Fig. 2

S. alveolata. Opercular papillae. a, b SEMs. c-h longitudinal TEM sections. a Finger-shaped opercular papillae (op) located underneath the opercular paleae (pa), circle of papillae surrounding entire head region, b Tip of opercular papilla showing several bundles of sensory cilia (arrows), c Glandular cells (gc) with electron-lucent content open through indistinct pores in the cuticle, connected to neighboring cells via zonula adherens (za), d Longitudinal section of a papilla with prominent epidermal layer surrounding strand of connective tissue (ct) extending from opercular connective tissue. Several glandular cells (gc) between epidermal supportive cells, arrows point to ciliated receptor cells; anterior to the left, figures c, g, h represent magnifications from ventral side of (d, e) ECM between epidermis and connective tissue (arrows). Longitudinal muscle fibers (mu) between connective tissue and ECM; arrowheads: hemidesmosomes, (f) Bundles of additional circular muscle fibers (mu) in the connective tissue (ct), (g, h) Details of receptor cells with cilia and accessory centriole (ac), microvilli (mv) penetrating the cuticle (cu), groups of receptor cells comprise monociliary and multiciliary cells, cuticle devoid of collagen fibers, receptor cells adjoined to each other and to adjacent supportive cells via zonulae adherentes (za). Abbreviations: ac - accessory centriole, bp - basal plate, cl - cilium, ct - connective tissue, cu - cuticle, gc - glandular cell, if - intermediate filaments, mi - mitochondrion, mu - musculature, mv - microvillus, nc - nucleus, op - opercular papilla, pa - opercular palea, za - zonula adherens