Fine Structural Observations on the Extracellular Matrix (ECM) of Xenoturbella bocki Westblad, 1949

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Abstract
Fine structural observations on the extracellular matrix (ECM) and connective tissue system of the enigmatic vermiform animal Xenoturbella bocki have demonstrated a complex and interesting organization of the ECM. Most conspicuous is the subepidermal membrane complex (SMC), and the major part of the ECM is present in this structure, which consists of a limiting basal lamina on each side of a central thick filamentous layer, probably of a collagenous nature. Distinct anchoring filaments are found as part of the SMC. A basal lamina is also observed in connection with the gastrodermal cells. The mesoderm is represented by a loose, ill‐defined parenchyma, without filaments. The SMC is discussed in relation to subepidermal basal matrices occurring in turbellarians and enteropneusts, the two groups to which a relationship to Xenoturbella previously has been suggested. Comparisons between the ECM in Xenoturbella and in turbellarians do not support the notion of a relationship between these groups. Conversely, the present study strengthens previous indications of distinct similarities between certain characteristics of the epidermis and the SMC present in both enteropneusts and in Xenoturbella.