Hydra is one of the few members of the Cnidaria (the type of animal that includes jellyfish and sea-anenomes) that is found in freshwater. Most Cnidaria can live as tree-like polyps, or jellyfish-like medusae, but Hydra exists only at the polyp form. It has a body that is a tube made of two layers of cells with an anterior opening. Around the opening are numerous tentacles. The tentacles possess nematocysts (also called thread cells or cnidia) that explode to discharge a thread that can inject poison into prey or can hold prey. Hydra mostly eats small animals such as Daphnia. A gelatinous layer called the mesogloea separates the two layers of cells of a Hydra. The outer layer of cells is called the epidermis or ectoderm, and generates the nematocysts. The inner layer is the endoderm or gastrodermis, and produces the enzymes which digest the Hydra's food. Many members of the Cnidaria establish symbiotic relationships with other organisms, and some species of Hydra are bright green because they have many zoochlorellae living inside the body. Hydra reproduces asexually most of the time by a process of budding, young polyps becoming detached from the parent when they are fully developed. Seasonal episodes of sexual reproduction also occur, mature polyps developing gonads on the external body wall. Fertilized eggs give rise to tiny larvae which swim, attach themselves and develop into polyps which continue to reproduce by budding. Hydras not only bud but also have remarkable powers of regeneration. Hydra gets its name from a mythical creature that lived in the swamps near to the ancient city of Lerna in Argolis. It was a terrifying monster which was the offspring of Echidna (half maiden - half serpent), and Typhon (had 100 heads). The Hydra had the body of a serpent and many heads. If any of the other heads were severed another would grow in its place (in some versions two would grow). Also the Hydra's breath was enough to kill man or beast.