I saw a special on box jellyfish the other night with my family on National Geographic (:

Box jellyfish  developed its  venom to instantly stun and kill its prey; this is so its tentacles would not be damaged, as the prey was trying to escape.  The venom of these fatal creatures is known as the deadliest in the world because of its power to attack the heart, skin cells, and nervous system. The sting is said to be so painful, that human victims go into shock and drown or experience heart failure before ever reaching shore. Survivors have pains for weeks and usually are left with extreme scarring.

Box jellies are also known as sea wasps and marine stingers and are found in the coastal waters off Northern Australia and throughout the Indo-Pacific. They are a pale transparent blue and received their name because of there cube shaped bell. These jellies would be a little tough to escape from considering that their tentacles can grow up to 10 feet long, and they can have up to 15 tentacles! Each tentacles consists of over 5,000 stinging cells which instead of being triggered by touch they are activated by a chemical on the prey.
Their eyes are in clusters of six on the four sides of their bell.

 Over 5,000 human deaths have been reported their stings since 1954.

scary, isn’t it?