Misplaced Pages

#83916

29-658: Austrolimulidae is an extinct family of horseshoe crabs belonging to the infraorder Limulina. Members of the family are known from the Permian to the beginning of the Jurassic , though one species has been reported from the end of the Cretaceous . Austrolimulids are known for amongst the most extreme morphologies among Xiphosurids, including large elongated genal spines. Unlike living Limulids, Austrolimulids were likely adapted for freshwater and brackish environments. They are considered to be

58-406: 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

87-418: 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 glands connected to 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

116-498: A derived clade of arachnids. 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 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

145-701: A group they have never showed much diversity in regard of species. Less than 50 fossil species are known from the Carboniferous period, when they were at their most diverse. The last common ancestor of modern limulids has been suggested to date to the Jurassic-Cretaceous boundary based on molecular clock dating though depending on phylogeny the fossil record may suggest a split as old as the Triassic. Xiphosuran classification as of 2018 : Order Xiphosura Latreille, 1802 Two groups were originally included in

174-437: 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 . The exoskeleton consist of a tough cuticle, but do not contain any crystalline biominerals. Like scorpions, xiphosurans have an exocuticular layer of hyaline which exhibits UV fluorescence. Xiphosurans have up to four eyes, located in

203-406: A sclerotised oesophagus , which leads to a crop and gizzard . After grinding up its food in the gizzard, the animal regurgitates any inedible portions, and passes the remainder to the true stomach . The stomach secretes digestive enzymes, and is attached to an intestine and two large caeca that extend through much of the body, and absorb the nutrients from the food. The intestine terminates in

232-405: A sclerotised rectum , which opens just in front of the base of the caudal spine. Xiphosurans have well-developed circulatory systems, with numerous arteries that send blood from the long tubular heart 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 ,

261-483: A single erect image. Apposition eyes can be divided into two groups. The typical apposition eye has a lens focusing light from one direction on the rhabdom , while light from other directions is absorbed by the dark wall of the ommatidium . The mantis shrimp is the most advanced example of an animal with this type of eye. In the other kind of apposition eye, found in the Strepsiptera , each lens forms an image, and

290-425: 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 (though this same term is also used for a different structure in crustaceans ). The spine is highly mobile, and is used to push the animal upright if it is accidentally turned over. The mouth opens into

319-454: A transparent gap but use corner mirrors instead of lenses. Good fliers like flies or honey bees, or prey-catching insects like praying mantises or dragonflies , have specialized zones of ommatidia organized into a fovea area which gives acute vision. In the acute zone the eye is flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. There are some exceptions from

SECTION 10

#1733084741084

348-408: A way that resembles a true compound eye. Asymmetries in compound eyes may be associated with asymmetries in behaviour. For example, Temnothorax albipennis ant scouts show behavioural lateralization when exploring unknown nest sites, showing a population-level bias to prefer left turns. One possible reason for this is that its environment is partly maze-like and consistently turning in one direction

377-491: Is a good way to search and exit mazes without getting lost. This turning bias is correlated with slight asymmetries in the ants' compound eyes (differential ommatidia count). The body of Ophiomastix wendtii , a type of brittle star , was previously thought to be covered with ommatidia, turning its whole skin into a compound eye, but this has since been found to be erroneous; the system does not rely on lenses or image formation. "Dragonfly eyes" (Chinese: 蜻蜓眼 qingting yan ]

406-470: Is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialized retina. The resulting eye is a mixture of a simple eye within a compound eye. Another version is the pseudofaceted eye, as seen in Scutigera . This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in

435-486: Is an order of arthropods related to arachnids . They are more commonly known as 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. Xiphosura contains one suborder, Xiphosurida, and several stem-genera. The group has hardly changed in appearance in hundreds of millions of years;

464-459: Is now thought 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

493-1321: The Xiphosura, but since have been assigned to separate classes: Cladogram after Lasmdell 2020. Carcinoscorpius rotundicauda (Mangrove horseshoe crab) Tachypleus gigas (Indo-Pacific horseshoe crab) † Tachypleus decheni † Tachypleus syriacus Tachypleus tridentatus (tri-spine horseshoe crab) † Heterolimulus gadeai † Volanalimulus madagascarensis Limulus polyphemus (Atlantic horseshoe crab) † Limulus coffini † Crenatolimulus paluxyensis † Crenatolimulus darwini † Keuperlimulus vicensis † Casterolimulus kletti † Victalimulus mcqueeni † Allolimulus woodwardi † Mesolimulus crespelli † Mesolimulus walchi † Mesolimulus tafraoutensis † Mesolimulus sibiricus † Tarracolimulus rieki † Yunnanolimulus henkeli † Yunnanolimulus luopingensis † Boeotiaspis longispinus † Shpineviolimulus jakovlevi † Panduralimulus babcocki † Tasmaniolimulus patersoni † Limulitella bronni † Limulitella tejraensis † Psammolimulus gottingensis † Batracholimulus fuchsbergensis Compound eye Compound eyes are typically classified as either apposition eyes, which form multiple inverted images, or superposition eyes, which form

522-420: The carapace. Two compound eyes are 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

551-411: The egg, the embryo goes through four molts before it hatches into a larva, often called a 'trilobite larva' due to its superficial 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, after which the telson first appears. Through a series of successive moults ,

580-520: The extinct group evolved. Now Eurypterida is classified within Sclerophorata together with the arachnids , and therefore, Merostomata is now a synonym of Xiphosura. Several recent phylogenomic studies place Xiphosura within Arachnida , often as the sister group of Ricinulei ; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as

609-470: The images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye (which, despite its name, is a form of the apposition eye). The superposition eye is divided into three subtypes; the refracting , the reflecting , and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to

SECTION 20

#1733084741084

638-436: 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 separates, and the female buries the eggs. The egg is about 2–3 mm (0.08–0.12 in) across. Inside

667-412: 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. 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

696-620: 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. As

725-421: The modern horseshoe crabs look almost identical to prehistoric genera 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 historically placed in the class Merostomata , although this term was intended to encompass also the eurypterids , whence it denoted what

754-512: The same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition eye , seen in arthropods such as mayflies , the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array . Long-bodied decapod crustaceans such as shrimp , prawns , crayfish and lobsters are alone in having reflecting superposition eyes , which also have

783-513: The sister group to Limulidae , the group that contains all modern horseshoe crabs. This prehistoric arthropod –related article is a stub . You can help Misplaced Pages by expanding it . This Chelicerata -related article is a stub . You can help Misplaced Pages by expanding it . Xiphosura Xiphosura ( / z ɪ f oʊ ˈ sj ʊər ə / ; from Ancient Greek ξίφος ( xíphos )  'sword' and οὐρά ( ourá )  'tail', in reference to its sword-like telson)

812-405: The types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp, Dioptromysis paucispinosa . The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there

841-412: 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

#83916