Examples of soft tissue developmental stages of S. purpuratus larvae as defined in Table1(roman numerals). All images (except as indicated) are close-up views of the rudiment in the same orientation as indicated in A, i.e., an abanal view of the larvae (sensu ). Insets in C-K show schematic views of the stages in question; see also Table 1. (A) Overview diagram of larva with corresponding DIC image of actual larva (B – anal view). (C-F) Black arrow- invaginating ectoderm (e); white arrow- hydrocoel (h). (C) Stage i, approximately 60% ectodermal invagination. (D) Stage i, approximately 90% ectodermal invagination (anal view). (E) Stage ii, contact of invaginating ectoderm with hydrocoel (anal view). (F) Stage iii, ectoderm flattening alongside hydrocoel (anal view). (G) Stage iv, 5-fold mesoderm (first visible sign of 5-fold symmetry); white arrowheads- 3 of the 5 primary podia anlage are visible in this view; black arrow- invaginating ectoderm. (H) Stage v, 5-fold ectoderm; arrowheads as in E. (I) Stage vi, primary podia stage, arrowheads indicate two of the forming podia. (J) Soft tissue Stage vii with folded primary podia. (K) Soft tissue Stage viii with primary podia bending at the tip and touching each other (anal view). Note that schematic in A does not show the pair of preoral arms for simplicity. Abbreviations: Ala – anterolateral arms; M – Mouth; St – stomach; PD – postdorsal arms; PO – postoral arms, PrO – preoral arms; h – hydrocoel; e – invaginating ectoderm (=vestibule); L – Left larval side; R – Right larval side; A – anterior; P – posterior; An – anal; Ab – abanal. Scale bars: B- 200 μm C, D, E – 35 μm; F - 25 μm; G, H, I - 40 μm, J - 30 μm; K - 40 μm.