The ExtraxiaI/ Axial Theory (EAT) of echinoderm skeletal homologies describes two major body wall types: axial and extraxial. The latter is subdivided into perforate and imperforate regions. Each of the regions has a distinctly different source in early larval development. Axial skeleton originates in the rudiment, and develops in association with the pentaradially arranged hydrocoel according to specific ontogenetic principles. Perforate and imperforate extraxial regions are associated with the left and right somatocoels respectively, are not governed by ontogenetic principles of plate addition, and are products of the non-rudiment part of the larval body. The morphology of even the most bizarre of the earliest echinoderms can be explored using the EAT. Among these, edrioasteroid-like taxa best fit the idea that forms expressing archimery in the sequential arrangement of axial, perforate extraxial, and imperforate extraxial regions are the first echinoderms. Metamorphosis is especially marked in clades that have a high axial to extraxial skeleton ratio because structures developing from the non-rudiment part are suppressed in favor of the developing axial elements during this process. However, in early echinoderms, extraxial skeleton makes up a far larger proportion of the body wall than axial, implying that metamorphosis was not as significant a part of the developmental trajectory as it is in more recently evolved taxa. Echinoderm radiation consists of a succession of apomorphies that reduced the expression of extraxial components but increased the influence of axial ones, with a concomitant increase in the prominence of metamorphosis.
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