All jellyfish are considered jellies, but not all jellies are jellyfish. What gives? It turns out that having a body made of jelly doesn’t necessarily mean you are a jellyfish. For example, the animals known as comb jellies look in many ways like true jellyfish, but are actually distant cousins. Comb jellies have different bodies than true jellyfish and don’t make the stinging cells that jellyfish do. These stinging cells are called nematocysts (Neh-MAT-oh-sistz).
Scientists are still trying to figure out a lot about the sea’s gooey creatures, and the different kinds of jellies can be hard to tell apart. The true jellyfish are called scyphozoans (Sigh-fuh-ZOH-unz) and are a kind of jelly. Then there are two groups of close relatives—box jellies and hydrozoans (HI-druh-ZOH-unz). While they are very close relatives of the true jellyfish and they have the same stinging cells, scientists don’t consider them true jellyfish.
Jellyfish and their relatives the box jellies and hydrozoans are very simple animals. They don’t have brains or hearts or lungs. But they do have a thin layer of muscle. They swim by squeezing this muscle, which forces water out the bottom of their bell, propelling them forward. Scientists think that jellyfish were the first animals on earth to use their muscles to swim.
Most small jellyfish eat plankton and floating bits of food. Larger ones will eat fish and other small animals that they stun or kill with their stinging cells. Then they bring food to their mouths with tentacle-like structures called oral arms. Many jellyfish will glow when they are bumped or disturbed. They make a special protein that gives off light.