Figure 13

Fine structure of the juvenile of Saccoglossus kowalevskii at 2 gill slit stage (~ 432 hpf). (A) Cells lining the proximal duct of the proboscis pore. These cells have developed circular myofilaments (myo). (B) Longitudinal section showing the musculature of the protocoel. Outer circular muscles cells alternate with inner longitudinal muscle cells. (C) Podocytes (po) resting on the basement membrane (bm) covering the glomerular sinus (gls). Pedicels (pe) form fenestrations between them. (D) SEM showing the highly flattened metacoelic cells resting on the intestinal region. The area containing the nucleus is protruding into the coelomic cavity. (E) Cross section of the pericardial cells (pdc) covering the heart sinus (hs). Basal myofilaments are orientated in longitudinal direction. (F) Cells lining the distal duct of the proboscis pore. These cells are flattened and highly microvillar (mv). (G) Cross section of the mesocoelic lining cells (msc). Myofilaments of the somatic lining are longitudinally arranged. Inset: in contrast, myofilaments of the visceral lining cells are orientated in circular direction. (H) The somatic lining of the metacoel is hyperdeveloped to form the substantial longitudinal strands of the ventral musculature within the trunk region. Inset: numerous adhesion plaques (adj) anchor the myofilaments to the extracellular matrix (ecm). ac accessory centriole, ci cilium, cr ciliary rootlet, cmyo circular myofilaments, lmyo longitudinal myofilaments, ms mesocoel, mt metacoel, ne neurites, nu nucleus, phc pharyngeal cell, za zonula adherens.