Jellyfish (also known as jellies or sea jellies) are free-swimming members of the phylum Cnidaria. They have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa . The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; medusa is another word for jellyfish !

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Since jellyfish are not actually fish, some people consider the term "jellyfish" a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead.The word "jellyfish" is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish).

In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term "gelatinous zooplankton", when referring to these, together with other soft-bodied animals in the water column.

The class name Scyphozoa comes from the Greek word skyphos denoting a kind of drinking cup and alluding to the cup shape of the organism.

A group of jellyfish is sometimes called a bloom or a swarm. Using "bloom" implies that larger numbers than usual are present.Using "swarm" implies some kind of active ability to stay together, which a few species like Aurelia, the moon jelly, demonstrate. Many jellyfish have a second part of their life cycle, which is called the polyp phase. When single polyps, arising from a single egg, develop into a multiple-polyp cluster, connected to each other by strands of tissue called stolons, they are said to be "colonial." A few polyps never proliferate and are referred to as "solitary" species.

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Check this links for more information ;;;

http://en.wikipedia.org/wiki/Jelly_fish
http://www.marine-medic.com.au/pages/biology/biologyBreakup/jellyfishNames.pdf

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Hope this helps !
Source(s):
wikipedia

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