SKELETAL ULTRASTRUCTURE OF THE EARLY ASTOGENETIC STAGES OF SOME CYCLOSTOME BRYOZOANS

The calcitic protoecia of cyclostome bryozoans have remarkably uniform skeletal ultrastructure in three suborders (Tubuliporina, Cancellata and Rectangulata). The basal wall or floor has a fine outer granular l ayer succeeded by planar spherulitic fabric, internally lined by irreg ular semi-nacre. The roof of the disc comprises an outer granular laye r with an inner lining of semi-nacre continuous with that of the floor ; planar spherulitic fabric is absent. Growth of the floor of the disc is initiated around the circular outer rim and continues centripetall y to the centre; the inner lining has no prevailing growth direction. The gently domed roof also initiates around the outer rim, and grows i n strips, which grow toward the centre, then distally toward the dista l tube of the ancestrula which has a fully adult ultrastructure. Proto ecial ultrastructure is independent of adult ultrastructure. The unifo rmity of skeletal ultrastructure in cyclostomes corresponds with the c lose similarity of larvae and post-settlement metamorphosis in the ord er. The fabric suite of the protoecium resembles the skeletal ultrastr ucture of Palaeozoic stenolaemates. The primitive fabric condition is retained by some tubuliporines and cancellates. Complex multilayered f abric suites may have evolved in the Mesozoic by addition of new fabri c types. (C) 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved.