SKELETAL ULTRASTRUCTURE IN SOME TUBULIPORINE CYCLOSTOME BRYOZOANS

The ultrastructure of the calcareous skeleton is described in twenty-o ne species of recent tubuliporine cyclostome bryozoans, using field em ission SEM. The succession of skeletal fabrics in interior walls may b e classified into four different fabric suites. The first-formed part of the calcitic skeleton in all species for which it has been observed is a precursory fabric of tiny, wedge-shaped crystallites. This is su cceeded in about half of the species studied by a fabric of transverse fibres, followed by foliated fabric and often semi-nacre (fabric suit e 1). Most of the remaining species lack transverse fibres and have in terior walls largely comprising semi-nacre (fabric suite 2). A few spe cies have skeletons consisting of predominantly distally-oriented, irr egularly or regularly foliated fabric (fabric suite 3). A single speci es has a skeleton of proximally-oriented foliated fabric (fabric suite 4). Basal exterior walls in all species have a precursory fabric of t iny wedge-shaped crystallites without a strong preferred orientation, deposited directly upon the organic cuticle, followed by a layer of pl anar spherulitic structure, which in turn is succeeded by a similar fa bric to that developed in the interior wall of the species concerned. Outermost layers of frontal exterior walls exhibit one of the followin g combinations of three fabrics: an outer layer of (1) finely granular or wedge-shaped crystallites; a thin dense granular layer followed by (2) distally accreting planar spherulitic fabric, or (3) obliquely ac creting planar spherulitic fabric growing partly towards the midline o f the frontal wall. Terminal diaphragms usually have outer layers domi nated by planar spherulitic ultrastructure with centripetal growth dir ections. The fabric suites present in tubuliporines encompass most kno wn fabrics found in the other cyclostome suborders and support the not ion that this species-rich suborder occupies a central position in cyc lostome evolution. (C) 1997 Published by Elsevier Science Ltd on behal f of The Royal Swedish Academy of Sciences.