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

[Radiolaria Phaeodaria] A large, dominant pore or opening in the shell (Phaeodarian cell features 2, Phaeodarian cell features 9).

[Hydromedusae] Opening of the manubrium to the exterior, may be simple and circular, or with simple or complicated lips; see also lips (Medusa, Medusa 2).

[Gastropoda] The opening of the buccal mass (never used for the shell opening.