Jellyfish
Most jellyfish have a bell-shaped body and long tentacles at the underside of the body. Tentacles are long "arms" with special stinging cells called cnidoblasts containing the structure of the nematocyst. The nematocysts are pockets in the tentacle that have a small stinger that is sealed with a cap. There is a microscopic hair, or “trigger” on the outside of the pockets and when the tentacle brushes against something it activates the hair which opens the cap and allows salt water to pour in. The inrushing salt water increases the pressure in the nematocyst pushing out the venomous tentacle or thread, thus stinging the jellyfish’s prey. They move by contracting their bodies, but they do not have much control over where they go: most of the time, they drift with the water current. The largest type of jellyfish is the Lion's mane jellyfish, which has tentacles that can be as long as 60 meters, but most jellyfish are much smaller.