Myogenesis in P. lamarckii. Visualised by phalloidin staining. Scale bars = 50 μm. Apical is towards the top in all panels unless indicated. A. Early gastrula, lateral view. A horseshoe-shaped strip of muscle can be seen along the invaginating gut (arrowhead). Blastopore (b) is present at the posterior of the embryo. Cell junctions (zonulae adhaerens) are reactive to phalloidin at this stage (and in B – out of focus in these images). B. Late gastrula/early neurula, left lateral view. Blastopore has split and mouth (m) migrates anteriorly leaving anus (a) at blastoporal pole. Developing gut muscle is indicated by arrowhead. In panels A and B there is faint staining along the cell membranes, presumably due to the phalloidin binding to the zonulae adhaerens cell junctions. C. Early trochophore, left lateral view. Oesophageal musculature (oem) has begun to form around the oesophagus and mouth. The prototroch muscle (pm) has formed and the intestinal muscle (im) runs along the intestine from stomach (st) to anus. D. Complete trochophore, left lateral view. Further development of features in C. A metatroch muscle (mm) has also developed. E. Elongating trochophore, left lateral view. Longitudinal muscles (lm) run in pairs ventro-laterally and dorso-laterally towards the posterior of the larva. These fibres converge apically in the anterior muscle mass (am). F. Metatrochophore, latero-ventral view. Longitudinal muscles are more prominent and chaetal muscles have developed corresponding to the three new body segments (arrowheads). A thin, transverse muscle is seen in each segment (arrows, anterior most muscle obscured in this image). G. Juvenile, dorsal view. Anterior to the left. Longitudinal, oesophageal and intestinal muscles are still present. Muscles can be seen running along each filament in the branchial crown (bc) and the operculum (op). V-shaped branchial crown retractors (bcr) are present in the pharyngeal region. Small muscles can be seen in the newly developed collar (cm) and a ring of muscle is visible around the anus (ar).