Hydridae

Hydridae is a family of freshwater Hydrozoans containing solely the genus Hydra. These animals are usually found in freshwater habitats such as lakes, where they live attached to the substrate by their adhesive basal disc. Hydra has a tubular body with radial symmetry. They posess a mouth at the end of their body, surrounded by thin tentacles, which can be moved and retracted.
These freshwater polyps feed on small plankton organisms such as crustaceans. Through expansion of their tentacles, they can grab these tiny invertebrates. This is assisted by the discharge of stinging cells (nematocysts) into the body of the prey animal, thus paralyzing it. Then, the tentacles move the prey into the mouth opening, engulfing it with the flexible body wall. Hydra species do not possess an intestine, they digest their food using secretory glands. After digestion, the cells take up the food.