Figure 2

Myogenesis in Argyrotheca cordata. CLSM maximum projection micrographs, anterior faces upward. F-actin is labelled in red and cell nuclei are labelled in blue to indicate the outline of the specimens. Scale bars equal 50 μm. (A) Early larva in dorsal view with the first F-actin signals from microvilli (mi) within the setal canals. (B) Early three-lobed larval stage, postero-dorsal view, showing apical lobe (AL), mantle lobe (ML), pedicle lobe (PL), first rudiments of the pedicle musculature (pm), and microvilli (mi) in the setae pouches. (C) Larval stage with fully differentiated lobes and short setae in ventral view (corresponding to the larval stage shown in Fig. 1E). Visible are the apical transversal muscle (atm), the apical longitudinal muscles (alm), the interconnecting apical muscles (iam), the interconnecting mantle muscles (imm), the longitudinal muscles (lm), the foregut rudiment (fg), the hindgut rudiment (hg), the pedicle muscles (pm), microvilli (mi), the setae pouch musculature (arrowheads), and the setae muscles (sm). (D) Lateral right view of a fully developed three-lobed larva with the U-shaped muscle (empty arrows) on the ventral side. At this stage, the setae pouches are interconnected by a circular mantle muscle (arrow). New at this stage are the central mantle muscles (empty arrowhead). Further indicated are the setae pouch musculature (arrowheads), the setae muscles (sm), the serial mantle muscles (double arrowheads), the pedicle musculature (pm), and the apical longitudinal muscles (alm). (E) Same stage as in D, ventro-lateral view. The U-shaped muscle (empty arrows) is directly connected to the pedicle muscles (pm). In addition, the apical transversal muscle (atm), the apical longitudinal muscles (alm), the serial mantle muscles (double arrowhead), the central mantle muscles (empty arrowheads), the setae pouch muscles (arrowheads), the setae muscles (sm), the circular mantle muscle (arrow), and the primordial hump (asterisk) are indicated. (F) Fully grown larva in ventral view with circular mantle muscle (arrows), serial mantle muscles (double arrowheads), setae pouch muscles (arrowheads), setae muscles (sm), pedicle muscles (pm), longitudinal muscles (lm), apical longitudinal muscles (alm), apical transversal muscle (atm), interconnecting apical muscles (iam), primordial hump (asterisk), and central mantle muscles (empty arrowheads).