STEVE LOWRY / SCIENCE PHOTO LIBRARY STEVE LOWRY / SCIENCE PHOTO LIBRARY
Polarised light micrograph of a spine of a sea urchin (Echinus sp.). The test and spines of sea urchins are composed of calcium carbonate. Each spine articulates with a tubercle on the test. The elaborate structure of the spines when seen in cross section has made them a popular subject for microscopists.
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