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In astronomy, Hydra (the sea monster) is a constellation. See Hydra (constellation). Sometimes Hydra is confused with the constellation Hydrus.

In Greek mythology, the Hydra is the many-headed serpent which Heracles defeated. See Lernaean Hydra

Hydra is one of the Saronic Islands in Greece.

Hydra is also the name of a distributed operating system developed at Carnegie Mellon University.

In zoology, Hydra is a of simple fresh-water animal. Found in the phylum Cnidaria, of the class Hydrozoa. Hydras' are radial in symmetry. [Cnidocytes] are unique cells used by hydras to make nematocytes. Nematocytes are stinging capsules that look like light bulbs with a wiggling string coming out of the middle. Nematocytes make an electrical discharge that is used to paralyze their prey. Hydras have an integument made of mesoglea, a gel like substance. The integument is made of an outer epidermis and internal gastrodermis that are separate from the other. The [colelentron] is in the middle of the gastrovascular cavity and is the place where the digestion process occurs. They have tenticles where the cnidocytes are held. Hydras go through a body change from a polyp to the adult form called the medusa. Hydras have nerve net for a nervous system which is primitive compared to mammalian nervous systems that they do not have a brain. Nerve nets are connecting sensory photoreceptor nerve cells that are found in the body wall of hydras. Photoreceptors detect light and send signals to the body.


    Gilberson, Lance, Zoology Lab Manual 4th edition. Primis Custom 
Publishing. 1999
    Solomon, E., Berg, l., Martin, D., Biology 6th edition. Brooks/Cole Publishing. 2002