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Topic: Chelicerates

	Palaeos Metazoa: Arthropoda: Chelicerata: Chelicerata
		The Chelicerates include among their number both the tiniest (the microscopic Acari or mites) and the hugest (the giant Paleozoic Eurypterida) of all the Arthropoda.
		There is a tendency therefore to split the Chelicerates up into a number of distinct classes, with the old Merostomata replaced by the class Xiphosura, and several additional minor groups, and the Arachnida retained sans the Scorpions, which are now given their own class.
		Finally, there is a little-known group of arthropods that may be Chelicerates, but are only very distantly related to all other forms (they have even been placed on occasion in a separate phylum!) the pycnogonids or "sea spiders".
	www.palaeos.com /Invertebrates/Arthropods/Chelicerata/Chelicerata.html (508 words)

	And Along Came a Spider by Ronald L. Shimek, Ph.D. - Reefkeeping.com
		All chelicerates have two major body regions, each of which is composed of several fused embryological segments or regions.
		Generally, the pedipalps are held up in front of the chelicerate as it moves, waving around and touching the substrate as needed to "taste the world" or "see which way the wind blows." The back region, called the abdomen, after-body or "opisthosoma," often lacks appendages (see Figure 1).
		Most chelicerates such as horseshoe crabs, spiders, mites and scorpions have four pairs of walking legs; other appendages are one pair each of chelicerae and pedipalps.
	www.reefkeeping.com /issues/2003-01/rs/index.php (2335 words)

	Marine Phyla Pages
		Chelicerates have six pairs of appendages, which are uniramous (unbranched).
		Still other chelicerates are tiny organisms which feed on detritus, the bits of decaying matter that accumulate on and below the ground.
		Parental care is not common among the chelicerates, but some scorpions will carry their young on their backs for a time.
	kingfish.coastal.edu /marine/302/phyla/chelicerata.htm (361 words)

	Shared Features (Site not responding. Last check: 2007-11-03)
		As is discussed in Evolution and Phylogeny, Arachnids belong to a subphylum of arthropods called chelicerates.
		Although most chelicerates besides arachnids are extinct, arachnids share a few characteristic features with their remaining relatives, such as horseshoe crabs (Campbell, Reece, and Mitchell 617).
		Chelicerates differ from other subgroups of arthropods in that they have simple eyes (eyes with a single lens) and no antenna.
	www.personal.psu.edu /sek154/page4.html (138 words)

	Chelicerates
		"Spiders, scorpions, mites, ticks and horseshoe crabs are all related and form a major branch called the chelicerates within the Arthropod Phylum.
		All chelicerates live on land except horseshoe crabs which are still living in the world's oceans.
		Parasites, predators, disease carriers • Spiders and scorpions control pest populations (insects, rodents) • Humans use horseshoe crabs as animal feed, as fertilizer, as a source of medicines, and in medical research; Ticks, spiders, and scorpions are sources of medicines; spiders are a source of silk
	www.amnh.org /exhibitions/hall_tour/spectrum/24.html (114 words)

	BioG 105/106 \| Autotutorial Introductory Biology
		Chelicerates, represented in lab by the horseshoe crab (Limulus polyphemus), modify the basic arthropod body plan by specialization of some appendages and loss of the appendages associated with some body segments.
		The chelicerate body is divided into two main regions - an anterior cephalothorax (made up of 8 segments) and a posterior abdomen.
		Their appendages are typically biramous (meaning they are split into two branches at the base, a robust locomotory branch and a smaller branch that may incorporate a gill-like structure), including the second pair of antennae (but not the first).
	instruct1.cit.cornell.edu /courses/biog105/labs/inverts/arthropoda.html (496 words)

	Digital Zoology: Chelicerata (Site not responding. Last check: 2007-11-03)
		A chelicerate's body consists of two tagmata, including an anterior prosoma (cephalothorax) with the six pairs of appendages and a posterior opisthosoma (abdomen) formed from the fusion of twelve or less segments.
		The respiratory pigment hemocyanin is used by some chelicerates, especially those that use book lungs or book gills for gas exchange; pigments are absent in spiders that use trachea to deliver air directly to the tissues.
		Chelicerates are dioecious, and depending on the species, fertilization may be internal with sperm transferred either directly by the male or in a
	salinella.bio.uottawa.ca /digitalzoology/ArthChel/default.php?Chel_Chel_Readabout.htm??E?Md1ChapterMcp1?Md1Ch2SectionMcp1 (410 words)

