A group of spotted lagoon jellyfish
-Photo by Matthew T. Ross-
The spotted jellyfish (Mastigias papua) belongs to the phylum cnidaria. This means they contain a specialized structure in their skin, called a cnidocyte, that the jellyfish uses to defend itself and attack its prey. The cnidocyte effectively functions as a toxin-filled pressurized harpoon that pierces into anything unlucky enough to come into contact with it.
source - wikimedia common
Pretty nasty to imagine it sticking into your skin.
In addition to being weaponized, the spotted lagoon jelly are particularly interesting because they have many small mouths in what's called their oral arms. The jellyfish use their many arm mouths to eat plankton. Imagine having a mouth at the end of each of your hands—it would certainly beat using a fork.
http://www.montereybayaquarium.org/animal-guide/invertebrates/spotted-jelly