SKELETAL ULTRASTRUCTURE IN SOME CYCLOSTOME BRYOZOANS OF THE FAMILY LICHENOPORIDAE

The ultrastructure of the skeleton is described in six species of lich enoporid cyclostome bryozoans using field emission SEM. Both interior walls (vertical, interzooidal walls, and brood chamber roofs and floor s) and exterior walls (basal walls) are initially secreted as tiny wed ge-shaped crystallites without a strong preferred orientation. These a re seeded directly onto pre-existing crystallites in the case of inter ior walls, but onto the organic cuticle in exterior walls, the bases o f the crystallites forming a tightly packed mosaic against the cuticle . With growth the wedges become longer, broader and relatively flatter , developing into platey crystallites. These crystallites grow predomi nantly distally (i.e. parallel to wall growth direction) and are close ly imbricated in a foliated fabric. Local disruptions to this pattern occur, especially in association with crystallite division along ''div ergent zones''. The pattern also breaks down in old walls where crysta llite growing edges become less evident and imbrication is poorly deve loped. Although conforming to this general model, some differences exi st between species of lichenoporids, and in the patterns found in diff erent parts of the skeleton (e.g. apertural spines). Lichenoporid ultr astructure differs from that of both cinctiporid and hornerid cyclosto mes: notably, lichenoporids lack the layer of transverse fibres found in cinctiporids, and their predominant distal growth direction of crys tallites contrasts with the proximal direction found in hornerids.