Phylum Cnidaria

Jellyfish are actually only one form of cnidarians, the other being a polyp
form like a sea anenome or coral.   They have stinging tentacles,
which produce their sting from cells called nematocysts which cover the body.
Jellyfish live mainly in the ocean, some can also live in fresh water.    Typically
eat other animals ensnared by the tentacles.