Hydridae

Hydridae is a family of freshwater cnidarians belonging to the order Hydroida. These small, simple invertebrates are commonly known as hydras. They are characterized by their radial symmetry, a sac-like body, and a ring of tentacles surrounding a central mouth. Hydras typically inhabit freshwater ponds, lakes, and slow-moving streams, often attaching themselves to submerged vegetation or rocks.

The body of a hydra consists of two primary cell layers, the epidermis and the gastrodermis, separated by a non-cellular layer called the mesoglea. Embedded within the epidermis are specialized stinging cells called nematocysts, which are used for capturing prey and defense. Their diet mainly consists of small invertebrates like daphnia, copepods, and insect larvae.

Hydras are remarkable for their regenerative abilities. They can regenerate entire bodies from small fragments and have achieved a form of biological immortality in laboratory settings due to their capacity to continuously replace damaged or aging cells. Reproduction in hydras can occur both asexually through budding, where a new individual grows from the parent's body, and sexually, by the formation of gametes. This family plays a role in aquatic ecosystems as both predators and prey for other small organisms.