Article: Ultrastructure of the protegulum of some acrotretide brachiopods

Electron microscopic studies of the exteriors of a number of acrotretacean genera show that their protegula were ornamented by shallow pits, usually with a coarser set, about 2 nm in diameter, partially or completely segregated from one another by groups of smaller ones about 300 nm across. The pattern is identical with the mould of a bubble raft, and consideration of the structure of the periostracum of living terebratellaceans suggests that the pits ornamenting the acrotretacean protegulum are the moulds of a highly vesicular periostracum up to 3 fan thick with a thin (10 nm) inner sealing membrane. Such a periostracum would have afforded larvae extra buoyancy immediately prior to their settlement on the substrate. The absence of pit ornamentation from the adult shell is believed to indicate the development of an inner sealing membrane which was sufficiently thick to mask the vesicular nature of the adult periostracum. Nothing comparable with this ornamentation is known in other Acrotretida, although the protegulum of Eoconulus, which may be an aberrant craniacean, included a mineral mesh of regular, alternating, circular holes about 8 nm in diameter which have been attributed to differential secretion beneath a non-vesicular periostracum comparable with that covering living Crania.