Although jellyfish have the word “fish” in their name, they aren’t actually fish at all. Jellyfish are invertebrates, meaning that their bodies have no bones. And unlike an actual fish, a jelly doesn’t have gills, a heart, blood, or a brain either. The bodies of these extremely simple animals consist of 95 percent water. Humans, for comparison, are 65 percent water. The rest of a jelly is made up of proteins and minerals that form cells, organs, and a flexible skeleton made of collagen. It’s the same material that allows the human ear and nose to bend.
Along with a bell-shaped body, a jellyfish usually has dangling tentacles that contain stinger cells called cnidae (NIGH-dee), which secrete venom. This toxic substance protects jellyfish from predators. Jellyfish, which are carnivores, also use their venom to stun prey, like small fish, shrimp, and other jellies. Even a dead jellyfish that washes up on the beach can still have functioning stingers.