Embryonic formation of the sucker. (A to C) Morphology of the sucker at hatching stage. (A) Sagittal histological section through oral part of arm stained with Masson’s trichrome stain. (B, C) Confocal projection of sucker in a sagittal position stained with phallacidin to visualize F-actin (B) and of the whole sucker stained with an antibody against acetylated tubulin to visualize nerve tracts (C). (D to O) Developmental stages for the suckers shown in each column are indicated at the top margin, staining/gene name of gene expression (purple) shown in each row is indicated on the left margin. (D to F) Sagittal histological sections through the oral part of the arms stained with toluidine blue (D, E) and Masson’s trichrome stain (F). (G to I) Ov-Mhc (J to L) Ov-Actin and (M to O) Ov-Tm expression during sucker development. Dotted lines in (A, B) encircle the position of the sphincter muscle. Dotted white line in (F) highlights the outline of the invaginated acetabulum and infundibulum. Dotted black line marks the border between acetabulum and infundibulum. Arrow points at extrinsic circular muscle. asterisk denotes the forming intrinsic sucker muscle. Abbreviations: ac, acetabulum; cil, cilia; cim, circular muscle; cin; circular infundibulum nerve; em, extrinsic musculature; ep, epithelium; inf, infundibulum; me, mesoderm; mm, meridional muscle; ngan-suc, nerves connecting ganglion and sucker; rm, radial muscle. Scale bar in A (refers to A to O): 20 μm.