PAUL D STEWART / SCIENCE PHOTO LIBRARY PAUL D STEWART / SCIENCE PHOTO LIBRARY
Horseshoe crab Limulus polyphemus clearly showing the strange ventral anatomy (including book gills) of this primitive arthropod. The horse shoe crab is often considered a living fossil because they exist in the fossil record largely unchanged for 450 million years (since the Ordovician) and are reminiscent of the famous extinct trilobite group. Though they resemble crustaceans like crabs, they actually belong to the subphylum Chelicerata which places them closer to the arachnids (spiders and allies). They do not have hemoglobin in their blood, but rather hemocyanin, which makes it blue. Their blood contains amebocytes which attack invading micro-organisms and coagulate around them. These amebocytes are harvested from P.Limulus blood commercially to make Limulus amebocyte lysate (LAL). It is used widely for detection of bacterial contamination in medical applications as part of FDA test.
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