A constriction site at the junction of the plasma, flagellar and flagellar pocket membranes where the flagellum emerges from the cell body. Observed in some unicellular eukaryotic species such as Chlamydomonas, Giardia and Trypanosoma.
Comment:
Note that a 'flagellar collar' is described in Choanoflagellates, but this is different from the ciliary pocket collar found e.g. in Giardia; in Choanoflagellates, a single apical flagellum is surrounded by a collar of 30-40 microvilli.