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· Abraham Trembly , a Swiss biologist discovered Hydra in 1744 AD. · Linnaeus gave the name Hydra, after the great serpentine nine-headed dragon from the ancient · Greek mythology  is based on its ability to regenerate the lost parts in 1758 AD. · The name Hydra is an old Greek word for "water-serpent". · Hydra is a solitary polyp found in freshwater (stagnant). · Hydra is diploblastic and radially symmetrical. · Separate coelom is absent in Hydra, so can be called acoelomate . Among coelenterates, Hydra is one of the smallest polyps. · Hydra has tissue grade of organization. · Body of Hydra has no segmentation. · Hydra has a cylindrical body with 6‑10 hollow tentacles. · The tentacles of Hydra help in locomotion and food. · The basal part of Hydra has a pedal or basal disc. · Mouth is situated on a manubrium or hypostome. It is the most...

· Study of Coelenterates is known as  Cnidology · The term Coelenterate  was given by Leuckart . · Coelenteron (Kilos + enteron); Kilos meaning cavity and enteron meaning intestine; thus coelenterates are those having a space which acts as both a cavity and an intestine (Gastrovascular cavity). · Also called Cnidaria (the term given by Barnes). · Cnidaria, because of the presence of Cnidoblast (Stinging cells). · The members of this phylum are commonly called stinging animals. VVI Examples of Phylum Coelenterata for Entrance Exams of Nepal: Hydra, Obelia (Sea fur), Physalia ( Portuguese  man of war), Bougainvillea Aurelia or Jellyfish, Pelogia, Poricolpa, Pilema, Rhizostoma Adamsia Metridium (Sea anemone) Edwardsia Pennatula (Sea pen) Gorgonia (Sea fan) Alcyonium (Dead man's finger) Characteristics of Phylum Coelenterata · They have a single central cavity, the coelenteron or gastrovascular cavity; having a single opening...