Muscle tissue may have played a central role in the early evolution of mesoderm. The first function of myocytes could have been to control swimming and gliding motion in ciliated vermiform organisms, as it still is in such present-day basal Bilateria as the Nemertodermatida. The only mesodermal cells between epidermis and gastrodermis in Nemertodermatida are myocytes, and conceivably the myocyte was, in fact, the original mesodermal cell type. In Nemertodermatida as well as the Acoela, myocytes are subepithelial fiber-type muscle cells and appear to originate from the gastrodermal epithelium by emigration of single cells. Other mesodermal cells in the acoels are the peripheral parenchyma (connective tissue) and tunica cells of the gonads, and these also arise from the gastrodermis. Musculature in many of the coelomate protostomes and deuterostomes, on the other hand, is in the form of epitheliomuscular (myoepithelial) cells, and this cell type may also have been an early form of the mesodermal myocyte. The mesodermal bands in the small annelid Polygordius and in juvenile enteropneusts have cells intermediate between mesenchymal and epithelial in their histological organization as they develop into myoepithelia. If acoelomates were derived from coelomates by progenesis, then the fiber-type muscles of acoelomates could be products of foreshortened differentiation of such tissue. The precise serial patterning of circular muscle cells along the anterior-posterior axis during embryonic development in the acoel Convoluta pulchra provides a model for early steps in the gradual evolution of segmentation from iterated organ systems.
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