Introduction
The Xiphosura are members of marine chelicerates that includes a large number of extinct lineages and only four extant species in the family Limulidae, which include the horseshoe crabs. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the Jurassic Mesolimulus, and are considered to be living fossils what are crabs. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.
Classification of Xiphosura
Xiphosura were traditionally placed in the class Merostomata, although this term was intended to encompass also theeurypterids, whence it denoted what is now known to be an unnatural (paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses) what are crabs. Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The Merostomata label originally did notinclude Eurypterida, although they were added in as a better understanding of the extinct group evolved (see Merostomatafor additional details). However, as the similarities, which were once thought to tie the group together, what are crabs are now understood to be symplesiomorphies, it seems that abandoning the term altogether to avoid any confusion arising from these historical usages of the term is the best strategy.
Description of Xiphosura
Their bodies are covered with a tough cuticle, but do not contain any crystalline biominerals, and are divided into an anterior prosoma and a posterioropisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircular carapace, while the underside bears five pairs of walking legs and a pair of pincer-like chelicerae. The mouth is located below the forward tip of the carapace, and lies behind a lip-like structure called the labrum, what are crabs.
Xiphosurans have up to four eyes, located in the carapace. A pair of compound eyes is on the side of the prosoma, with one or two median ocelli towards the front. The compound eyes are simpler in structure than those of other arthropods, with the individual ommatidia not being arranged in a compact pattern. They can probably detect movement, what are crabs but are unlikely to be able to form a true image. In front of the ocelli is an additional organ that probably functions as a chemoreceptor.
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 2–3 mm (0.08–0.12 in) across, and hatches into a larva that superficially resembles a trilobite. Indeed, it is often referred to as the ‘trilobite larva’. 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, what are crabs.