SKELETAL ULTRASTRUCTURE IN SOME ARTICULATE CYCLOSTOME BRYOZOANS

The ultrastructure of the calcareous skeleton is described in Il speci es of articulate cyclostome bryozoans with elastic joints. Ten species have interior walls comprising semi-nacreous and pseudofoliated fabri cs without a precursory granular layer. Exterior walls consist of oute r, finely granular and planar spherulitic layers, succeeded by semi-na creous and pseudofoliated fabrics like those of interior walls. Outer fabrics are calcified as longitudinal strips, each corresponding to a planar spherulitic unit. Articulation surfaces comprise ring diaphragm s of very fine granular fabric with concentric laminations. The semi-n acre of walls adjacent to ring diaphragms contains minute holes. Crisu lipora occidentalis is unique in having interior walls of transverse f ibres succeeded by pseudofoliated fabric, articulation surfaces festoo ned with deep pits but lacking well-differentiated ring diaphragms, an d pseudopores containing sieve-like closure plates. The ultrastructure of most articulates resembles tubuliporine cyclostomes with dominantl y semi-nacreous walls, although the lack of precursory granular fabric in the interior walls and the presence of subcircular tablets of semi nacre (without six-fold sectoring) may be peculiar to articulates. In contrast, Crisulipora is more similar to other tubuliporines with tran sverse fibres, evidence which, together with other skeletal characters , suggests that Crisulipora evolved jointing independently of the rest of the articulate cyclostomes. (C) 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved.