Jellyfish range from about one millimeter in bell height and diameter to nearly two meters in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension. Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net," Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species. Certain species of jellyfish, such as the box jellyfish, have been revealed to have more advanced vision than their counterparts.