Jellyfish (also called jellies or sea jellies as they are not true fish) are animals that belong to Phylum Cnidaria, included in the class Scyphozoa. Almost all jellyfish live in the seas and though they lack true organ structures they feature specialized tissues. The adult forms of these creatures are composed of 95-99% water. All species are found in each of the world's oceans, with a few species living in fresh water.
Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion. They are Coelenterates which means "hollow gut," and are made up of a layer of epidermis, gastrodermis, and a thin jelly-like layer called mesoglea that separates the epidermis from the gastrodermis.
The jellyfish have two major body forms throughout their life. The first form is called the polyp stage and is characterized by a either a non-moving (sessile) stalk that catches food drifting by or a similar form that is free-floating. Their mouth and tentacles are located anteriorly, facing upwards. The second form looks like a saucer is called the medusa stage and is characterized by a round (radially symmetric) dome-shape body plan with food catching tentacles hanging down. It is this form which is most able to respond to and interact with its environment and is also the form most people are familiar with.
Dangers of the JellyfishLike all other cnidarians, jellyfish have stinging cells called cnidocytes which contains the stinging nematocysts on their tentacles. Whenever a prey comes in contact with a tentacle, hundreds to thousands of nematocysts fire one or another type of "hook and line" into the prey's direction. These stinging cells are thus able to latch onto the prey and the tentacles bring the prey item into their large "mouth" for digestion. These cells are activated by a simple but precise nervous system called a nerve net which is located in the epidermis of the jellyfish. Impulses to these nerve cells are sent from the nerve rings that have collected information from the environment of the jellyfish through the rhopalial lappet, which is located around the animal's body.
Most jellyfish are not dangerous to humans but a few are highly toxic, such as the Cyanea capillata. Contrary to popular belief, the menacingly infamous Portuguese Man O' War (Physalia) isn't actually a jellyfish, but a colony of hydrozoan polyps.
This is a sample file for Google: Optimizing Your Site, a tutorial offered by the Networked Information Services at Boston University. Information on this page is taken from http://en.wikipedia.org/wiki/Jellyfish.