SKELETAL ULTRASTRUCTURES IN SOME CERIOPORINE CYCLOSTOME BRYOZOANS

The ultrastructure of the calcareous skeleton is described in nine spe cies of Recent cyclostome bryozoans belonging to the suborder Ceriopor ina. Two species of Heteropora have interior zooecial walls comprising a granular precursory layer followed by a thick layer of transverse f ibres and a subordinate foliated fabric with, in mature proximal walls , a semi-nacreous layer. The remaining seven species have interior wal ls with no transverse fibres and instead predominantly comprise a dist ally-imbricated. regularly foliated fabric overlying a granular precur sory layer. Older, proximal surfaces often have abundant screw disloca tions, but true semi-nacre is absent. Basal walls comprise an outer fi nely granular precursory fabric and planar spherulitic layer, succeede d by the same ultrastructural succession seen in the interior zooecial walls of the respective groups. Exterior walled diaphragms, peristome s and gonozooids similarly comprise an external fabric of planar spher ulitic calcite, lined internally by the predominant fabric seen in the interior walls. Ultrastructurally, therefore, cerioporines may be spl it into two groups with different fabric suites, the first resembling cinctiporids and many tubuliporines in having interior walls with fabr ics of transverse fibres, foliated crystallites and semi-nacre; and th e second resembling the rectangulates Lichenopora and Disporella in ha ving interior walls comprising only the foliated fabric. These finding s support the close phylogenetic relationship between cerioporines and other cyclostomes but suggest that the cerioporines may constitute ei ther a diphyletic or a paraphyletic group. Copyright (C) 1996 Publishe d by Elsevier Science Ltd on behalf of the Royal Swedish Academy of Sc iences