Xiphosurida

(invertebrate zoology) A subclass of primitive arthropods in the class Merostomata characterized by cephalothoracic appendages, ocelli, book lungs, a somewhat trilobed body, and freely articulating styliform telson.

An order of the subclass Xiphosura in the class Merostomata. These “sword-tailed” arthropods have three distinct body divisions: a prominent horseshoe-shaped, fused head and thorax, the prosoma; a segmented “abdomen,” or opisthosoma; and a spikelike terminal segment, the telson. The Xiphosurida have an extensive fossil record. The two suborders, Synziphosurina and Limulina, span over 500,000,000 years of evolution. The Synziphosurina usually lacked compound eyes. Some species had a distinct axial, or cardiac, region, free segments in the opisthosoma, and a broad, short telson. Superfamilies Belinuracea and Limulacea of the Limulina have the clearly recognizable horseshoe “crab” form.

The carapace (test or shell) is composed of chitin, strengthened internally in the adults by bridgelike struts within the shell margins. Sensory structures on the carapace include chemoreceptors for taste on the bases of the legs and a pair of simple eyes, or ocelli. The multilensed compound eyes form mosaic images and are sensitive to polarized light. The organ systems, except the peripheral blood vessels and nerves, digestive, and reproductive glands, which branch extensively into the lateral portions of the prosoma, are mainly within the prominent axial region. This region contains the musculature of the paired appendages, hinge, and telson, the digestive tube, the coxal (excretory) gland, and the respiratory, central circulatory, and nervous structures. The dorsally situated tubular heart lies within a sinus which receives venous blood from the book gills, the respiratory structures, and the body. The central nervous system and the major nerve branches are sheathed by arteries. A plate of “cartilaginous” chitin, the endocranium, forms a roof over the brain. See also Merostomata.