Fig. 4

The process of sucker formation of the second arm in S. lycidas during embryonic development. Arms are oriented with distal to the right. a-c Confocal stacks of arms from oral view. Arrows indicate sucker field ridges on the arm tips. Arrowheads indicate the proximal constriction of the sucker field ridges. a St. 30–33 (n = 3). b St. 34–36 (n = 4). c St. 39 (n = 5). d A sagittal optical section at St. 39 (n = 5). e, f Optical sections of the base of sucker buds at St. 34–36 (n = 4). White dotted frames indicate the gourd-shaped actin localization. g-i the magnification of sagittal section; distal (g), middle (h), and proximal (i) parts. White dotted frames indicate the constricted sucker bud (h) and the primordial sucker with differentiated structures (i). j-n Histological sections in sagittal planes, stained with hematoxylin and eosin at St. 34–36 (j) and St. 39 (k). l-n Magnified images focusing on the primordial area at St. 39; distal (l), middle (m), and proximal (n) parts. Brackets indicate the regions where epithelial tissues are composed of one cell layer. Arrowheads indicate proximal constriction of sucker field ridges. Scale bars: 50 μm. Abbreviations: ac, acetabulum; in, infundibulum; p, peduncle