Parenchyma fine structure in Hypomyzostoma jasoni Summers et Rouse, 2014 and Myzostoma alatum Graff, 1884 (Annelida: Myzostomida)

Myzostomida are annelids with a reduced coelom and bodies filled with parenchyma containing muscle bundles and internal organs.Coelomic cavities are not visible even during the early developmental stages of larvae or during metamorphosis.Our TEM data on the fine structure of the parenchyma of adult Hypomyzostoma jasoni and Myzostoma alatum show that the parenchymatous cells of the species studied are polarized, their basal parts are contractile and connected by hemidesmosomes to the basal lamina of the ectodermal epithelium or the epithelium of the internal organs, and the proximal parts of these cells form numerous extensive sinuous projections that deeply penetrate into adjacent parenchymal cells; the cell membranes are connected by adherens junctions.The presence of cell junctions of these types indicates the myoepithelial origin of parenchymal cells in Myzostomida, which aligns with the mesodermal tissues in several other annelids like Phyllodocidae and Cossuridae.Thus, Myzostomida have a typical annelid body organization; the conception that myzostomid and Plathyhelminthes parenchymas may be homologous is not supported by our data.