Fig. 5

S. alveolata. Tentacular filaments, TEM, cross-sections. a Three multiciliated cells with long and densely arranged (cc) located on each side of the frontal ciliary band, groups of cilia distinctly separated from each other by the unciliated periphery of ciliated cells, cuticle (cu) devoid of collagen fibers, penetrated by microvilli (mv), note distal branching of microvilli (mv) (boxed and inset a’), b Right part of u-shaped hyaline cartilage structure (hc) surrounded by ECM and unciliated supportive cells (suc); groups of sensory cilia (sc) located on abfrontal side of filament; boxed area enlarged in (g, c) Three receptor cells on abfrontal side, sensory cilia with basal body and short rootlet; cells adjoined by zonulae adherentes (arrows) and septate junctions (sj), d Central strand of tissue laterally located in concave part of u-shaped cartilaginous (hyaline) structure; tissue primarily comprises longitudinal muscle fibers (lm) with a few supplementary transverse fibers around a coelomic cavity (coe), e Apices of coelothelial cells forming coelomic cavity (coe), cells connected by zonulae adherentes (arrows), f Detail of longitudinal and transverse muscle fibers, boxed area of (d, g) Enlargement of an outer section from b (boxed) showing u-shaped structure surrounded by ECM (ecm), in contact with intermediate filaments (if) via hemidesmosomes (arrows), note small longitudinal muscle fiber outside central strand of tissue (arrowhead), asterisks: intercellular spaces basally between supportive cells, h Detail (boxed in g) of material comprising u-shaped hyaline cartilaginous structure. Abbreviations: afs - abfrontal side, cc - ciliated cell, coe - coelom, cu - cuticle, ecm - ECM, mv - microvillus, nc - nucleus, sc - sensory cilium, sj - septate junction, za - zonula adherens