Fig. 5

Histological details of the apertural area of Monobryozoon ambulans. A Cross-section of the distal apertural area showing a massive collar within the vestibulum and dense apertural muscles. B Longitudinal muscles showing the dense orificial sphincter and the distal, thick cuticle. C Diaphragmatic sphincter showing the distinct vestibular folds. D Detail of the diaphragm showing lateral expansions of the vestibular epithelium and corresponding muscles. Note the more prominent cells lining the vestibular fold. am—apertural muscles, at—atrium, c—collar, cut—cuticle, d—diaphragm, o—orifice, os—orifical sphincter, t—tenacle, ts—tenacle sheath, v—vestibulum, vf—vestibular fold, vw—vestibular wall