From the Greek metamórphosis - transformation.

Metamorphosis is the process of transformation of a larva into a fry. Metamorphosis ends with the appearance of scales on the fry.

For example, in protozoa, in sucking infusoria, there are elements of metamorphosis: the budding new individuals have ciliated cover and swim, then lose cilia, become attached-living and feed with the help of extending tubes. Lower invertebrates (sponges, coelenterates) are characterized by metamorphosis, in which free-swimming larvae (parenchymula, amphiblastula, planula) perform the function of dispersal of the species. In many cases, such metamorphosis is complicated by the change of generations (developmental phases), reproducing asexually or sexually (e.g., in scyphomedusae, many flatworms).

Necrotic metamorphosis is peculiar in nemertines, in which the future adult develops inside the larva, and the main body mass of the larva dies off. In metamorphosis without alternation of generations (in many invertebrates), a larva with a dispersal function emerges from the egg (e.g., trochophora of marine polychaetes, veliger of marine mollusks). In the adult animal, larval segments (preserved from the first larva) and postlarval segments (appearing later) are distinguished; for example, in crustaceans, antennules, antennae and mandibles develop from appendages of the nauplius and correspond to larval segments.