Description of Difflugia lacustris:
The shell is transparent or hyaline, elongate, cylindrical or slightly pyriform. It is composed of small to medium pieces of quartz, diatom frustules and small siliceous flagellate cysts blended together to form a thin structure intermediate between smooth and rough. Only small areas of organic cement occur at the junction of the shell components. The cement is in the form of thick-walled rings, between 0.7-0.8 microns in diameter, perforated with either three or four holes, 0.12-0.16 microns in diameter, which gives these units a similar shape to a button. The cement may occasionally be seen either as rings with a slight indentation or as a network of joined rings. When organised as a network the walls of individual rings may be fused together but the typical button-like form are usually seen at the edges. The aperture is usually circular and surrounded by small particles so that the margin is smooth. Length 140-231 microns, breadth 63-94 microns, diameter of aperture 26-42 microns.