Fig. 5

Transmission electron (a-f, h-j) and light (g) micrographs of sample sections of the epidermis in 3–4 d post-fertilization (dpf) larvae of Quasitetrastemma stimpsoni. a Panoramic view showing the longitudinal section through the apical organ. b Transversal section of the apical plate (ap). c Longitudinal section of multiciliated cells of the apical plate (asterisk). d Middle region of multiciliated cells of the apical plate with longitudinal arrays of microtubules (arrows). e Transversal section through the sphincter (arrows) located in the basal half of the apical plate. f Longitudinal section of the basal half of the apical plate. Arrowheads point to circular musculature rows of the sphincter. High-magnification inset shows the basal extensions formed by apical plate cells which are attached to sphincter wall. g Panoramic view showing the anterior end of the larva with secretory part (sp) and ducts (d) of frontal gland. h Granular cell (asterisk) of frontal gland. i Bacillary type I (black asterisks) and type II (white asterisk) cell extensions. j Secretory extensions of bacillary type I cells extend through pore (p) in extracellular matrix (ecm). Abbreviations: ap–apical plate; ar–axial rootlet; ct–ciliary tuft; d–ducts of frontal gland; dbcI–frontal gland ducts of bacillary cell type I; dbcII–frontal gland ducts of bacillary cell type II; dgc–frontal gland ducts of granular cell; dmc–frontal gland ducts of mucoid cell; ecm–extracellular matrix; hr–horizontal rootlet; p–pore in extracellular matrix; sp–secretory part of frontal gland