Fig. 6

Ultrastructure of somatic musculature, the parafrontal sections of the early metatrochophore (A, C, D) and parasagittal sections of late metatrochophore (B) of Siboglinum fiordicum. A The opisthosomal tagma with the incomplete first septum and with the growing second septum. Through the lumen in the first septum the gut rudiments passes. B Tubiparous glands secreting the tube material, note the cup-shaped microvilli facing he lumen of the gland. Thin layer of the muscle surrounding the gland, especially around the developing glandular duct, which supposedly pushes the tube secrete outside when muscles contract. C Left ventrolateral longitudinal muscle (mvl) strand extending from the trunk segment to the first segment of opisthosoma, and septal longitudinal muscle (mls) bands extending from the septum I to the opisthosome. Coelom of opisthosoma lined with the large myoepithelial cells (marked in purple). D Circular muscles (mc) lie inner to the longitudinal muscles, at the prototroch level. aj—adherens junction, co—opisthosomal chaetae, ecm—extracellular matrix, gr—gut rudiment, flo—finger-like outgrowths, mc—circular muscles, mls—longitudinal muscles of the first septum, mrl—rootlets of the mesotroch cilia, mtg—musculature of the tubiparous gland, mvl—ventrolateral longitudinal muscle strand, sI—the first septum, sII—the second septum, tg—pouch of the tubiparous gland