Investigated morphological features of different semaphoronts of
(Branchiopoda). External morphology and developing musculature of A. franciscana larvae. (a) Overview of semaphoront Af II. Postnaupliar muscle precursors are not present, except for visceral musculature of the gut and proctodeum. (b) Higher magnification image of the region demarcated by a white rectangle in (a). Appendage anlagen are seen in the first [T1] and second thoracic segment, but not in the first and second maxilla segment (demarcated with a bracket). (c) Semaphoront Af III. Appendage buds become visible in the first and second maxillary segment. Longitudinal muscle precursors extend from the first maxilla segment into the anterior thoracomeres [lmp-mx1], [lmp-mx2], [lmp-t1]. Medial and lateral extrinsic appendage muscle precursors appear in the first maxilla segment and the first thoracopod segment [mx1-m], [mx1-l], [t1-m], [t1-l]. (d) Semaphoront Af IV. A lateral extrinsic muscle precursor appears in the second maxillary segment [mx2-l]. (e) Semaphoront Af V. All muscle precursor groups show advanced differentiation and a medial extrinsic muscle precursor of the second maxilla has appeared [mx2-m]. (a, c, d) Maximum intensity projection of phalloidin-signal (red) and nuclear signal (TOPRO-3), (blue). In (a), also autofluorescence signal of the cuticle is shown (green). (b) Blend projection of phalloidin-signal (orange) and nuclear signal (TOPRO-3), (light blue) and cuticle autofluorescence signal (green). (e) Blend projection of phalloidin-signal (light grey). In (c, d, e), extrinsic appendage muscle precursors are reconstructed and highlighted. Medial muscle precursors are shown in yellow and orange, lateral extrinsic precursors are shown in blue and light blue. Abbreviations: A1–T4 appendage anlagen of respective segments, lmp-mx1, lmp-mx2, lmp-t1 longitudinal muscle precursors of the respective segments; md-m, mx1-m, mx2-m, t1-m medial extrinsic appendage muscle precursors of the respective segments, md-l, mx1-l,mx2-l, t1-l medial extrinsic appendage muscle precursors of the respective segments. Scale bars are 100 μm in all panels.