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Definition: horseshoe crab from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Marine arthropod of the order Xiphosura, class Merostomata, distantly related to spiders, which lives on the Atlantic coast of North America and the coasts of Asia. The upper side of the body is entirely covered with a rounded shell, and it has a long, spinelike tail. Horseshoe crabs grow up to 60 cm/2 ft long. They crawl along the bottom in coastal waters and lay their eggs in the sand at the high water mark.

Summary Article: horseshoe crab
From The Columbia Encyclopedia

large, primitive marine arthropod of the family Limulidae, related to the spider and scorpion and sometimes called a king crab (a name also used for the largest of the edible true crabs). 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 trilobites and eurypterids of the Paleozoic era. They are classified in the phylum Arthropoda, subphylum Chelicerata, class Merostomata, order Xiphosura.

The Columbia Encyclopedia, © Columbia University Press 2018

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