An Ocean Jellyfish.
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Jellyfish live mainly in the ocean, 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. Some plankton are microscopic, single-celled organisms, while others are several feet long. Jellyfish can range in size from less than an inch to nearly 7 feet long, with tentacles up to 100 feet long.
Jellyfish are also members of the phylum Cnidaria, (from the Greek word for “stinging nettle”) and the class Scyphozoa (from the Greek word for “cup,” referring to the jellyfish’s body shape). All cnidarians have a mouth in the center of their bodies, surrounded by tentacles. The jellyfish’s cnidarian relatives include corals, sea anemones and the Portuguese man-o’-war.
Jellyfish are about 98 percent water. If a jellyfish washes up on the beach, it will mostly disappear as the water evaporates. Most are transparent and bell-shaped. Their bodies have radial symmetry, which means that the body parts extend from a central point like the spokes on a wheel. If you cut a jellyfish in half at any point, you’ll always get equal halves. Jellyfish have very simple bodies — they don’t have bones, a brain or a heart. To see light, detect smells and orient themselves, they have rudimentary sensory nerves at the base of their tentacles.