Cubozoa includes the 51 species of box jellyfish, which get their name from their distinctive box-shaped bells in the medusa stage. They are strong swimmers compared to the other cnidarians and many of them have extremely toxic venom. They can grow up to 30 cm in diameter with tentacles reaching up to 3 meters long. Each corner of their bell has roughly 15 tentacles, each containing over 500,000 cnidocytes. They have two types of eyes: 4 relatively developed eyes called rhopalium, which allow them to detect light and dark, and 20 simpler eyes called ocelli. Their nervous system is also more developed than other cnidarians, possessing a nerve ring at the base of their bell which allows them to have faster and more coordinated movements. This, combined with their vision, gives them more directional fish-like carnivorous behavior. Some of these species have been linked to human deaths and routinely trigger beach closings.