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Abstract: Echinoderms are so highly derived compared with other deuterostomes, including their sister group, hemichordates, that comparisons of body plans are sometimes accompanied by points of view enjoying varying levels of morphological, paleontological, and especially, embryological support. No echinoderm taxon has been the subject of more contentious debate than the carpoids, a disparate assemblage of non-pentaradial, flattened echinoderms that includes the Cincta, Ctenocystoidea, Soluta, and Stylophora. Because of their unusual morphologies, the phylogenetic position and significance of carpoids concerning the origins of the Echinodermata are still being evaluated. A detailed review of carpoid research over the past century and a half reveals that the debate largely results from methodological issues employing two basic, but very different models. Conceptual models, usually imbued with Haeckelian principles, consider the absence of a single character (pentaradial symmetry) as a recapitulation of the pre-metamorphic larval stage of echinoderms, forcing unusual taxa that also lack pentaradiality down the phylum's phylogenetic tree. Such scenarios assume that first echinoderms had a bilaterian-type anterior-posterior axis. Empirical models rely on comparison of non-pentaradial early forms with a wide array of data obtained from extant and fossil echinoderms. These data support a view in which larval morphologies of echinoderms are not represented in the fossil record of echinoderms, and that pentaradial symmetry was secondarily lost in carpoids, just as it was in many other coeval types of echinoderms.