Anatomy
Box jellyfish most visibly differ from the "true" or Scyphozoan jellyfish in that they are umbrella shaped, rather than domed or crown-shaped. The underside of the umbrella includes a flap, or velarium, concentrating and increasing the flow of water expelled from the umbrella. As a result, box jellyfish can move more rapidly than other jellyfish. In fact, speeds of up to six metres per minute have been recorded.
The box jellyfish's nervous system is also more developed than that of many other jellyfish. Notably, they possess a nerve ring around the base of the umbrella that coordinates their pulsing movements; a feature found elsewhere only in the crown jellyfish. Whereas some other jellyfish do have simple pigment-cup ocelli, box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are located on each of the four sides of their bell in clusters called rhopalia. This enables them to see specific points of light, as opposed to simply distinguishing between light and the dark. Box jellies also retain the lesser type of eye, because the strong eyes are only one of four subsets. They therefore have 24 eyes. Another interesting addition is that a box jellyfish actually has the closest thing a known jellyfish has to a brain. Tests have shown that they have a limited memory, and have a limited ability to learn.
The tentacles of some species can reach up to 10 ft (3 metres) in length. Box jellyfish can weigh up to 4.4 lb (2 kg)
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