Fig. 3

Ultrastructure of the protonephridial complex and excretory ducts of Gyrocotyle urna. a Longitudinal section through the flame tuft; note zonulae adherentes between the internal and external ribs (insert: scale-bar: 0.5 µm). b Oblique section through distal extremity of the flame tuft; note septate junction between neighbouring terminal and proximal cells. c Terminal flame tuft showing septate junction at the distal extremity. d Adjacent proximal duct cell. e, f Larger excretory duct; note numerous surface microvilli, cytoplasmic processes and septate junction. g Epithelial wall of small excretory duct; note short surface microvilli and septate junction. h Luminal surface with microvilli; note thin electron-dense band in its central area. i Epithelial wall of larger excretory duct; note basal extracellular matrix supporting basal membrane. j, l Epithelial wall of the largest excretory duct showing large lateral flame, two distinct zones of the epithelial cytoplasm, septate junction and thick layer of extracellular matrix. k Inserted striated conical rootlets of the cilia of the lateral flame. Abbreviations: b, medial band of microvilli; bb, basal body of the cilium; cp, cytoplasmic process; dl, duct lumen; ec, epithelial cytoplasm; em, extracellular matrix; er, external rib; fr, fibrous rootlet; ft, flame tuft; i, invaginations of the basal membrane; ir, internal rib; lf, lateral flame; mf, muscle fibres; mv, microvilli; n, nucleus; sj, septate junction; sr, striated rootlet; v, vesicles; za, zonula adherens