The "primary" cell of chrysophytes contains two
specialized flagella. The active, "feathered" (with
mastigonemes) flagellum is oriented toward the moving direction. The smooth passive flagellum, oriented toward the opposite direction, may be present only in rudimentary form in some species. An important characteristic used to identify members of the class Chrysophyceae is the presence of a siliceous cyst that is formed endogenously. Called
statospore,
stomatocyst or
statocyst, this structure is usually globose and contains a single pore. The surface of mature cysts may be ornamented with different structural elements and are useful to distinguish species. • Most members are unicellular
flagellates, with either two visible flagella, as in
Ochromonas, or sometimes one, as in
Chromulina. The
Chromulinales as first defined by Pascher in 1910 included only the latter type, with the former treated as the order
Ochromonadales. However, structural studies have revealed that a short second flagellum, or at least a second basal body, is always present, so this is no longer considered a valid distinction. Most of these have no cell covering. Some have loricae or shells, such as
Dinobryon, which grows in branched colonies. Most forms with silicaceous scales are now considered a separate group, the
synurids, but a few belong among the Chromulinales proper, such as
Paraphysomonas. • Some members are generally
amoeboid, with long branching cell extensions, though they pass through flagellate stages as well.
Chrysamoeba and
Rhizochrysis are typical of these. There is also one species,
Myxochrysis paradoxa, which has a complex life cycle involving a
multinucleate plasmodial stage, similar to those found in
slime molds. These were originally treated as the order
Chrysamoebales. The superficially similar
Rhizochromulina was once included here, but is now given its own order based on differences in the structure of the flagellate stage. • Other members are non-motile. Cells may be naked and embedded in
mucilage, such as
Chrysosaccus, or coccoid and surrounded by a cell wall, as in
Chrysosphaera. A few are filamentous or even
parenchymatous in organization, such as
Phaeoplaca. These were included in various older orders, most of the members of which are now included in separate groups.
Hydrurus and its allies, freshwater genera which form branched gelatinous filaments, are often placed in the separate order
Hydrurales, but may belong here. == Classifications ==