SKELETAL ULTRASTRUCTURE IN THE CYCLOSTOME BRYOZOAN HORNERA

Scanning electron microscopy of calcified walls in two species of the cyclostome bryozoan Hornera has revealed previously undescribed detail s of skeletal morphology and growth. The calcitic interior walls of bo th H. robusta MacGillivray and H. squamosa Hutton have a laminated str ucture. Walls are extended at distal growing edges where the formation of new crystallites is concentrated and wall fabric is nacreous or se mi-nacreous. New crystallites are seeded on the surface of existing cr ystallites as six-sided rhombs. At the centres of the rhombs in H. rob usta there are often three 'spikes' which point towards alternate side s of the rhomb. Screw dislocations resulting in spiral overgrowths are also common at these distal wall edges. Wall thickening occurs furthe r proximally where walls develop a regularly foliated structure of imb ricated laths growing towards the colony base. Although often thought to be ubiquitous in cyclostomes, the division of walls into three laye rs (an inner, primary layer flanked on both sides by secondary layers) is absent in Hornera. Wall ultrastructure contrasts strongly with the lamellar-fibrous-lamellar structure recently described from cinctipor id cyclostomes. The c-axes of the crystallites are orientated perpendi cular to the wall surface in Hornera, unlike cinctiporids in which the y are orientated within the plane of the wall. Apparent similarities i n ultrastructure suggest that Hornera may provide a good model for wal l growth in extinct trepostome bryozoans.