Jellyfish are very simple sea creatures that have lived in the oceans since before the time of the dinosaurs. Found in every ocean around the world, some jellyfish live in deep, cold water, while others prefer warm, shallow coastal waters. Scientists believe jellyfish have been around for at least 500 million years.

Members of the scientific phylum Cnidaria, jellyfish are considered to be plankton and are closely related to sea anemones and corals. Since jellyfish aren’t really fish, many scientists prefer to call them “jellies” or “sea jellies” instead.

Floating along an ocean current, jellyfish can be mesmerizing to watch. Their elegant and colorful bodies seem so mysterious.

Take them out of the water, though, and they become boring blobs. Why? Their bodies are more than 90 percent water!

Jellyfish don’t have bones, brains, hearts, blood, or a central nervous system. Instead, they sense the world around them with a loose network of nerves called a “nerve net.”

Jellyfish consist of three basic layers. The outer layer, called the “epidermis,” contains the nerve net.

The middle layer is made of “mesoglea,” the thick, elastic stuff that looks like jelly. The final, inner layer is called the “gastrodermis.”

Inside their bell-shaped bodies, jellyfish have a large “mouth” where food comes in and waste goes out. Jellyfish eat fish, shrimp, crabs, small plants, and sometimes even other jellyfish. They squirt water from their mouths to move through the water.

The most recognizable feature of a jellyfish is its tentacles that hang down from its body. Fascinating to look at, these tentacles can be dangerous to touch.

Jellyfish can sting with their tentacles. They use them to stun prey before they eat them.

Jellyfish don’t purposefully attack humans. Most jellyfish stings occur when someone accidentally touches a jellyfish. Even a dead jellyfish can sting!

How harmful a jellyfish sting depends on the type of jellyfish. Some jellyfish stings have little or no effect on humans, while others may cause minor discomfort to extreme pain.

The sting of a few types of jellyfish, though — such as the Australian sea wasp, the Irukandji, and the Portuguese man-of-war — can be potentially fatal.

WONDERful jellyfish facts:

A group of jellyfish can be called a “bloom,” a “swarm,” or a “smack.”

The lion’s mane jellyfish might be the longest animal in the world. Its thin tentacles can reach up to 120 feet long.

The Nomura’s jellyfish might be the largest jellyfish. Average specimens weigh 330 pounds, and the largest can reach 440 pounds.