A pronounced change in both the internal and external morphology of an animal that takes place in a short amount of time, triggered by some combination of external and internal cues. The extent of morphological change varies considerably among species. Even when morphological changes are relatively slight, metamorphosis typically brings about a pronounced shift in habitat and lifestyle. Consider, for example, the transformation of a sluggish leaf-eating caterpillar into a nectar-drinking flying butterfly; or of a nonfeeding but free-swimming microscopic larva in a bivalved shell into a suspension-feeding sedentary barnacle; or of an aquatic suspension-feeding tailed tadpole larva into a carnivorous semiterrestrial frog or toad. In many species, metamorphosis is truly a morphological, ecological, and physiological revolution. The precise morphological, physiological, and biochemical changes that constitute metamorphosis; the neural, hormonal, and genetic mechanisms through which those changes are controlled; and the ecological consequences of those changes and when they take place continue to be studied in a wide variety of animals. The hormonal and genetic control of metamorphosis has been best examined in a few species of insect, amphibian, and fish (such as flounder), but other aspects of metamorphosis have been investigated for other insect, amphibian, and fish species as well as for crabs, barnacles, gastropods, bivalves, bryozoans, echinoderms, sea squirts—indeed, representatives of essentially every animal phylum.