The vast majority of modern animals are triploblastic and bilaterians, meaning that they possess bodyplans that develop from three primary germ layers which subsequently differentiate into a body that is bilaterally symmetrical along the anterior-posterior axis.
Triploblastic organisms possess an internal, fluid-filled cavity that supports their three-dimensional morphology by providing both selective flexibility and rigidity.
Development and arrangement of the fluid-filled cavity, or coelom, in triploblastic bilaterians takes on one of three forms:
- Acoelomates - possessing an incomplete digestive tract and some cannot fill coelom with fluid.
- Psuedocoelomates - seperate sexes as well as possessing a complete digestive tract with both mouth and anus. They possess lateral muscles but can only move side to side because cannot manipulate the coelomic fluid.
- Coelomates -
All bilaterians share a common set of HOX genes that direct the development of the anterior-posterior axis of the animal.
Early on, Coelemate Bilaterians split into two major divisions based upon their mode of embryological development:
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Protostomes, meaning "mouth first", include most of the bilaterian animal kingdom (i.e. Arthropods, Mulluscs, Annelids, etc...)
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Deuterostomes, meaning "mouth second", are the much smaller branch of the animal kingdom to which we, as chordates, belong - along with echinoderms and hemichordates.
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Interestingly, recent genetic and developmental evidence presented by Katharina Nübler-Jung and Detlev Arendt (, ) has revived a hypothesis proposed by Geoffroy St. Hilaire in 1822 suggesting that the chordate bodyplan is to inverted along it´s dorsal-ventral axis relative to all other bilaterians ( ).
Arendt and Nübler-Jung were able to demonstrate that the BMP-Chordin genetic network that establishes the Dorsal-Ventral Axis of the bodyplan was inverted when bilateral Protostomes and Deuterostomes are compared.
The Dorsal-Ventral axis of chordates position the nerve cord on the dorsal side of the animal and the circulatory system on the ventral side - A condition opposite to that of the other great branch of bilaterians, the arthropods.
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Such findings point to the deeply conserved nature of the genetic networks that regulate animal bodyplan evolution and the ways in which these networks can be harnessed and altered to express a diverse array of phenotypes.
Subsequent evolutionary transformations of the bilaterian animal bodyplans have been improvisations on genetic and developmental themes long established but capable of being expressed in a multitude of ways.
Nonetheless, these deeply conserved features within each phyla unite the diverse range of organisms around a common bodyplan architecture within a given phyla.
