|Major events in the evolution of echinoderms viewed by the light of embryology|
David, B.; Mooi, R. (1998). Major events in the evolution of echinoderms viewed by the light of embryology, in: Mooi, R. et al. (Ed.) Echinoderms: San Francisco: Proceedings of the Ninth International Echinoderm Conference, San Francisco, California, USA, 5-9 August, 1996. pp. 21-28
In: Mooi, R.; Telford, M. (Ed.) (1998). Echinoderms: San Francisco: Proceedings of the Ninth International Echinoderm Conference, San Francisco, California, USA, 5-9 August, 1996. A.A. Balkema: Rotterdam. ISBN 90-5410-929-7. XXXII, 923 pp., more
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The extraxtial/axiaI theory (EAT) for skeletal homologies in the Echinodermata recognizes two major body wall components: axial elements associated with the water vascular system, architecturally organized according to the "ocular plate rule" (OPR), and extraxial elements. The EAT shows that the amazing diversity recorded in body wall organization of adult echinoderms can be interpreted as a balance between axial and extraxial elements. At the time of its introduction, the EAT relied chiefly on the manner in which skeletal elements were organized, their association with specific somatic features, and observations of their ontogeny. Here we determine whether the axial/extraxial distinction is validated through integration of embryological information with the sources of data lists above. Sea urchins comprise an appropriate model depicting major events in echinoderm development; 1) the hydrocoel produces five primary lobes that elaborate according to the OPR to form radial water vessels; 2) larval ectoderm, the primary lobes and the somatocoel interact to form the rudiment; 3) the rudiment everts as the larval structures are resorbed. We show that axial and extraxial elements display radically different origins: axial elements are formed in relation to the rudiment whereas extraxial elements are inherited from the non-rudiment part of the larval body. Other echinoderms follow the same generalized developmental trajectory leading to 2-part somatic construction, and the degree of evagination of newly formed structures (rudiment) is directly related to degree of expression of axial elements in the adult body. Application of the EAT invicts reinterpretation of some groups and re-examination of major events in evolution of the phylum. Among the most significant proposals: edrioasteroids are the most primitive echinoderms (not carpoids and relatives); echinoid interambulacra are not homologous with the interradii of other echinoderms (a reaffirmation of conclusions reached by L. Agassiz, 1841; Mooi et al., 1994; inter alia); crinoid and asteroid arms are homologous; holothuroid radii can no longer be considered homologous with those of other echinoderms; the basic somatic structure of echinoderms can be viewed as a linear arrangement of major components rather than a radial arrangement or repeated units.