Schematic of embryonic and larval development in the sipunculan, Themiste lageniformis. (A) Timeline showing stages of development and corresponding age intervals in hours post-activation (HPA), along with a series of major developmental events that characterize the transition to each stage. (B) Diagrams of developmental stages 1 to 9, which depict prominent morphologic-characters occurring from first cleavage through formation of the non-feeding, pre-metamorphic pelagosphera larva. Stage 1, animal-pole view; stage 2, lateral view with animal hemisphere up; stages 3, 6, 7, 8 and 9 are lateral views with ventral to the left and anterior up; stages 4 and 5 are ventral views with anterior up. ag, apical groove; bm, blastomere; bp, blastopore; cu, cuticle; ee, egg envelope; en, endoderm; es, esophagus; ey, eye; fg, foregut; in, intestine; mc, metatroch cells; mg, midgut; mt, metatroch; n, nucleus; pb, polar body; pc, prototroch cell; pt, prototroch; rm, retractor muscle; st, stomodeum; to, terminal organ; vn, ventral nerve cord. Light gray shading in stages 3 to 6, prototroch cells; dark gray shading in stages 3 to 9, gut tissue; ventral nerve cords also are shaded gray in stages 7 to 9.