Fig. 4

Early mitraria larvae and the beginning of organogenesis. CLSM images of early larval stages from a 11 hpf to k 27 hpf. Images on the left column are ventral views with anterior facing up, while those on the right are lateral views with anterior facing left, except for (b) which is an apical view. a–c Ciliogenesis starts at 11 hpf with the formation of the apical tuft and the prototroch dividing the embryo into an apical episphere and a vegetal hyposphere. d, e A short neurotroch then forms on the posterior side of the blastopore by 13 hpf. By this stage the blastopore elongates to form the presumptive mouth, but still remains open. f, g By 18 hpf, the early mitraria now has a secondary ciliary band, the metatroch, in addition to a complete gut, larval muscles and chaetae. h–k Soon after, the larva grows and expands the metatroch throughout the hyposphere, while more muscle develops, including circular muscles around the foregut. an: anus; at: apical tuft; bl: blastocoel; bp: blastopore; cb: chaetoblast; cht: chaetae; cs: chaetal sac; dl: dorsal levator; em: esophageal muscle; fg: foregut; fgm: foregut circular muscle; hg: hindgut; mg: midgut; mo: mouth; mt: metatroch; np: nephridia; nt: neurotroch; pt: prototroch; rg: refringent globules; rm: retractor muscle; tm: membrane between chaetal sac and blastocoel. Scale bar is 50 µm