Answer:
jellyfish have special nematocyst (a specialized cell in the tentacles of our jelly friend, containing a venomous coiled or barbed thread that can be projected in self defense or to capture prey) located along the tentacles, which act like harpoons (harpoon is a long spear used in fishing to catch large fish or marine animals such as whales) and can sting prey (or humans) jelly fish do not go looking to sting humans!!
