There Are Many Kinds of Jellyfish the World Over

Jellyfish have drifted along on ocean currents for millions of years, even before dinosaurs lived on the Earth. They’re jellylike creatures pulse along on ocean currents and are abundant in cold and warm oceans, in deep water, and along coastlines. But despite their name, jellyfish aren't actually fish—they're invertebrates, or animals with no backbones. They come in a variety of shapes, sizes, and colors. Jellyfish are characterized by their bell-shaped or umbrella-shaped bodies and their long, trailing tentacles that are used for capturing prey. Jellyfish have tiny stinging cells in their tentacles to stun or paralyze their prey before they eat them. They primarily feed on small fish, shrimp, crabs, tiny plants, and plankton, and are themselves preyed upon by larger fish, sea turtles, and other marine animals. Inside their bell-shaped body is an opening that is its mouth. They eat and discard waste from this opening. Jellyfish digest their food very quickly. They wouldn't be able to float if they had to carry a large, undigested meal around.  

As jellyfish squirt water from their mouths, they are propelled forward. Tentacles hang down from the smooth baglike body and sting their prey. Jellyfish stings can be painful to humans and sometimes very dangerous. Some jellyfish are clear, but others have vibrant colors such as pink, yellow, blue, and purple, and often are luminescent. They have a lifespan of about three to six months and can grow up to 7 feet in size. They don’t have a brain. Instead, the body is made up of a complex nervous system, which is where all of its motor functions and sensory activity occurs. Through the neurons in this system, their body tells the muscles when to contract, which is how they swim.

Jellyfish are aquatic creatures belonging to the phylum Cnidaria, which includes other organisms like sea anemones and corals. Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. The four major classes of medusozoan Cnidaria are: Scyphozoa are sometimes called true jellyfish, though they are no more truly jellyfish than the others. Cubozoa (box jellyfish) have a (rounded) box-shaped bell, and their velarium assists them to swim more quickly. Hydrozoa also have tetra-radial symmetry, with a velum (diaphragm used in swimming) attached just inside the bell margin and a much smaller central stalk-like structure. Staurozoa(stalked jellyfish) are characterized by a medusa form that is generally oriented upside down and with a stalk emerging from the apex of the "calyx" (bell). There are over 200 species of Scyphozoan, about 50 species of Staurozoan, about 20 species of Cubozoan, and the Hydrozoa which has about 1000–1500 species.