Figure 4

Neuromuscular development in Axiothella rubrocincta. Confocal micrographs showing serotonergic immunoreactivity (red), muscles (green), and cell nuclei (blue). Anterior is to the left (A-E) or up (F-G). Scale bars: 75 μm (A-C), 135 μm (D-E), 150 μm (F), 55 μm (G). (A) Pre-segmental larva, left lateral view, right side is omitted. The cerebral ganglion (cg) with associated dorsal serotonergic perikarya (arrowheads), the circumesophageal connectives (cc), and the ventral neurite bundle (vnb) with linked ventral serotonergic perikarya (arrowheads) have already formed. The first longitudinal muscle fibers (lm) are located in a dorso- and ventro-lateral position. (B) 4-setiger stage, right lateral view. A dorsal (dlm) and ventral (vlm) longitudinal muscle band, three groups of setal muscles (sm), and an anal sphincter (as) are present. (C) Same individual as in B. (D) 5-setiger juvenile, ventral view. The anterior diagonal (d-vlm) and the longitudinal retractor sheath muscles (lm-rs) of the pharynx are derived from the ventral longitudinal muscles (vlm). (E) Same individual as in D. The body is elongated and the setigers (1s-7s) differentiate behind the head (h) region from anterior to posterior. (F-G) 7-setiger juvenile. (F) Right lateral view. Note the pair of ventral (vlm) and the single dorsal (dlm) longitudinal muscle strands. (G) Ventral view, depth-coded confocal image. The circular (cm-rs) and longitudinal (lm-rs) muscles of the retractor sheath form a basket-like structure. Anteriorly, straight ventral longitudinal muscles (s-vlm) are present in addition to the diagonal ventral longitudinal muscles (d-vlm). peristomium (per), prostomium (pro), setigers (1s-7s), pygidium (pyg).