Hydra viridissima is a species of cnidarian which is commonly found in still or slow-moving freshwater[2] in the Northern temperate zone.

Hydra are diploblastic organisms, the body is composed of two embryonic cell layers; the ectoderm and the endoderm.

The endoderm lines the gastrovascular cavity, which is a water-filled sac, this acts as a hydroskeleton and site for food digestion.

They also have a simple nervous system that consist of a nerve net that covers the entire body.

An individual can extend and contract with a mixture of muscle movement and water (hydraulic) pressure created inside the digestive cavity.

Hydra can also move by floating upside down using a gas bubble produced on the basal disc which carries the creature to the water surface.

[6] H. viridissima is currently the only known Hydra species that has a permanent symbiotic relationship with photosynthetic unicellular Chlorella algae.

[3] During long period of darkness, such as storms or blooms that block sunlight, algae loss starts from the tentacles, hypostome and growth region.

When light conditions return, the algae undergo rapid multiplication and can repopulate the host in approximately two days.

Uniquely to Hydra, the medusa stage is absent and only the polyps will reproduce sexually and asexually.