Cnidaria











Cnidaria is a phylum to which to which jellyfish belong. The phylum consists of over 9000 species of aquatic animals, 2000 or more of which are jellyfish. Apart from sea anemones and corals, jellyfish are perhaps the most well known and recognized members of the Cnidaria phylum. 

Cnidarians can be classified into four main groups, although recent studies show that some of the species in the phylum are can be divided into another group based on their physical similarities, but no conclusions have been reached with regard to that. The four main groups of the Cnidaria phylum as we know them now are:
  • Anthozoa: the species in this group are characterized by their sessile or stationary nature. Members of this group include sea anemones, corals and sea pen
  • Scyphozoa: the species of this group are free-swimming and mostly include jellyfish
  • Cubozoa: these are marine invertebrates named for their cube-shaped medusae. The most famous member of this group is the deadly box jellyfish found in the waters surrounding Australia.
  • Hydrozoa: this is a diverse group of species. Both freshwater and marine species can be found in this group. This group of Cnidaria has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War.
Jellyfish are perhaps the most typical members of the Cnidaria phylum. The phylum is characterized by the presence of cnidocytes on the bodies of its members. Cnidocytes are specialized cells used by the member species to procure food and protect themselves from predators. Like the jellyfish, members of the phylum Cnidaria have bodies that contain thick gelatinous material called the mesoglea. The mesoglea is a non-living jelly like substance, and also the cause for jellyfish to be so named. The mesoglea is usually contained in a single or double cell layer, both on the inside and the outside, called epithelium. 

Like the jellyfish, most Cnidarians have two basic body forms. The two body forms are sessile polyps or free swimming medusae. The radial symmetry of the jellyfish is common to both the sessile and free swimming forms of the member species. The mouth is at the centre of the body and it is surrounded by tentacles on the margin of the body. These tentacles are the bearers of thousands of cnidocytes, which give their name to the phylum. 

Whether a species of the Cnidaria phylum is sessile or free swimming, they are characterized by a single orifice that enables both the ingestion of food and excretion of waste material. On the other hand, they also have a single cavity within this orifice with a rudimentary digestive system, mostly comprising only of the gullet, the stomach and the guts. The single orifice is also responsible for the respiration of the creature, and in many cases like the jellyfish, also responsible for movement. Jellyfish are only capable of vertical movement which is enabled by thrusting water out of this orifice for an upwards thrust. Apart from the rudimentary digestive and respiratory systems Cnidarians only have an elementary nervous system made up of a simple nerve net and individual receptors. 
Normen FAdel
Author of the article
writer and blogger, founder of jellyfish .

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