairly unharmful to humans,
except those who have extreme allergies to the poison.
Hydras are able to sexually or asexually reproduce, the former involves fertilization of eggs by sperm floating in
the water, and the latter involves budding of the Hydra, and each individual bud develops into a young hydra.
Colonial hydrozoans have a base, a stalk, and at least one polyp, which during reproduction buds, producing a
medusae. These medusae then mature and release gametes, which fuse to form zygotes, which grow into free-swimming
planula larvae upon settling on a suitable substrate develop into a hydroid colony by asexual reproduction. There
are about 3,000 known Hydrozoan species.