Phylum Arthropoda Class Chelicerata Subclass Merostomata Number of families 1
Marine creatures distinguished by a large, hard exoskeleton that includes an arched, horseshoe-shaped shield in front (prosoma), a middle portion (opisthosoma), and a thin tail (telson); they are among the oldest living organisms
Photo: A horseshoe crab (Limulus polyphemus) trying to turn itself right side up. (Photo by Gilbert S. Grant/Photo Researchers, Inc. Reproduced by permission.)
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