Fig. 4

Competent larva of Siboglinum fiordicum, by SEM (A) and phalloidin and DAPI staining visualized by CLSM (B–H). A Dorsal view of the competent larva having the prostomium (pr), peristomium (pe), forepart segment (fr), trunk segment (tr), ca 6 opisthosomal segments (o1, o2-6). C DAPI staining marks the accumulation of cells in the region of the tentacle rudiment (tb), frenulum (f), and vice versa, the rare location of large cells, such as trochoblasts of the prototroch (p) and mesotroch (m), and myoepithelial cells that form the muscles of the first septum (sI). C–E Dorsal, lateral and ventral views of the competent larvae with the circular musculature in the anterior end and almost complete layers of the prominent longitudinal musculature in the posterior end of larva. Note in (C, D) the periglandular musculature (mtg) which are equipped with the radial fibers extending from the main longitudinal strand (mrtg). F, G DAPI and phalloidin stainings the postannular area of the trunk segment (ttr), which will encompass the trophosome in juveniles. H Ventral view of the opistosoma, the seventh circular muscle added (moc7) at the posteriormost end, pygidium (py) is hardly distinguished. ca—annular chaetae, co—opisthosomal chaetae, f—frenulum, fr—forepart, m—mesotroch, mc—circular musculature, mca—muscles of annular chaetae, mcc—lateral circular muscle fibers interconnecting the longitudinal strands, mcch—circular muscles of the chaetal apparatus, md—dorsal longitudinal muscle strand, mf—anterior circular muscle of the sIII, mls—longitudinal muscle of the first septum, mlvl—lateral component of the ventrolateral longitudinal muscle strand, moc—circular muscles in the opisthosoma, mrtg—radial muscles interconnecting mtg and longitudinal muscle strands, mtg—muscles of the tubiparous glands, mv—ventral longitudinal muscle strand, mvvl—ventral component of the ventrolateral longitudinal muscle strand, o1-o2—the first-sixth segments of the opisthosoma, p—prototroch, pe—peristomium, pr—prostomium, py—pygidium, sI—the first septum in the larval ontogenesis, sII—the second septum in order of formation, sIII—the third septum in order of formation, tb—tentacular anlage, tm—muscle bundles of the tb, tr—trunk segment