The phenomenon of metamorphosis is described as a process during development which involves a remarkable change in morphology and physiology of the larva, so that it is transformed into an adult with totally different morphology and physiology, often for life in a different habitat. In many such type of animals undergoing metamorphosis, the larva is usually very different from the adult. The best known instances are tadpoles of frogs, caterpillar of butterflies and moths, tadpole larva of ascidians, several larval types of crustaceans, ciliated trochophores of marine annelids and molluscs etc; actually sow times the difference between the larva and adult is so great that with no knowing the origin of the egg, or without following the young one by its full development, it would be next to impossible to know that the young and the adult are of similar species.
In the past such differences have sometimes led to the larva and adult of similar species being assigned to different taxonomic groups. For instance the larva and adult of the axolotl Ambystoma (Urodele - amphibia) and Tribegulians of blister beetle (Inasecta) till quite freshly were mistakenly assigned to different species. Much More than a century ago, only the study of metamorphosis of the tadpole larva of ascidians could decide that the ascidians belong to Phylum Chordata. The changes that take place due to metamorphosis relate often to a change in habitat with a corresponding alter in the organisms structure and other features. For example, in sea urchins, there take place a change from planktonic to benthic existence, in frogs and toads from an aquatic to a terrestrial mode of life and in insects from a non-flying to a flying life.