Figure 6

Stage 7 (Postnaupliar segments; 7.4 (a-d); 7.5 (e-l)): (a, b, e, h, j) Sytox; (c, f, g, i, k) SEM; (d, l) Schematic drawing. (a) Stage 7.4, ventral view: The first antennae are located closer to the labrum anlage, which appears as a roundish protrusion and covers the stomodeum. The arrow points to the proctodeum. (b) Lateral view: The differentiation of the first and second thoracopods is indicated by slight indents (arrows). The first thoracopod shows one indentation, which marks the subdivision into later exo- and endopodite (arrow), second thoracopod shows two indentations (arrows) dividing the limb bud into one basal and two distal segments. The black arrowhead indicates the endopodite; the white arrowhead indicates the exopodite of the second antennae. (c) Ventro-lateral view: the SEM shows clearly the indentation of the first and second thoracopod (arrows); the depression medially to the indentation (asterisk) of first thoracopod is an artefact from mounting the embryo. (d) Schematic drawing of stage 7.4. (e, f) Stage 7.5, ventral view: the insertion of the first antennae abuts the insertion of the slightly more rectangular labrum. The first maxillary bud is positioned in a longitudinal line with the mandible, whereas the second maxilla lies slightly more laterally. The asterisk marks an artifact as a1 was lost during mounting. The arrow points to the proctodeum. (g) The tips of the endopodite of the second antenna shows three setae-buds, the exopodite of the second antenna shows two setae-buds (arrow heads). In addition, one bud of a developing seta is evident at the medial side of the endopodite approximately half way through its length. (h) The endo- and exopodite of the second antenna show each two slight indentations (arrowheads), which indicate the final three-partite segmentation. At the proximal segment of the second antenna an outgrowth becomes evident postero-laterally (asterisk). (i) The area around the slit-like proctodeum becomes bulky. The arrowheads point to a pair of setae posterior to the proctodeum. (j, k) The thoracopods show their continuing differentiation with a developmental gradient from anterior to posterior. (l) Schematic drawing of stage 7.5. a1, a2 = first and second antennae; ca = carapace; lb = labrum; md = mandible; ml = midline; mx1, mx2 = first and second maxillae; pro = proctodeum; t = thoracic segment; thp = thoracopod; vne = ventral neuroectoderm.