A larva (plural larvae) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
In addition to the anatomo-physiological characteristics, the larvae often have different habitats, locomotion, and dietary regimen from the adult, so that they occupy a different ecological niche. Significant examples can be found among Insects that spend almost the entire period of growth in the larval stage. Among the Ephemeroptera there are species whose larvae, aquatic, live for several years, while the adults live a few days and are even deprived of the possibility to feed. In Insects are generally distinguished four basic types of larvae: protopod (head not always well separated from the thorax and stubby appendages), eruciform (with limbs on the thorax and abdomen), oligopod (with six thoracic limbs) and apod (without limbs).
The larvae of aquatic animals can also help to spread the species: there are many fixed organisms (Porifera, madrepores, corals, Polychaete annelids, Bryozoa, etc..) whose larvae are transported by the currents to other places, unreachable by the parent. But even bottom animals that are not properly sessile (such as starfish or holothurians) can spread with greater ease thanks to their tiny larvae that travel routes immensely longer than those possible for the adult. Often, moreover, some animals pass through several larval stages: this is the case of Crustaceans, Echinoderms, some Molluscs, etc..
Numerous are the larval forms of animals, some of which are simple variations of a common model, others are forms more proper to a particular taxon. In addition to the larval forms of Vertebrates, such as the ammocete (Petromyzonts), the larvae of Fishes (e.g., the leptocephalus of the eel), and the tadpole (Anurans), some of the most common larvae of Invertebrates are: the amphiblastula and parenchymula, or parenchymella (Porifera), the planula and actinula (Coelenterates), the cidippus or cidippid larva (Ctenophores), the miracidium (Digenei), the oncomiracidium (Monogenei), and the hexacanth larva (Cestodes), the decacanth or lycophora larva (Cestodarians), Müller’s larva and Götte’s larva (Polyclades) the pilidium and Desor’s larva (Nemertini), the trocophora (Anellidae), the cyphonaut (Bryozoa), the dipleurula (Echinoderms), the auricularia and brachiolaria (Asteroidea), the echinoplutean (Echinoidea), the ophioplutean (Ophiuroidea), the vitellaria (Crinoidea), the veliger (Bivalves, Scaphopods, Gastropods), the glochidium (Unionid Lamellibranchs), the cypridiform larva (Cirripedi), the nauplius (Crustacea), the zoea (Malacostraci), the copepodite (Copepodidae), the trilobitiform larva (Merostomi), the tardigradiform larva (Pentastomidae), the tornaria (Hemicordates), etc. The study of larval forms is of great importance because the similarity of the larvae provides useful hints about the relatedness of apparently well-diverse animal groups.