But what exactly is a jelly? It’s is a general term for any kind of transparent, gelatinous (or jellylike) animal that floats in the ocean. Jellies belong to two different groups, cnidarians and ctenophores, and while members of the two groups may sometimes look alike, they are not all closely related.
Hundreds of jelly species live in oceans around the world, from shallow bays to the deep sea. Some even live in fresh water. The most common jellies are true jellyfish (cnidarians) and comb jellies (ctenophores).
Most jellyfish have long stinging tentacles and have oral-arms that help catch and eat food. Comb jellies have oval bodies lined with rows of fluttering cilia. Instead of stinging, they use their tentacles to pull prey into their large mouths.
Whether they’re cnidarians or ctenophores, jellies have bodies that are made of two transparent layers—an outer one for protection and an inner one for digesting food. Between the two layers, you’ll find nothing but a watery gel—in fact, their bodies are more than 95% water! Aside from these few parts, there’s not much more to them. These amazing animals get along with no bones, no head, no legs—not even a brain!