Jellyfish are Cnidaria, a group of animals that also includes corals, hydras, and sea anemones. Cnidaria are all radially symmetrical. Their body parts are roughly symmetrical when viewed around a central axis. Jellyfish have no left or right side, when viewed side-on, they look approximately the same from every angle.
The top of the jellyfish is referred to as the bell. The shape of the bell varies from species to species, with some species having round bells and others having flat or oval shaped bells. The bell consists of an outer layer of cells known as the epidermis. The mouth of a jellyfish is located on the underside of the bell and opens into a gastrovascular cavity within the bell. The gastrovascular cavity is lined with a layer of cells known as the gastrodermis. Between the epidermis and the gastrodermis is the mesoglea, a jelly-like substance that gives the jellyfish much of its bulk and shape.
Jellyfish have thin tentacles that grow from the bottom edge of the bell. The number and size of these tentacles varies from species to species. Jellyfish also have four (or in some species, eight) oral arms that trail from the jellyfishes' mouth.
Jellyfish have a complex life cycle throughout which they take on a number of different forms. The dominant (and most familiar form) of jellyfish is the medusa. But there are several other forms including planula, polyp, and ephyra forms.
Jellyfish are carnivores. They feed on zooplankton, small fish, and invertebrates. When unsuspecting fish or other small sea creatures becomes entangled in the jellyfish tentacles, specialized cells within the tenticals called cnydocites fire little barbs and threads (which are contained in organelles referred to as nematocysts) into their prey. These threads penetrate the skin of the victim and deliver venom which immobilizes the animal. The jellyfish then uses its oral arms to bring the prey to its mouth and ingest it.
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Jellyfish live in oceans throughout the world. Most jellyfish live in marine environments, a few species inhabit saltwater lakes. All species of jellyfish that are said to be freshwater dwelling are in fact Hydrozoans (not Scyphozoans). Some jellyfish prefer warm, tropical waters while other species tend to prefer cold, arctic or antarctic waters.