Dipleurula is a hypothetical

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

of the ancestral

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

. It represents the type of basis of all larval forms of, at least, the

eleutherozoa Eleutherozoa is a subphylum of echinoderms. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort. It includes all living echinoderms except for crinoi ...

ns (all echinoderms except crinoids), where the

starfish Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...

sea urchins Sea urchins or urchins () are echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body covered by a spiny p ...

sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asi ...

and

brittle stars Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

belong. The dipleurula is a bilaterally symmetrical,

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

ted echinoderm larva (cilia devoted to movement, feeding and perception).

Etymology

Derives from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

''di'', meaning 'two' and the small form of ''pleura'' (''pleurula''), meaning 'little side', i.e. 'little, two-sided arva.

History

Although the term dipleurula stems from Semon (1888), it was first systematically introduced, described and drawn by Bather (1900) in his monography on the echinoderms. The name dipleurula, two-sided, was given to stress the fact that the larva of the typically five-rayed, (approximately) radially symmetric

adults An adult is an animal that has reached full growth. The biological definition of the word means an animal reaching sexual maturity and thus capable of reproduction. In the human context, the term ''adult'' has meanings associated with social and ...

show a bilateral structure. It was this bilateral structure of the larvae that identified echinoderms as

bilaterian Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...

animals. The original doliolaria schema shows a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

, crawling, larva. However Bather could not have known yet that larval echinoderms are typically pelagic (free-floating plankton).

Structure

The hypothetical dipleurula larva bears resemblance to stages of all extant echinoderms, such as the bipinnaria and the

brachiolaria A brachiolaria is the second stage of larval development in many starfishes. It follows the bipinnaria. Brachiolaria have symmetry (biology)#Bilateral_symmetry, bilateral symmetry, unlike the adult starfish, which have a symmetry (biology)#Pentame ...

of the

, the auricularia of the

, the echino pluteus larva of the

, and the ophiopluteus of the

. Also the doliolaria of the

crinoids Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

(sea-lilies and feather stars) can be attributed to the same basic pattern. Note, that the extant echinoderms represent just a small window on the extraordinary diversity of early echinoderms as known from their rich fossil record. Although there is current research on fossilized larval skeletons, the doliolaria remains a hypothesis. The doliolaria shows a three-partite body, each of which develop paired coelomic spaces which originate from the enterocoel. This, and their close resemblance to the tornaria larva of

hemichordates Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and includ ...

, identifies them as

deuterostomes Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...

. The anterior coeloms are known as axocoel, the medial ones as hydrocoel and the posterior ones as somatocoels. In extant echinoderm larva, the coeloms on the right side of the larva are typically much smaller than the ones on the left side, or even rudimentary. The right hydrocoel may fuse with the right axocoel.

References

External links

Information on forn
* {{cite web , url=http://www.spektrum.de/lexikon/biologie/dipleurula/18496 , title=Dipleurula , accessdate=2019-12-30 , year=1999 , work= :de:Lexikon der Biologie , publisher=Spektrum Akademischer Verlag. Heidelberg , language=de Larvae Echinoderm biology