Triploblastic is a condition of the ovum in which there are three primary germinal layers: the ectoderm, mesoderm, and endoderm. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers. In higher and intermediate animals, all organisms arise from a triploblastic ovum.
Triploblastic organisms are organisms which evolve from such an ovum. Unlike diploblastic animals, in which cells are proximate to the organism's environment, triploblastic animals have internal cells that are not adjacent to external food sources. Since all cells must be fed and waste removed, these tasks are handled by various organs and circulatory and digestive systems in any triploblastic animal.
In anthozoan and scyphozoan but not in hydrozoan polyps the presumptive mesodermal elements include amoeboid cells, the mesentery retractor muscles and scleroblasts, all of which are embedded or deeply rooted in the extracellular matrix (mesoglea) and derive from the ectoblastemal cells invading the extracellular matrix from the gastrulation site during or shortly after endoderm formation.
These data lend further support to the cnidarian mesodermate hypothesis, whereby cnidarians and bilaterians share a common triploblast ancestor, the Urtriploblast, a small, motile, possibly medusa-like organism that did not feature a sessile polyp stage in its life cycle.
As a consequence the diploblasty of the hydrozoan polyps may represent a derived morphology resulting from heterochronic modulations of the gastrulation process after endoderm formation.
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