[Foraminifera] Opening (or multiple openings) in the chamber through which cytoplasmic strands emerge. Shells may have one aperture or several; in the latter case there usually is one main (larger) opening and several smaller ones (secondary or supplementary apertures).
(Fo5c)
(Fo5d)
(Fo5e)
(Fo5f)
(Fo5g)

[Radiolaria Polycystina] A large basal opening in the last segment of the Nassellaria (also called mouth), which can be open (shell structure) or obliterated by a porous plate or velum (A. denticulata group? intro).
(Fo5h)

[Radiolaria Phaeodaria] Usually odd, major hole or opening in the shell-wall (see mouth).

[Gastropoda, general] The opening of the shell through which the animal can expand (never used for the mouth in the soft parts).

[Pteropoda] The opening of the shell through which the animal can expand (not to be confused with the mouth of the soft body). In the Limacinidae and Clioinae the aperture is rounded or triangular and continuous. In the remaining Cavoliniidae the aperture is composed of the main central part and two lateral slits. The height of the aperture is equal to the maximum height of the shell aperture, in straight ventral view or oral view.

Limacina in oral view
Cavolinia in ventral view
Cuvierina detached protoconch
Peraclis in oral view

Alternative forms for Aperture : Aperture sinusaperture base, [Pteropoda] Shell aperture, [p][/p].