Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal’s body structure through cell growth and differentiation.
Present metamorphosis are organisms that at birth are quite different both morphologically and functionally from the adult: for example, the pluteus, larva of echinoderms, pelagic, has bilateral symmetry, while the adult has ray symmetry, more consistent with benthic life. In addition to anatomical and functional organization, the environment is often different for larvae and adults: winged insects often have aquatic larvae.
Metamorphosis can be rapid or slow and gradual. While the physiology of metamorphosis is poorly understood in most metamorphosing animals (porifera, coelenterates, platelminths, crustaceans, etc.), it is well understood in amphibians and insects.
In amphibians, the thyrotrophic hormone secreted by the anterior lobe of the pituitary gland stimulates the thyroid gland to produce thyroxine, which acts at the tissue level and is thus responsible for metamorphosis; depending on the time in which it acts and its concentration, it either causes some organs to regress and be destroyed (e.g., gills of tadpoles) or causes the differentiation or growth of others (e.g., lungs of frogs).
In insects, metamorphosis is practically a complication of molting: a hormone produced by secretory cells of the protocerebrum stimulates the prothoracic gland to produce ecdysone, a hormone that causes molting, if at the same time the corpora allata (endocrine glands on either side of the pharynx) secrete abundant neotenin, a hormone that promotes larval growth. When this hormone, called “juvenile”, is absent or scarce, ecdysone causes metamorphosis.