Jellyfish are the informal common names given to the Medusa-phase of certain gelatinous members of the Subphylum Medusozoa, Jellyfish are mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although a few are not mobile, being anchored to the seabed by stalks
Jellyfish have been in existence for at least 500 million years, and possibly 700 million years or more, making them the oldest multi-organ animal group
Jellyfish range from about one millimetre in bell height and diameter, to nearly 2 metres (6.6 ft) in bell height and diameter
Jellyfish live all over the world, from surface waters to the deep sea, but they aren’t actually fish — they’re plankton. These plants and animals either float in the water or possess such limited swimming powers that currents control their horizontal movements.
All Jellyfishes has a mouth in the center of their bodies, surrounded by tentacles
Some jellyfish are clear, but others are in vibrant colours such as pink, yellow, blue, and purple, and often are luminescent
Jellyfish are about 95 percent or more water.
Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. To see light, detect smells and orient themselves, they have rudimentary sensory nerves at the base of their tentacles.
Most feed on copepods, fish larvae, and other small animals that they catch in their tentacles, which have stinging cells. They hunt passively using their tentacles as drift lines, or sink through the water with their tentacles spread widely