Fig. 3

Light (a) and transmission electron (b-f) micrographs of sample sections highlighting the epidermis in 2 d post-fertilization (dpf) larvae of Quasitetrastemma stimpsoni. a Panoramic view showing larva with small pit (arrow) on the anterior end and a well-developed frontal gland (fg). b Panoramic view showing the longitudinal section through the apical organ. High-magnification inset shows the nerve tract. c The transversal section of the extreme anterior end of larva with a central lumen (lm) surrounded by the papillae of secretory cells. d Transversal section of the apical plate consisting of double cell rings with a centrally situated lumen (lm). High-magnification inset on the bottom left shows the apicolateral junction (arrow). High-magnification inset on the top right shows the cilia root of the apical plate cells. e Transversal section of secretory tract (st) with centrally situated basal processes (bp) of apical plate cells. High-magnification inset shows the basal processes of apical plate cells. f Transversal section through rudiments of the brain (b) and rhynchocoel (rc). Abbreviations: ap–apical plate; bcp–bacillary cell papilla; bp–basal process; br–brain rudiment; fg–frontal gland; gcp–granular cell papilla; hr–horizontal rootlet; il–inner layer of apical plate; lm–lumen of apical plate; mcp–mucoid cell papilla; ol–outer layer of apical plate; rc–rudiment of rhynchocoel; st–secretory tract