Chelicerata

Segmentation and cuticle The Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo; a much reduced coelom; a hemocoel through which the blood circulates, driven by a tube-like heart. Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the prosoma or cephalothorax, and the rear tagma is called the opisthosoma or abdomen. However, in the Acari (mites and ticks) there is no visible division between these sections. which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1, but later investigations reveal that it is retained and corresponds to a pair of chelicerae or chelifores, small appendages that often form pincers. Somite 2 has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments (somite 4 to 6) have pairs of legs. while those of horseshoe crabs (Xiphosura) form gills. Chelicerae and pedipalps Chelicerae and pedipalps are the two pairs of appendages closest to the mouth; they vary widely in form and function and the consistent difference between them is their position in the embryo and corresponding neurons: chelicerae are deutocerebral and arise from somite 1, ahead of the mouth, while pedipalps are tritocerebral and arise from somite 2, behind the mouth. Respiratory systems These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both book lungs, which deliver oxygen and remove waste gases via the blood, and tracheae, which do the same without using the blood as a transport system. The living horseshoe crabs are aquatic and have book gills that lie in a horizontal plane. For a long time it was assumed that the extinct eurypterids had gills, but the fossil evidence was ambiguous. However, a fossil of the long eurypterid Onychopterella, from the Late Ordovician period, has what appear to be four pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs. Feeding and digestion The guts of most modern chelicerates are too narrow to take solid food. and many supplement their diets with nectar and pollen. Many of the Acari (ticks and mites) are blood-sucking parasites, but there are many predatory, herbivore and scavenger sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of the exoskeleton, and which forms a preoral cavity for pre-processing food. Harvestmen are among the minority of living chelicerates that can take solid food, and the group includes predators, herbivores and scavengers. Horseshoe crabs are also capable of processing solid food, and use a distinctive feeding system. Claws at the tips of their legs grab small invertebrates and pass them to a food groove that runs from between the rearmost legs to the mouth, which is on the underside of the head and faces slightly backwards. The bases of the legs form toothed gnathobases that both grind the food and push it towards the mouth. Excretion Horseshoe crabs convert nitrogenous wastes to ammonia and dump it via their gills, and excrete other wastes as feces via the anus. They also have nephridia ("little kidneys"), which extract other wastes for excretion as urine. Most terrestrial chelicerates cannot afford to use so much water and therefore convert nitrogenous wastes to other chemicals, which they excrete as dry matter. Extraction is by various combinations of nephridia and Malpighian tubules. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolved independently in insects and several groups of arachnids. If one assume that chelicerates lose the first segment, which bears antennae in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two. and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia. Living chelicerates have both compound eyes (only in horseshoe crabs, as the compound eye in the other clades has been reduced to a cluster of no more than five pairs of ocelli), mounted on the sides of the head, plus pigment-cup ocelli ("little eyes"), mounted in the middle. These median ocelli-type eyes in chelicerates are assumed to be homologous with the crustacean nauplius eyes and the insect ocelli. The eyes of horseshoe crabs can detect movement but not form images. able to see in both colors and UV-light. Reproduction Vaejovis cashi carrying its young (white) Horseshoe crabs use external fertilization; the sperm and ova meet outside the parents' bodies. Despite being aquatic, they spawn on land in the intertidal zone on the beach. The female digs a depression in the wet sand, where she will release her eggs. The male, usually more than one, then releases his sperm onto them. Their trilobite-like larvae have full sets of appendages and eyes. Initially the horseshoe crab larvae begin with two pairs of book-gills, later gaining three more pairs of book-gills as they molt. Except for Opiliones and some mites, where the male has a penis used for direct fertilization, fertilization in arachnids is indirect. Indirect fertilization happens in two ways: the male deposits his spermatophore (package of sperm) on the ground, which is then picked up by the female, or the male stores his sperm in appendages modified into sperm transfer organs, such as the pedipalps in male spiders, which are inserted into the female genital openings during copulation. Female pseudoscorpions carry their eggs in a brood pouch on the belly, where the growing embryos feeds on a nutritive fluid provided by the mother during development, and are therefore matrotrophic. Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the first molt, and in a few semi-social species the young remain with their mother. Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back, ==Evolutionary history==

