Are Jellyfish really fishes?

Jellyfish are almost entirely made of jelly. Jellyfish are actually not fishes, fishes are vertebrates. That is fishes have backbones. But jellyfish do not have backbones. Their bodies are bags of jelly with a hollow inside. They are shaped like an overturned bowl. The bowl of the jellyfish is made up of two thin layers of tissue with jelly-like material between them. If a jellyfish is removed from the water, it dries up very quickly because 98% of its body is water.

The jellyfish has tentacles, hanging from the edge of the bowl, which gathers food and is sometimes used for swimming. Some of these tentacles are barbed and pierce the body of the prey. These barbed cells are connected to poison glands. The jellyfish protect themselves with a sting that can sometimes be dangerous. If the jellyfish is quite small, being stung by one may not be too dangerous. But when it comes to the big ones, it may be dangerous. 

There are big ones with a bowl of nearly 4 meters in diameter and with tentacles more than 30 meters long! If it embraces us, we might even become paralyzed. The Portuguese man-of-war which is one of the largest jellyfish can kill and eat a full-sized mackerel. It can cause serious injury to human beings. There is a kind of jellyfish found off the coast of Australia called "the sea wasp" which has been known to cause death in many cases.