The most abundant constituents of marine phytoplankton are the diatoms (Bacillariophyceae), single-celled algae. The delicate pattern of the cleaned frustule (shell) of diatoms like this Actinoptychus splendens makes them excellent subjects for microscopy. The cell walls are elaborately ornamented with patterning that is very accurate. Diatoms are often used to calibrate and test microscope optics.