The acritarch wall consists of a complex of polymers known as sporopollenin. Most acritarchs are 20-150 |im across consisting of a vesicle enclosing a central cavity from which may project spine-like processes and crests. The shape of the vesicle, presence or absence of processes and of ornamentation are important criteria for defining species and genera. Compression, pyrite growth and other diagenetic processes and extraction techniques can considerably modify the original shape.
Many acritarchs have a wall composed of a single layer, whilst double and complex wall ultrastructures are not uncommon. Wall thickness can also vary considerably from <0.5 |im in Leiosphaeridium (Fig. 9.1a) through 2-3 |im in Baltisphaeridium (Fig. 9.1b) and up to 7 mm in Tasmanites (Fig. 9.3d, a prasinophyte). Wall ultrastructure is very poorly known. In the prasinophytes and Baltisphaeridium the vesicle ultrastructure comprises a single layer penetrated by narrow canals, usually only discernible by SEM and TEM. A thin two-layer wall structure separated by a zone comprising 0.5-2-|im-diameter pores has been described in Acanthodiacrodium (Fig. 9.1c) and is similar to that found in some dinoflagellates. A double wall occurs in Visbysphaera (Fig. 9.1d), which develops processes from the outer layer.
The exterior surface of the vesicle may be smooth, granulate, or may bear a variety of spinose or reticulate ornaments, indentations or micropores. These processes may be hollow and connected with the central cavity open (e.g. Diexallophasis, Figs 9.1e, 9.3a) or closed at the base, or solid. The tips of the processes can be simple, bifurcated, branched or connected by a thin membrane, the trabeculum (e.g. Tunisphaerid-ium, Fig. 9.1f). Processes on an individual vesicle are termed homomorphic if all are similar or heteromor-phic if more than one type is developed. Processes may be variously branched, smooth or bear a secondary ornament of granules.
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