Xiphosura

Xiphosuran is an order of arthropods related to arachnids,
 or, according to one recent study, actual arachnids. They are
sometimes called horseshoe crabs (a name applied more specifically to
the only extant family, Limulidae). They first appeared in the Hirnantian (Late Ordovician).
 Currently, there are only four living species. They are members of the
order Xiphosura, which contains two suborders, Xiphosurida and Synziphosurina.
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.
 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.
Xiphosura were traditionally placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic)
 group (although this is a grouping recovered in some recent cladistic
analyses ). 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 not include
 Eurypterida, although they were added in as a better understanding of
the extinct group evolved. Now Eurypterida is classified within Sclerophorata together with the arachnids, and therefore, Merostomata is now a synonym of Xiphosura. One recent study places Xiphosura within the Arachnida as the sister group of Ricinulei.

Modern xiphosurans reach up to 60 cm (24 in) in adult length, but the Paleozoic species were often far smaller, some as small as 1 to 3 cm (0.39 to 1.18 in) long.
Their
 bodies are covered with a tough cuticle, but do not contain any
crystalline biominerals, and are divided into an anterior prosoma and a posterior opisthosoma, 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 on underside of the center of the prosoma, between
 the bases of the walking legs, and lies behind a lip-like structure
called the labrum.
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, 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.
The
 first four pairs of legs end in pincers, and have a series of spines,
called the gnathobase, on the inner surface. The spines are used to masticate
 the food, tearing it up before passing it to the mouth. The fifth and
final pair of legs, however, has no pincers or spines, instead having
structures for cleaning the gills and pushing mud out of the way while
burrowing. Behind the walking legs is a sixth set of appendages, the
chilaria, which are greatly reduced in size and covered in hairs and
spines. These are thought to be vestiges of the limbs of an absorbed
first opisthosomal segment.
The opisthosoma is divided into a
forward mesosoma, with flattened appendages, and a metasoma at the rear,
 which has no appendages. In modern forms, the whole of the opisthosoma
is fused into a single.

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.