The body of a horseshoe crab is covered by a smooth greenish to dark brown exoskeleton. The exoskeleton con sists of three major parts: an arched, horseshoe-shaped shield in the front, the prosoma; a middle portion, the opisthosoma; and a thin tail, the telson. The prosoma bears two pairs of simple eyes on the top and a pair of compound eyes on ridges laterally along the outside. Under the exoskeleton, eight pairs of appendages are aligned along the lengthwise axis of the prosoma. The first seven pairs function in feeding. The eighth pair is fused and covers five pairs of book gills in the opisthosoma. The book gills maintain water flow for respiration, movement, and reproduction. Spines protrude from the outer edge of the opisthosoma; the number of spines varies by species. The long, thin telson extends from the back of the body.
Horseshoe crabs must shed their exoskeleton, or molt, to grow. Individuals molt 16 or 17 times during their lives. Six of these molts occur within the first year. As adults, females
are larger than males. In the smallest species, Carcinoscorpius rotundicauda, females reach 15 in (38 cm) in length and 5 in (12.5 cm) in width. In Tachypleus tridentatus, the largest species, females attain a length of 33.5 in (85 cm) and a width of 15.5 in (39.3 cm).
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