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30-556: Belinuridae is an extinct family of arthropods belonging to the order Xiphosura . The family consists of several genera. 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)

60-406: 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

90-479: A proton and a bicarbonate ion. The response is that the respiratory centre (in the medulla), sends nervous impulses to the external intercostal muscles and the diaphragm , via the intercostal nerve and the phrenic nerve , respectively, to increase breathing rate and the volume of the lungs during inhalation. Chemoreceptors that regulate the depth and rhythm of breathing are broken down into two categories. The response to stimulation of chemoreceptors on

120-418: 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

150-498: 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

180-701: 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

210-437: 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

240-515: A predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been. Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. It passes in either chemical or electrochemical form to the central processor, the brain or spinal cord . The resulting output from the CNS ( central nervous system ) makes body actions that will engage

270-406: 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

300-405: 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 ,

330-425: 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

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360-685: Is an indicator that chemoreceptors play a heightened role in the sensing of cytosolic signals in archaea. Primary cilia , present in many types of mammalian cells , serve as cellular antennae . The motile function of these cilia is lost in favour of their sensory specialization. Plants have various mechanisms to perceive danger in their environment. Plants are able to detect pathogens and microbes through surface level receptor kinases (PRK). Additionally, receptor-like proteins (RLPs) containing ligand binding receptor domains capture pathogen-associated molecular patterns (PAMPS) and damage-associated molecular patterns (DAMPS) which consequently initiates

390-486: 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;

420-403: Is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals. Chemoreception is important for the detection of food, habitat, conspecifics including mates, and predators. For example, the emissions of

450-459: 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

480-1232: 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 Chemoreceptor In bacteria , chemoreceptors are essential in

510-420: 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

540-411: 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 ,

570-520: 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

600-409: The food and enhance survival. Particular chemoreceptors, called ASICs , detect the levels of carbon dioxide in the blood. To do this, they monitor the concentration of hydrogen ions in the blood, which decrease the pH of the blood. This can be a direct consequence of an increase in carbon dioxide concentration, because aqueous carbon dioxide in the presence of carbonic anhydrase reacts to form

630-558: The heart and a corresponding increase in heart rate and contractility in most cases. These factors include activation of stretch receptors due to increased ventilation and the release of circulating catecholamines. However, if respiratory activity is arrested (e.g. in a patient with a high cervical spinal cord injury), then the primary cardiac reflex to transient hypercapnia and hypoxia is a profound bradycardia and coronary vasodilation through vagal stimulation and systemic vasoconstriction by sympathetic stimulation. In normal cases, if there

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660-419: The heart rate is complicated. Chemoreceptors in the heart or nearby large arteries, as well as chemoreceptors in the lungs, can affect heart rate. Activation of these peripheral chemoreceptors from sensing decreased O 2 , increased CO 2 and a decreased pH is relayed to cardiac centers by the vagus and glossopharyngeal nerves to the medulla of the brainstem. This increases the sympathetic nervous stimulation on

690-436: 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

720-412: 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

750-669: 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

780-423: The mediation of chemotaxis . Bacteria utilize complex long helical proteins as chemoreceptors, permitting signals to travel long distances across the cell's membrane. Chemoreceptors allow bacteria to react to chemical stimuli in their environment and regulate their movement accordingly. In archaea , transmembrane receptors comprise only 57% of chemoreceptors, while in bacteria the percentage rises to 87%. This

810-421: 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

840-538: The plant's innate immunity for a defense response. Plant receptor kinases are also used for growth and hormone induction among other important biochemical processes. These reactions are triggered by a series of signaling pathways which are initiated by plant chemically sensitive receptors. Plant hormone receptors can either be integrated in plant cells or situate outside the cell, in order to facilitate chemical structure and composition. There are 5 major categories of hormones that are unique to plants which once bound to

870-469: The receptor, will trigger a response in target cells. These include auxin , abscisic acid , gibberellin , cytokinin , and ethylene . Once bound, hormones can induce, inhibit, or maintain function of the target response. There are two main classes of chemoreceptor: direct and distance. When inputs from the environment are significant to the survival of the organism, the input must be detected. As all life processes are ultimately based on chemistry it

900-412: 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

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