horseshoe crab

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horseshoe crab,

large, primitive marine arthropod of the family Limulidae, related to the spiderspider,
organism, mostly terrestrial, of the class Arachnida, order Araneae, with four pairs of legs and a two-part body consisting of a cephalothorax, or prosoma, and an unsegmented abdomen, or opisthosoma.
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 and scorpionscorpion,
any arachnid of the order Scorpionida with a hollow poisonous stinger at the tip of the tail. Scorpions vary from about 1/2 in. to about 6 in. (1–15 cm) long; most are from 1 to 3 in. (2.5–7.6 cm) long.
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 and sometimes called a king crab (a name also used for the largest of the edible true crabscrab,
crustacean with an enlarged cephalothorax covered by a broad, flat shell called the carapace. Extending from the cephalothorax are the various appendages: five pairs of legs, the first pair bearing claws (or pincers), are attached at the sides; two eyes on short, movable
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). The heavy dark brown exoskeleton, or carapace, is domed and shaped like a horseshoe. The body is divided into a broad, flattened, semicircular front part (the prosoma), a tapering middle part (the opisthosoma), and a pointed, spiky taillike part (the telson).

Horseshoe crabs have no jaws, and the mouth is flanked by a pair of pincerlike chelicera that are used to crush worms and other invertebrates taken as food. They have two compound, primary eyes and five simple, secondary eyes on top of the carapace, and two simple eyes near the mouth, under the carapace; additionally, the telson has photoreceptors. Five pairs of walking legs attached to the prosoma enable the animals to swim awkwardly or burrow through the sand or mud. They swim upside down, with the carapace forward and angled upward. The respiratory organs are called book gills and are unique to horseshoe crabs. Each book gill is made of about 100 thin leaves, or plates; these are fitted like pages of a book onto one pair of flaplike appendages on the opisthosoma. Rhythmic movement of the appendages circulates water over the gill surfaces and drives blood into and out of the gill leaves.

Horseshoe crabs first appeared in the Upper Silurian period, and a number of fossil species have been described. Four species still survive; three of these are found along the Pacific coast of Asia. The American species, Limulus polyphemus, is common along the Atlantic coast from Nova Scotia to Florida. It lives in shallow water, preferring soft or sandy bottoms, and reaches a maximum length of nearly 2 ft (61 cm). The shores of the Delaware Bay form the largest spawning ground of the species, and their eggs make the bay a critical feeding stopover for migrating shorebirds.

Horseshoe crabs are considered living fossils; they resemble fossil trilobitestrilobite
, subphylum of the phylum Arthropoda that includes a large group of extinct marine animals that were abundant in the Paleozoic era. They represent more than half of the known fossils from the Cambrian period.
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 and eurypterids of the Paleozoic era. They are classified in the phylum ArthropodaArthropoda
[Gr.,=jointed feet], largest and most diverse animal phylum. The arthropods include crustaceans, insects, centipedes, millipedes, spiders, scorpions, and the extinct trilobites.
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, subphylum ChelicerataChelicerata
, subphylum of Arthropoda, including the horseshoe crabs (order Xiphosura), the arachnids (class Arachnida), and the sea spiders (class Pycnogonida). The extinct giant water scorpions (order Eurypterida, not true scorpions) also are chelicerates.
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, class Merostomata, order Xiphosura.

horseshoe crab

[′hȯr‚shü ‚krab]
(invertebrate zoology)
The common name for arthropods composing the subclass Xiphosurida, especially the subgroup Limulida.