FIGURE 11. Scanning electron micrographs of the ctenidium of Thyasira excavatus Dall, 1901 [= Conchocele excavata (Dall, 1901) in Coan & Valentich-Scott, 2012]. A, cross section showing full reflection of both demibranchs; B, transverse section of a single demibranch; C, frontal surface; D, frontal surface partly torn away to reveal laminar filaments and inter-lamellar junctions; E, lateral view of a filament showing lozenge shaped bacteriocytes; F, transverse section through the abfrontal zone showing bacteriocytes and bacteria. abs, abfrontal surface; asc, ascending lamella of demibranch; bc, bacteriocyte; dsc, descending lamella of demibranch; fs, frontal surface; ifj, inter-filamentar junction; id, inner demibranch; ilj, inter-lamellar junction; lc (b), base of lateral cilia; od, outer demibranch; uf abs, unfused abfrontal filaments.