A space or gap in the apical silica of the partectal ring, between the apex and the subapical projections on the sides of the ring. May have a pyriform shape (having the shape of a pear).

Specific to the genus Mastogloia. Visible with SEM; usually not discernable with LM. From Latin for gap. Plural form is lacunae. An example species having a lacuna is Mastogloia calcarea.

Mcalcarea Lacuna
Image Credit: Bart Van de Vijver
Scanning electron micrograph showing the internal view of a valve of Mastogloia calcarea, with the partectal ring in the foreground. The ring has a lacuna, or gap, near each apex.