Cubomedusae by Ernst Haeckel from Kunstformen der Nature 1904.♥
Box jellyfish (class Cubozoa) are cnidarian invertebrates distinguished by their cube-shaped medusae. Box jellyfish are known for the extremely potentvenom produced by some species: Chironex fleckeri, Carukia barnesi and Malo kingi are among the most venomous creatures in the world. Stings from these and a few other species in the class are extremely painful and sometimes fatal to humans.ox 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-cupocelli, 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).