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The cubozoans are similar to the Scyphozoa ("true jellyfish") but the bell is square in cross section, with a velum-like structure called the velarium. The velarium restricts the size of the opening through which water is expelled when the bell contracts, thus increasing thrust and making them stronger swimmers than the Scyphozoa. There are four clusters of tentacles, one at each corner of the bell. The rhopalia of the Cubozoa differ from those of the Scyphozoa in possessing very complex eyes with lenses, corneas and retinas. The lens is capable of producing very sharp images, as good as human eyes but the focal length is longer than the distance between the lens and the retina making box jellyfish strangely far-sighted. Development is also different in the cubozoans. Each scyphistoma forms a single medusa via complete metamorphosis. Cubozoa includes the highly toxic box jellyfish, Chironex fleckeri, found in tropical regions and often in swarms which can drift into bays, disrupting human activities.