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Miniature marine organism

Spike Walker

Download this image from Wellcome Collection.

Light micrograph of a miniature organism found in the sea, part of a group called Foraminifera (pronounced for-am-in-if-er-ra). This organism is approximately 1.4 mm long and has an outer shell made of calcium carbonate called a test. When the organism dies, its test is eventually turned into rock such as chalk or limestone on the seabed. This specimen came from the South China Sea and has a narrow neck with an external corkscrew-like thread.

Why is there a black cross on the organism?

Polarised light microscopy involves illuminating a sample using light waves that are all vibrating along the same plane. If a biological sample is placed between two polarising filters that are 'crossed', or at right angles to each other, light that passes through the first filter cannot pass through the second filter unless it is bent (refracted) to be in the same plane of orientation as the second filter. Some biological materials with an orderly molecular structure (such as starch and certain minerals) can refract light like this (the effect is called birefringence). A tell-tale sign of birefringence is the appearance of a black cross in the image of the sample, similar to a Maltese cross. Spike explained: "The black cross is an artefact resulting from the use of crossed polarising filters in producing the image, and indicates that the test was built up from concentric layers. If the object happens to be more or less spherical and has grown by the addition of concentric layers, as in bladder sand or starch grains or the calcium carbonate of the foraminiferan test, a diagnostic dark ‘Maltese cross’ appears in the image in the planes of polarisation of the two filters.”

Polarised light microscopy with crossed polars can be used to enhance contrast and colour in samples that are almost colourless or transparent. See Spike's award-winning image of vitamin C crystals for another example of this.

What is the corkscrew-like neck for?

Fossilised shells from Foraminifera form the bulk of the world's chalk and limestone deposits, from the White Cliffs of Dover to the Himalayas. They have distinctive shapes and as such are sometimes used by geologists as markers to identify different layers of rock. The foraminiferan in this image has a narrow neck with an external corkscrew-like thread through which it extends thin thread-like structures (pseudopodia) to capture its prey.