Hydra are relatively simple animals without respiratory systems that perform gas exchange wherever their bodies are in contact with water, distributing it through their tissues by simple diffusion. They have only two layers of tissue, the ectoderm that coats their outer bodies and the endoderm that lines their guts, with a gelatinous substance known as the mesoglea between them. The mesoglea is relatively thin in hydra species.
Hydra are simple predatory animals that attach to vegetation at the base of their thin body stalks. They are cnidarians, relatives of jellyfish and sea anemones. Like these organisms, hydras are armed with stinging tentacles that ensnare and kill prey. These stings are from special cell structures known as nematocysts, which are one of the defining characteristics of cnidarians. Unlike sea anemones and jellyfish, which are restricted to marine environments, hydra are also found in freshwater environments.
Some hydra enter into symbiotic relationships with algae, which colonize their bodies. The algae supply the hydra with sugars as an additional energy source. The hydra, in turn, provide nitrogen compounds to the algae from the prey they catch. Once prey are caught by hydra, leaking body fluids activate a reflex in the hydra that opens its mouth and causes it to engulf the prey for digestion.