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Metamorphosis is a process used by certain arthropods, amphibians, mollusks, cnidarians, echinoderms, and tunicates to develop from a juvenile, larval stage into an adult stage. The larva may resemble miniature versions of the adult, or look entirely different, but in most cases have fundamentally different physiology, including special organs.
One of the most popular conceptions of metamorphosis is the transformation of a caterpillar into a butterfly. This has long been seen culturally as a metaphor for transformation and rebirth, the emergence of a beautiful butterfly from an ugly caterpillar. Before engaging in metamorphosis, the caterpillar wraps itself in a sheath known as a cocoon. Cocoons may have commercial value -- the cocoons of silkworms, for instance, are used to make silk. No accurate reproduction of silk has yet been created in the lab.
Metamorphosis can permanently change the organism's capabilities. For instance, tadpoles, the larval form of amphibians, are purely aquatic, but once they metamorphisize, they turn into salamanders, newts, frogs, or toads, and gain the ability to travel on land. Toads, a type of frog adapted to prevent itself from drying out, can even spend hours on land without exposure to water, and survive just on the moisture from burrowing in dirt.
Sometimes the difference between the larval and adult form is so extreme that the most fundamental of characteristics, such as a notocord (a kind of primitive backbone) is present in the larval stage but not with adults, as in the case of tunicates. It is thought that vertebrates may have in fact evolved from the larval form of stationary animals such as tunicates. In a display of a phenomenon known as neotony, all vertebrates may be fancy versions of long-living tunicate larva.
Insects all undergo metamorphosis. There are two main types: incomplete metamorphosis and complete metamorphosis. In the case of incomplete metamorphosis, each instar (exoskeleton shedding stage) only changes slightly from the last, and the organism never enters into a sealed cocoon, also known as a pupa. In complete metamorphosis, the entire insect encloses itself in a pupa, and changes its bodily form considerably. One example would be a maggot transforming itself into a fly. There are many other examples of metamorphosis, in several very large animal phyla. The phenomenon of metamorphosis was apparently lost after amphibians, instead being replaced with further growth inside the womb.