The ability of certain plants and animals to emit light has long been a source of fascination to humans. Why do certain species of mushrooms glow? Why are midwater squids designed with ornate light-emitting organs underneath their eyes and ink glands? Are these light displays simply an example of nature in its most flamboyant mode ―a case of  “if you've got it, flaunt it”―or do they serve any practical purposes? 　　Actually all of the known ways in which bioluminescence functions may be classed under three major rubrics: assisting predation, helping escape from predators, and communicating. Many examples of the first two uses can be observed in the ocean's midwaters. Almost all of the animals that inhabit the murky depths are capable of producing light in one way or another. Certain animals, when feeding, are attracted to a spot of light as a possible food source. Hence, other animals use their own luminescence to attract them.    Just in front of the angler fish's mouth is a dangling luminescent ball suspended from a structure attached to its head. What unwitting marine creatures see as food is really a bait to lure them into the angler fish's gaping maw. The uses of luminescence to elude prey are just as sophisticated and various. 　　Some creatures take advantage of the scant sunlight by using bioluminescence as a form of camouflage. The glow generated by photophores on the undersides of some fishes and squids acts to hide them through a phenomenon known as countershading: the weak downward lighting created by the photophores effectively erases the animals' shadows when viewed from below against the lighted waters above. 　　Some marine animals use bioluminescence more actively in their own defense, turning predators into prey. For instance, there is the so-called “burglar alarm effect,” in which an animal coats an advancing predator with sticky glowing tissue that makes the would-be attacker vulnerable to visually cued hunters―like bank robbers marked by exploding dye packets hidden in stolen currency.