Fig. 12
Ultrastructure of chaetal muscles of the late metatrochophore of Siboglinum fiordicum. A–C Ultrastructure of the muscle fibers of the chaetal sacs at the parasagittal sections. A Overview of the chaetae position in the first opisthosomal segment; arrowheads show the non-muscular second septum. B Close-up of the chaetal sac formed by the longitudinal muscle bundles and follicle cells chaetoblasts. C The chaetae raised by muscles, the muscles are attached to the cuticle through tonofilaments. fc—follicle cell, co—opisthosomal chaetae, mlch—longitudinal muscles of the chaetal apparatus, tf—tonofilament