Leuckart (1847) first coined the name Cnidaria. The term ‘Cnidaria’ is derived from two words; Gr. Knide=nettle and L. aria=like or connected with. Cnideraia are also known as Coelenterata. They have diverse colorful, radially symmetrical bodies which are known as the flower of the sea. The phylum Cnidaria contains more than 10,000 living species, among them, mostly are marine animals whereas only 20 species inhabit in freshwater. The notable cnidarians include hydras, the corals, Portuguese men-of-war, jellyfish, sea anemones, sea whips, sea pens, and sea fans. Their variety, diverse colorful symmetry body forms, and complex life histories attract the scientists. They are diversely and abundantly distributed in tropical waters. They have calcareous skeletons which form the framework of the reefs and atolls in most tropical seas. The famous reef is the Great Barrier Reef which is more than 2000 km along the northeastern coast of Australia, made by the cnidarians.
General Characteristics Of Phylum Cnidaria
Classification of Phylum Cnidaria/Coelenterata
The phylum Cnidaria is composed of the following three classes on the basis of development of zooids:
Class-1: Hydrozoa (Hydra; water; zoon: animal)
Examples: Hydra oligactis, Obelia bidentata, Physalia physalis (portuguese man of war) etc.
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Class 2: Scyphozoa
Example: Aurelia aurita (Jelly fish), Rhizostoma pulmo etc.
Class-3: Anthozoa (Anthos: flower; zoios: animal ‘flower-like animals')
Example: Gorgonia ventalina (the purple sea fan), Pennatula phosphorea (common Sea Pen), Metridium parvulum (sea anemone), etc
The representatives of Phylum Cnidaria have radial or biradial symmetrical body. About 99 % cnidarians are marine species and few inhabit in freshwater. They are diploblastic animals and have a more complex level of organization than Porifera. In their body wall, outer ectoderm, inner endoderm layers are present but in between these two layers non-cellular mesoglea is found. In their life cycle, they show two distinct morphological forms such as polypoid and medusoid.