ABSTRACT
The bewildering array of echinoderm diversity has made it difficult to base assessments of relationships on a generalized model of skeletal homologies. This is in stark contrast to other taxa, such as the Vertebrata, in which evolutionary change can be explored in the context of known homologies correlated with cranial, pelvic, pectoral, etc. skeletal systems. We have been working to provide such a system for the Echinodermata, and have been using the early results of our skeletal model to identify homologous elements within each of the major groups. The model recognizes two major body wall components: the axial and extraxial skeletons. The latter can be further subdivided into perforate and imperforate extraxial. Axial skeleton comprises primarily the paired plate columns of the ambulacra, but can include associated elements such as cover plates and spines. New axial elements are formed within the body wall in an area adjacent to the oculars (echinoids) and terminals (other echinoderms) so that the oldest plates are next to the mouth, and the youngest next to the ocular/terminal. Extraxial skeleton is essentially all body wall outside of the axial system, and comprises skeletal elements harboring (among other variously developed structures) the periproct, genital pores, and madreporite. In most cases we have examined, extraxial skeleton is characterized by the addition of skeletal elements anywhere within this part of the body wall, and not just at localized growth regions. We have concentrated on analysis of the Echinoidea and related taxa, but we also outline some of the criteria for recognizing axial and extraxial skeletons, and trace the major events of their evolution in the Echinodermata. Application of the growth zone criteria that we have established to recognize skeletal homologies leads to the conclusions that: 1) the “interambulacra” of echinoids are not homologous with those of any other echinoderm class, 2) echinoids are composed entirely of axial skeleton, except for the genital and periproctal plates in the apical system.
