It contains the sea spiders , arachnids including scorpions , spiders , and potentially horseshoe crabs  , and several extinct lineages, such as the eurypterids.
The Chelicerata originated as marine animals in the Middle Cambrian period; the first confirmed chelicerate fossils , belonging to Sanctacaris , date from million years ago. Like all arthropods , chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate bauplan consists of two tagmata , the prosoma and the opisthosoma , except that mites have lost a visible division between these sections.
The chelicerae , which give the group its name, are the only appendages that appear before the mouth. In most sub-groups, they are modest pincers used to feed. However, spiders' chelicerae form fangs that most species use to inject venom into prey.
The group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel , which is the major body cavity. Marine chelicerates have gills, while the air-breathing forms generally have both book lungs and tracheae. In general, the ganglia of living chelicerates' central nervous systems fuse into large masses in the cephalothorax, but there are wide variations and this fusion is very limited in the Mesothelae , which are regarded as the oldest and most primitive group of spiders.
Most chelicerates rely on modified bristles for touch and for information about vibrations, air currents, and chemical changes in their environment. The most active hunting spiders also have very acute eyesight.
Chelicerates were originally predators, but the group has diversified to use all the major feeding strategies: predation, parasitism , herbivory , scavenging and eating decaying organic matter.
Although harvestmen can digest solid food, the guts of most modern chelicerates are too narrow for this, and they generally liquidize their food by grinding it with their chelicerae and pedipalps and flooding it with digestive enzymes.
To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules , structures that also evolved independently in insects. While the marine horseshoe crabs rely on external fertilization , air-breathing chelicerates use internal but usually indirect fertilization.
Many species use elaborate courtship rituals to attract mates. Most lay eggs that hatch as what look like miniature adults, but all scorpions and a few species of mites keep the eggs inside their bodies until the young emerge. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.
The evolutionary origins of chelicerates from the early arthropods have been debated for decades.
Subphylum chelicerata pdf to jpg
Although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has recently been questioned see below , and the exact position of scorpions is still controversial, though they were long considered the most primitive basal of the arachnids. Venom has evolved three times in the chelicerates; spiders, scorpions and pseudoscorpions, or four times if the hematophagous secretions produced by ticks are included.
In addition there have been undocumented descriptions of venom glands in Solifugae. Although the venom of a few spider and scorpion species can be very dangerous to humans, medical researchers are investigating the use of these venoms for the treatment of disorders ranging from cancer to erectile dysfunction.
Pages in category "Chelicerata"
The medical industry also uses the blood of horseshoe crabs as a test for the presence of contaminant bacteria. Mites can cause allergies in humans, transmit several diseases to humans and their livestock , and are serious agricultural pests. 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.
The prosoma is formed in the embryo by fusion of the ocular somite referred as "acron" in previous literatures , which carries the eyes and labrum  , with six post-ocular segments somite 1 to 6 ,  which all have paired appendages.
It was previously thought that chelicerates had lost the antennae-bearing somite 1,  but later investigations reveal that it retain and correspond to a pair of chelicerae or chelifores,  small appendages that often form pincers.
The opisthosoma consists of thirteen or fewer segments, may or may not end with a telson.
Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must molt to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden.
Until the new cuticle hardens the animals are defenseless and almost immobilized. 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.
The chelicerae "claw horns" that give the sub-phylum its name normally consist of three sections, and the claw is formed by the third section and a rigid extension of the second. In basal chelicerates, the pedipalps are unspecialized and subequal to the posterior pairs of walking legs.
As in all arthropods, the chelicerate body has a very small coelom restricted to small areas round the reproductive and excretory systems. The main body cavity is a hemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward.
Although arteries direct the blood to specific parts of the body, they have open ends rather than joining directly to veins , and chelicerates therefore have open circulatory systems as is typical for arthropods. 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.
For a long time it was assumed that the extinct eurypterids had gills, but the fossil evidence was ambiguous.
However, a fossil of the 45 millimetres 1. The guts of most modern chelicerates are too narrow to take solid food. 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.
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.
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. 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. Chelicerate nervous systems are based on the standard arthropod model of a pair of nerve cords , each with a ganglion per segment, and a brain formed by fusion of the ganglia just behind the mouth with those ahead of it.
There is a notable but variable trend towards fusion of other ganglia into the brain. The brains of horseshoe crabs include all the ganglia of the prosoma plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia. As with other arthropods, chelicerates' cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system.
In fact, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch and vibration sensors, mostly bristles called setae , respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell , often by means of setae.
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.
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These median ocelli-type eyes in chelicerates are assumed to be homologous with the crustacean nauplius eyes and the insect ocelli. Horseshoe crabs , which are aquatic, use external fertilization , in other words the sperm and ova meet outside the parents' bodies.
Their trilobite -like larvae look rather like miniature adults as they have full sets of appendages and eyes, but initially they have only two pairs of book-gills and gain three more pairs as they molt.
Being air-breathing animals, the living arachnids excluding horseshoe crabs use internal fertilization , which is direct in some species, in other words the males' genitalia make contact with the females'. However, in most species fertilization is indirect. Male spiders use their pedipalps as syringes to "inject" sperm into the females' reproductive openings,  but most arachnids produce spermatophores packages of sperm which the females take into their bodies.
Most arachnids lay eggs, but all scorpions and a few mites keep the eggs inside their bodies until they hatch and offspring rather like miniature adults emerge. 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. However, cladistic analyses that consider wider ranges of characteristics place neither as chelicerates.
Another Cambrian fossil, Kodymirus , was originally classified as an aglaspid but may have been a eurypterid and therefore a chelicerate. Sanctacaris , member of the family Sanctacarididae from the Burgess Shale of Canada , represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age.
The eurypterids have left few good fossils and one of the earliest confirmed eurypterid, Pentecopterus decorahensis , appears in the Middle Ordovician period The Late Silurian Proscorpius has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had gills rather than book lungs or tracheae ; its mouth was completely under its head and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs.
Chelicerata, Myriapoda, Crustacea
The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups crustaceans ; hexapods , which includes insects ; and myriapods , which includes centipedes and millipedes than these other groups are to each other.
Recent research since , using both molecular phylogenetics the application of cladistic analysis to biochemistry , especially to organisms' DNA and RNA and detailed examination of how various arthropods' nervous systems develop in the embryos , suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives.
However, these results are derived from analyzing only living arthropods, and including extinct ones such as trilobites causes a swing back to the "traditional" view, placing trilobites as the sister-group of the Tracheata hexapods plus myriapods and chelicerates as least closely related to the other groups.
28.4D: Subphyla of Arthropoda
Acariformes mites. It is generally agreed that the Chelicerata contain the classes Arachnida spiders , scorpions , mites , etc. However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in to combine analysis of RNA 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.
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.
A phylogenetic analysis the results presented in a cladogram below conducted by James Lamsdell in 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.
Lamsdell noted that 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 xiphosurans. Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77, living species that have been described in scientific publications.
However, many species feed as parasites , herbivores , scavengers and detritivores. 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. Because spider silk is both light and very strong, but large-scale harvesting from spiders is impractical, work is being done to produce it in other organisms by means of genetic engineering.
In the 20th century, there were about reliably reported deaths from spider bites,  compared with 1, from jellyfish stings. Most of these incidents are caused by accidental human "invasions" of scorpions' nests.