a bladder that opens near the base of the last pair of walking legs. The brain is relatively large, and, as in many arthropods, surrounds the oesophagus. In both sexes, the single gonad lies next to the intestine and opens on the underside of the opisthosoma. Reproduction Xiphosurans move to shallow water to mate. The male climbs onto the back of the female, gripping her with his first pair of walking legs. The female digs out a depression in the sand, and lays from 200 to 300 eggs, which the male covers with sperm. The pair then separates, and the female buries the eggs. The egg is about across. Inside the egg the embryo goes through four molts before it hatches into a larva often called a 'trilobite larva', due to its superficially resemblance to a trilobite. At this stage it has no telson yet, and the larva is lecithotrophic (non-feeding) and planktonic, subsisting on the maternal yolk before settling to the bottom to molt, which is when the telson first appear. Through a series of successive moults, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form. Modern xiphosurans reach sexual maturity after about three years of growth. Evolutionary history The oldest known stem-Xiphosuran, Lunataspis, is known from the late Ordovician of Canada, around 445 million years ago. No xiphosurans are known from the following Silurian. Xiphosurida first appears during the late Devonian. A major radiation of freshwater xiphosurids, the Belinuridae is known from the Carboniferous, with the oldest representatives of the modern family Limulidae also possibly appearing during this time, though they only appear in abundance during the Triassic. Another major radiation of freshwater xiphosurans, the Austrolimulidae, is known from the Permian and Triassic. Classification Xiphosuran classification : Order Xiphosura Latreille, 1802 †Maldybulakia Tesakov & Alekseev, 1998 (Devonian) †Willwerathia Størmer, 1969 (Devonian) †Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian) Suborder Xiphosurida †Infraorder Belinurina †Belinuridae Zittel & Eastman, 1913 (Middle Devonian to Upper Carboniferous) Infraorder Limulina †Bellinuroopsis Chernyshev, 1933 (Carboniferous) †Rolfeiidae Selden & Siveter, 1987 (Early Carboniferous to Early Permian) Superfamily †Paleolimuloidea Anderson & Selden, 1997 †Paleolimulidae Raymond, 1944 (Carboniferous to Permian) Superfamily Limuloidea †Valloisella Racheboeuf, 1992 (Carboniferous) †Austrolimulidae Riek, 1955 (Early Permian-Early Jurassic) Limulidae Zittel, 1885 (Carboniferous to recent) Limulinae Zittel, 1885 (Late Jurassic-Present) Tachypleinae Pocock, 1902 (Late Cretaceous-Recent) Taxa removed from Xiphosura Two groups were originally included in