Not all jellies are jellyfish
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.