Pinnularia is a genus of freshwater
algae in the class Bacillariophyceae, the
diatoms. An elongated elliptical
unicellular organism, it is usually found in ponds and moist soil. The
cell walls are composed chiefly of
pectic substances on a rigid
silica framework, and are composed of two halves known as
thecae (or, less formally, valves). This
Pinnularia major specimen, about 0.3 millimetres (0.012 in) in length, was photographed through a light microscope.
Photograph credit: Anatoly Mikhaltsov