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.