Hydra, a genus of small, freshwater polyps. The hydra is one of the simplest forms of many-celled animals. Hydras are related to the jellyfish, sea anemones, and corals.
The hydra is usually found attached to a submerged leaf or stick. Its body is a simple, threadlike tube 1/8 to 3/4 of an inch (3 to 19 mm) long. One end is closed; the other consists of a mouth surrounded by five to eight tentacles. These tentacles have nettling (poison) cells with which the hydra paralyzes its prey of larval insects and tiny crustaceans. The hydra's activities are regulated by sensitive nerve cells, for the animal has no brain or vision. Hydras move by attaching their tentacles to an object, then dragging or somersaulting their bodies forward.
The hydra has amazing regenerative (body-repairing) power. Each piece cut from a hydra soon develops into a complete polyp. Hydras reproduce from eggs or from buds that develop on the side of the parent, then become detached.
Most hydras (HY druhz) spend their whole lives as tube-shaped polyps. Still, they have a lot in common with jellyfish. A hydra has two cell walls with a thin layer of jelly in between. It has a mouth, tentacles, and lots of stingers. Hydras live in freshwater ponds. They are very small—and look like bits of string with frayed ends. A typical hydra is only 1/4 to 1/2 inch (6 to 13 millimeters) long.
To grow, a hydra attaches itself to the bottom of a pond or to a water plant. But it can and does move. A hydra can slide along on its cuplike foot. It can somersault or pull itself along by its tentacles. It can also let go and float free on an air bubble.
Hydras belong to the class Hydrozoa of the phylum Cnidaria.