Larva

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 anatomical and physiological characteristics, larvae often have completely different habitats, locomotion, and feeding habits than adults, allowing them to occupy a different ecological niche. Important examples can be found among insects that spend almost their entire growth period in the larval stage.

Among the Ephemeroptera, there are species whose larvae, aquatic, live for several years while the adults live only a few days and are even deprived of the ability to feed. In insects, four basic types of larvae are generally distinguished: protopod (head not always well separated from thorax and stubby appendages), eruciform (with limbs on thorax and abdomen), oligopod (with six thoracic limbs), and apod (without limbs).

The larvae of aquatic animals can also contribute to the dispersal of species: there are many sessile organisms (porifera, madrepores, corals, polychaete annelids, bryozoans, etc.) whose larvae are carried by currents to other places unreachable by the parent. But even non-sessile benthic animals (such as starfish or holothurians) can disperse more easily because their tiny larvae can travel much longer distances than the adult. In addition, certain animals often go through several larval stages: this is the case with crustaceans, echinoderms, some mollusks, etc.

There are numerous larval forms of animals, some of which are simple variations of a common pattern, while others are forms more peculiar to a particular taxon. In addition to the larval forms of vertebrates, such as the ammocete (Petromyzonts), the larvae of fishes (e.g, leptocephalus of the eel) and the tadpole (anurans), some of the most common invertebrate larvae are the amphiblastula and paramecium: The amphiblastula and parenchymula or parenchymella (Porifera), the planula and actinula (Coelenterates), the cidippus or cidippid larva (Ctenophores), the miracidium (Digenei), the oncomiracidium (Monogenei), the hexacanth larva (Cestodes), the decacanth or lycophore larva (Cestodarians), Müller’s larva and Götte’s larva (Polyclades), the pilidium and Desor’s larva (Nemertinians), the trochophore (Annelida), the cyphonaut (Bryozoans), the dipleurula (Echinoderms), the auricularia and brachiolaria (Asteroidea), the echinoplutea (Echinoidea), the ophioplutea (Ophiuroidea), the vitellaria (Crinoidea), the veliger (Bivalvia, Scaphopoda, Gasteropoda), the glochidium (Unionid lamellibranches), the cypridiform larva (Cirripedes), the nauplius (Crustacea), the zoea (Malacostracians), the copepodite (Copepoda), the trilobitiform larva (Merostomes), the tardigradiform larva (Pentastomidae), the tornaria (Hemichordates), etc.

The study of larval forms is of great importance because the similarity of larvae provides useful clues to the relationships of apparently very diverse groups of animals.

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