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Jellyfish are not fish. The are actually invertebrates which means they do not have a backbone. The jellyfish is made up of ninety-five per cent water. Jellyfish are marine invertebrates belonging to the Scyphozoan class, and in turn the phylum Cnidaria. The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creatures tentacles suspend.
Jellyfish come in all different shapes and colours and range in size from 3 millimetres to 3 metres in diameter. Although Jellyfish often appear clear or pale bluish in colour, they can also be yellow, deep blue, bright purple, pale lilac, bright orange, deep red. Some Jellyfish, when they are disturbed at night, give off a cold bright light called luminescence.
Jellyfish are made up of a layer of epidermis, gastrodermis and a thick jellylike layer called mesoglea that separates the epidermis from the gastrodermis.
Jellyfish live in the ocean. They drift at the mercy of the currents, often accumulating in sheltered bays and estuaries. Jellyfish swim in an unusual way, a pumping action. Muscles in their bodies contract and propel them through the water. This does little more than allow them to move up or down in the water. However, the four sided box jellyfish are powerful swimmers with good control and speed.
Depending on the number and length of tentacles, jellyfish feed in various ways. Most catch their food sith their tentacles as it drifts by and is then carried to the mouth, which is located in the centre of the bell shaped body. Some jellyfish eat small crustaceans and fish, some like minute planktonic organisms and some are like coral that have algae in their tissues which produces food by photosynthesis.
If you have never been stung by a Jellyfish, then you are very lucky as they have a painful sting and some can even kill you. The tentacles on a Jellyfish are covered with stinging cells (cnidocytes) that sting or kill other animals: most jellyfish use them to secure prey or as a defence mechanism. Others, such as Rhizostomae, do not have tentacles at all.