Xiphosura is an order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of marine chelicerates
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...

 which includes a large number of extinct lineages and only four recent species in the family Limulidae
Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating...

, which include the horseshoe crab
Horseshoe crab
The Atlantic horseshoe crab, Limulus polyphemus, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...

s. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 Mesolimulus, and are considered to be living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...


Xiphosura are traditionally placed in the class Merostomata
Merostomata is the name given to a grouping of the extinct Eurypterida and the Xiphosura . The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups....

, although this term was originally intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

) group. Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged.


Modern xiphosurans reach up to 60 centimetres (23.6 in) in adult length, but the Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 species were often far smaller, some as little as 1 to 3 cm (0.393700787401575 to 1.2 ) long.

The body is covered with heavy mineralized cuticle, and is divided into an anterior prosoma and a posterior opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...

, or abdomen. The upper surface of the prosoma is covered by a semicircular carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

, 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
Labrum (arthropod mouthpart)
The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant euarthropods, the general exception being provided by the probable chelicerate-relatives the pycnogonids. It has proved to be by far the most controversial of all arthropod head structures. It is...


Xiphosurans have up to four eyes, located in the carapace. There are a pair of compound eyes on the side of the prosoma, and 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
Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by...

 the food, tearing it up before passing it to the mouth. The fifth and final pair of legs, however, have no pincers or spines, instead having structures for cleaning the gills and pushing mud out of the way while burrowing. Behind the walking legs are 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 unsegmented structure. The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.

The opisthosoma terminates in a long caudal spine, commonly referred to as a telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

 (though this same term is also used for a different structure in crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s). The spine is highly mobile, and is used to push the animal upright if it is accidentally turned over.

Internal anatomy

The mouth opens into a sclerotised oesophagus which leads to a crop
Crop (anatomy)
A crop is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion that is found in many animals, including gastropods, earthworms, leeches, insects, birds, and even some dinosaurs.- Bees :Cropping is used by bees to temporarily store nectar of flowers...

 and gizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

. After grinding up its food in the gizzard, the animal regurgitates any inedible portions, and passes the remainder to the true stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

. The stomach secretes digestive enzymes, and is attached to an intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

 and two large caeca that extend through much of the body, and absorb the nutrients from the food. The intestine terminates in a sclerotised rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

, which opens just in front of the base of the caudal spine.

Xiphosurans have a well-developed circulatory system, with numerous arteries
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....

 that send blood from the long tubular heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 to the body tissues, and then to two longitudinal sinuses next to the gills. After being oxygenated, the blood flows into the body cavity, and back to the heart. The blood contains haemocyanin, a blue copper-based pigment performing the same function as haemoglobin in vertebrates, and also has blood cells that aid in clotting.

The excretory system consists of two pairs of coxal gland
Coxal gland
The coxal gland is a gland found in some arthropods, for collecting and excreting urine. They are found in all arachnids , and in other chelicerates, such as horseshoe crabs. The coxal gland is thought to be homologous with the antennal gland of crustaceans...

s connected to a bladder
Bladder usually refers to an anatomical hollow organBladder may also refer to:-Biology:* Urinary bladder in humans** Urinary bladder ** Bladder control; see Urinary incontinence** Artificial urinary bladder, in humans...

 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
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

 lies next to the intestine and opens on the underside of the opisthosoma.


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 separate, and the female buries the eggs.

The eggs are about 2–3 mm (0.078740157480315–0.118110236220472 ) across, and hatch into a larva that superficially resembles a trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

. Indeed it is often referred to as the 'trilobite larva'. Through a series of successive moults
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

, 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.

Classification of the Xiphosura

Order Xiphosura Latreille, 1802
  • †Suborder Synziphosurida
    • Weinberginidae Richter & Richter, 1929 (Lower Devonian
      The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

    • Bunodidae Packard, 1886
      • Bunodinae Packard, 1886 (Upper Silurian
        The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

         to Downtonian)
      • Limuloidinae Størmer, 1952 (Upper Silurian)
    • Pseudoniscidae Packard 1886 (Upper Silurian)
    • Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian)
  • Suborder Xiphosurida
    • †Infraorder Bellinurina
      • Elleriidae Raymond, 1944 (Upper Devonian to Upper Carboniferous
        The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

      • Euproopidae Eller, 1938 (= Liomesaspidae Raymond, 1944) (Upper Carboniferous to Lower Permian)
      • Bellinuridae Zittel & Eastman, 1913 (Middle Devonian to Upper Carboniferous)
    • Infraorder Limulina
      • †Rolfeiidae Selden & Siveter, 1987 (Early Carboniferous to Early Permian)
      • †Paleolimulidae Raymond, 1944 (Carboniferous to Permian)
      • †Moravuridae Pribyl, 1967 (Mississippian)
      • †Austrolimulidae + †Heterolimulidae (Middle Triassic
        The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

      • Limulidae Zittel, 1885
        • †Mesolimulinae Størmer, 1952 (Lower Triassic to Cretaceous
          The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

        • Limulinae Zittel, 1885
          • Tachypleini Pocock, 1902 (Miocene
            The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

             to Recent)
          • Limulini Zittel, 1885 (Recent)

Taxa removed from Xiphosura

There are two groups originally included in the Xiphosura, but which have been assigned to separate classes:
  • Aglaspida
    The Aglaspida, more correctly termed Aglaspidida, were a group of small, horseshoe crab-like arthropods that were once regarded as basal or ancestral horseshoe crabs. However they cannot be accommodated within the chelicerates. Now, though, they are regarded as a distinct group, possibly being...

     Walcott, 1911 (Cambrian to Ordovician)
  • Chasmataspida
    Chasmataspidida are a rare, extinct group of chelicerate arthropods. They are probably related to either horseshoe crabs and/or sea scorpions...

    Caster & Brooks, 1956 (Lower Ordovician)

Further reading

  • B. B. Rohdendorf (ed.) Fundamentals of Paleontology, vol. 9, Arthropoda-Tracheata and Chelicerata: 894 pp. [1991 English translation of Russian original, Smithsonian Institution Libraries and National Science Foundation].
  • R. E. Snodgrass. 1952. A Textbook of Arthropod Anatomy. Hafner Publishing Company, New York.

External links

  • Peripatus - an overview of arthropod relationships.
  • Paleos - a site with a synoptic account of the Xiphosura, focused on fossils.
  • Xiphosura - the article from the UCMP Web Taxa project.
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