Hydra

Hydra is a genus of small, tubular, freshwater animals belonging to the phylum Cnidaria, renowned for their simple structure, predatory behavior, and remarkable regenerative capacity.​

Structure and External Morphology

Hydra has a radially symmetric, cylindrical body, typically 1–30 mm long, anchored at the base by an adhesive structure called the basal disc.​

The body wall comprises two layers: the outer epidermis and the inner gastrodermis, separated by a non-cellular mesoglea.​

The basal disc secretes a sticky fluid, enabling the Hydra to attach to submerged surfaces such as aquatic plants or rocks.​

Specialized Features

At the free end is the hypostome (mouth), surrounded by 6–12 long, flexible tentacles equipped with specialized stinging cells called cnidocytes.​

Cnidocytes contain nematocysts—capsules with coiled, dart-like threads and neurotoxin—used to immobilize and capture prey.​

The tentacles can extend, retract, and grasp prey, transferring food to the mouth for digestion in the central gastrovascular cavity.​

Internal Organization

Hydra is diploblastic—its two tissue layers contain simple, unspecialized cells including muscle, gland, nerve, and stem cells for continuous regeneration.​

Respiration and waste excretion occur across the body surface; the mouth opening is used for both ingestion and eliminating undigested remains.