Skeletal homologies of echinoderms

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Abstract
The impressive array of disparity within the Echinodermata can be explained by the interplay of components (particularly skeletal elements) making up two major body wall regions: axial and extraxial. Axial skeleton comprises paired plate columns of the ambulacra, formed according to the Ocular Plate Rule (OPR) and in association with the water vascular system. Extraxial skeleton (subdivided into two subtypes: perforate and imperforate) is not formed according to the OPR, and new elements can be added anywhere and at any time within extraxial body wall. Recent work on early development of echinoderms reveals that axial skeleton is formed as an integral part of the rudiment, but that extraxial skeleton is derived from the non-rudiment part of the larval body. In addition to displaying such fundamental embryological and ontogenetic differences, the body wall regions have distinctive distributions and topologies that can be used to formulate criteria for their identification in any echinoderm regardless of how esoteric their morphology might be. Like the system of homologies that has long been established for vertebrates, the model of axial and extraxial skeletal types can be used to explore relationships among Recent and fossil taxa alike. Application of the model also leads to reassessment of previously published morphological characters and phylogenies.

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