Dipleurula is a hypothetical
larva of the ancestral
echinoderm. It represents the type of basis of all larval forms of, at least, the
(all echinoderms except crinoids), where the
starfish,
sea urchins,
sea cucumbers and
brittle stars belong. The dipleurula is a bilaterally symmetrical,
echinoderm larva (cilia devoted to movement, feeding and perception).
Etymology
Derives from
Ancient Greek di, meaning 'two' and the small form of
pleura (
pleurula), meaning 'little side', i.e. 'little, two-sided larva'.
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)
radial symmetry adults show a bilateral structure. It was this bilateral structure of the larvae that identified echinoderms as
bilaterian animals.
The original doliolaria schema shows a benthic, 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 of the
starfish, the auricularia of the
sea cucumbers, the echino
pluteus larva of the
sea urchins, and the ophiopluteus of the
brittle stars. Also the
doliolaria of the
crinoids (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 Enterocoely. This, and their close resemblance to the tornaria larva of hemichordates, identifies them as deuterostomes.
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