Fossil record '' (Habeliida), one of the oldest known chelicerate relatives from the mid-Cambrian Burgess Shale, around 508 million years ago. ,'' the oldest arthropod known to bear true chelicerae, dating to just over 500 million years ago The split of chelicerates from the other major living group of arthropods, Mandibulata, is suggested to have occurred during the early Cambrian. It is disputed whether the megacheirans, which appeared by around 520 million years ago, are early chelicerate relatives or stem-group arthropods. The mollisoniids and habeliids, which emerged by the mid-Cambrian (~508 million years ago), are widely agreed to be stem-group chelicerates, These taxa, while widely agreed to be more closely related to chelicerata than to other arthropod groups, do not unambiguously possess the titular chelicerae. The oldest arthropod to unambiguously possess true chelicerae is Megachelicerax dating to around 504-500 million years ago. The earliest eurypterids have a sparse fossil record, with the oldest fossils of eurypterids dating also dating to the early Ordovician around 479 million years ago, though the fact the remains belong to an advanced subgroup indicate that the group had already undergone significant cryptic diversification by this time. The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps. Attercopus fimbriunguis, from in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider, but it lacked spinnerets and hence was not a true spider. The extinct Chasmataspidida may be a sub-group within Eurypterida. The Pycnogonida (sea spiders) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved. However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis of DNA features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY). An analysis in 2007 using only anatomical features produced the cladogram on the right, but also noted that many uncertainties remain. In recent analyses the clade Tetrapulmonata is reliably recovered, but other ordinal relationships remain in flux. The position of scorpions is particularly controversial. Some early fossils such as the Late Silurian Proscorpius have been classified by paleontologists as scorpions, but described as wholly aquatic as they had gills rather than book lungs or tracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. Cladistic analyses have recovered Proscorpius within the scorpions, This is reflected also in the reinterpretation of Palaeoscorpius as a terrestrial animal. A 2013 phylogenetic analysis (the results presented in a cladogram below) on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera Eurypterus, Parastylonurus, Rhenopterus and Stoermeropterus) concluded that the Xiphosura, as presently understood, was paraphyletic (a group sharing a last common ancestor but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group Sclerophorata within the clade Dekatriata (composed of sclerophorates and chasmataspidids). This work suggested it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is in turn part of the Prosomapoda, a group including the Xiphosurida (the only monophyletic xiphosuran group) and other stem-genera. A recent phylogenetic analysis of the chelicerates places the Xiphosura within the Arachnida as the sister group of Ricinulei, but others still retrieve a monophyletic Arachnida. ==Diversity==

Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77,000 living species that have been described in scientific publications. Some estimates suggest that there may be 130,000 undescribed species of spider and nearly 500,000 undescribed species of mites and ticks. While the earliest chelicerates and the living Pycnogonida (if they are chelicerates Like their ancestors, most living chelicerates are carnivores, mainly on small invertebrates. However, many species feed as parasites, herbivores, scavengers and detritivores. ==Interaction with humans==

''. In the past, Native Americans ate the flesh of horseshoe crabs, and used the tail spines as spear tips and the shells to bail water out of their canoes. More recent attempts to use horseshoe crabs as food for livestock were abandoned when it was found that this gave the meat a bad taste. Horseshoe crab blood contains a clotting agent, Limulus amebocyte lysate, which is used to test antibiotics and kidney machines to ensure that they are free of dangerous bacteria, and to detect spinal meningitis and some cancers. Cooked tarantula spiders are considered a delicacy in Cambodia, and by the Piaroa Indians of southern Venezuela. Spider venoms may be a less polluting alternative to conventional pesticides as they are deadly to insects but the great majority are harmless to vertebrates. Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia, strokes, and erectile dysfunction. Spider silk proteins have been successfully produced in transgenic goats' milk, tobacco leaves, and bacteria, and recombinant spider silk is now available as a commercial product from some biotechnology companies. compared with 1,500 from jellyfish stings. Scorpion stings are thought to be a significant danger in less-developed countries; for example, they cause about 1,000 deaths per year in Mexico, but only one every few years in the USA. Most of these incidents are caused by accidental human "invasions" of scorpions' nests. On the other hand, medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases. Ticks are parasitic, and some transmit micro-organisms and parasites that can cause diseases in humans, while the saliva of a few species can directly cause tick paralysis if they are not removed within a day or two. A few of the closely related mites also infest humans, some causing intense itching by their bites, and others by burrowing into the skin. Species that normally infest other animals such as rodents may infest humans if their normal hosts are eliminated. Three species of mite are a threat to honey bees and one of these, Varroa destructor, has become the largest single problem faced by beekeepers worldwide. Mites cause several forms of allergic diseases, including hay fever, asthma and eczema, and they aggravate atopic dermatitis. Mites are also significant crop pests, although predatory mites may be useful in controlling some of these. ==References==