In bordered pits the secondary wall bends as an arch over 
the pit chamber which opens into the cell by a small 
aperture at the centre.this is called pit aperture.

The thin membrane present in the pit chamber is called 'pit 
membrane'.
In the pitmembrane there is a thick lens like structure 
called "TORUS".

The thin part of the membrane present around the base of 
the torus is called "margo".

the margo exhibits movements due to which the torus is 
closely appressed to the pit aperture.