horseshoe crab(redirected from Book gill)
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horseshoe crab(hŏrs′shoo″, hŏrsh′oo krăb)
Limulus polyphemus, a species of saltwater arthropod that is a member of the Chelicerates, the subphylum containing scorpions and spiders, rather than the Crustaceans, the subphylum containing true crabs. The Limulus body structure has remained nearly unchanged for 450 million years, longer than almost any other living animal. Its blood is blue rather than red because oxygen is carried by a copper-based compound rather than hemoglobin, which is an iron-based compound; Limulus blood is used in testing drugs for bacterial contamination.