	A CLADISTIC ANALYSIS OF TRILOBITOMORPHA TO CHELICERATA AND CRUSTACEA (Site not responding. Last check: 2007-11-03)
		The Trilobites, the Crustaceans, and the Chelicerates are all grouped in the phylum Arthropoda, which are believed to have evolved from annelid ancestors.
		Holt (2003) proposed that the Chelicerates were more closely related to the Trilobites because cladistic evidence suggested that the Trilobites were closely related to spiders, which are classified under the subphylum Chelicerata.
		The major distinguishing factors that separate the Chelicerates from the Trilobites and Crustaceans are shown in Table 3.
	www.susqu.edu /students/b/bradleyl/Trilobites.htm (1521 words)

	Chelicerata showcase
		The chelicerates' first pair of legs are developed as pincer-like claws.
		The chelicerate's hind body is segmented in aglaspids and sea scorpions, while in horseshoe crabs and spiders the segments are fused into a shield.
		The chelicerates are known from the Cambrian to the present day.
	www.toyen.uio.no /palmus/galleri/montre/english/m_klosaks_e.htm (409 words)

	The Paleontology Portal
		Chelicerates are named for the pair of feeding appendages in front of the mouth, called chelicerae.
		Less familiar chelicerates include the sea spiders (pycnogonids) and the extinct relatives of horseshoe crabs called sea scorpions (eurypterids).
		Eurypterids first appear in the Ordovician and were extinct by the Permian, while most of the chelicerates began to appear in the Devonian and are still around today.
	www.paleoportal.org /index.php?globalnav=fossil_gallery§ionnav=taxon&taxon_id=126 (94 words)

	Phylum Arthropoda: Chelicerates (Site not responding. Last check: 2007-11-03)
		Chelicerae-Either the first pair of fanglike appendages near the mouth of an arachnid, often modified for grasping and piercing (pinchers).
		Chelicerates are mostly predatory arthropods that are, for the most part, unable to digest anything solid, so they either drink the prey's blood, or spit digestive enzymes on them.
		A chelicerate's legs are often adapted for both walking and swimming, and often have a very reduced gill branch (yes, on the legs).
	www.angelfire.com /tx6/sakernstarla/chel.html (72 words)

	Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment -- Telford and Thomas 95 ...
		Posterior boundary of otd and anterior boundaries of Dfd, Scr, and Antp are identical in insects and chelicerates.
		observation that the anteriormost chelicerate appendages are not
		Molecular phylogenetic studies show that the chelicerates are an outgroup to the insect/crustacean clade (33-36).
	www.pnas.org /cgi/content/full/95/18/10671 (3877 words)

	Columbia Encyclopedia- Chelicerata - AOL Research & Learn
		Chelicerates are characterized by the absence of antennae and jaws and the presence of feeding structures (chelicera), which are modified pincerlike appendages used mainly for grasping and fragmenting food.
		Nearly all the merostomates are extinct, the only living representative being Limulus, the horseshoe crab (subclass Xiphosura) and its relatives, which inhabits the soft bottom mud of shallow, coastal seas.
		The largest class of chelicerates, class Arachnida, includes orders Aranaea (spiders), Acarina (ticks and mites), Opiliones (daddy longlegs, or harvestmen), and Scorpionida (scorpions), among the most important.
	reference.aol.com /columbia/_a/chelicerata/20051205215509990020 (311 words)

	Defining characters of Chelicerata (Site not responding. Last check: 2007-11-03)
		Chelicerates have adapted to many ways of life, ranging from marine predators to air-breathing parasites, and can look remarkably different as a result.
		Chelicerates are also defined by the arrangement and specialisation of their appendages.
		Often it is difficult to identify chelicerate characters (particularly the presence or absence of chelicerae) in fossil specimens.
	palaeo.gly.bris.ac.uk /Palaeofiles/Fossilgroups/Chelicerata/characters.html (303 words)

	A CLADISTIC ANALYSIS OF THE ARTHROPODS
		The out-groups are shown with the chelicerates in Clade A. Since the onychophorans and tardigrades did not behave as out-groups we reject this figure.
		The chelicerates are represented in Clade A. The separation of the chelicerates from the groups in Clade B shows them evolving separately with the emergence of chelicerae and no antennae.
		Through an analysis of Figure 3, we propose that the chelicerates should be in their own phylum, Chelicerata, while the crustaceans and the uniramids should be united into 1 phylum, we named Mandibulata.
	comenius.susqu.edu /bi/202/Journal/vol10/number1/ELF.htm (1511 words)

	Early history and phylogeny of chelicerates
		The Ordovician Lemoneites appears very close to the origins of the chelicerates, but is too poorly preserved to resolve whether it is a primitive synziphosurine, sister group to chasmataspids, eurypterids and arachnids, or a sister group to all other chelicerates.
		Presence of chelicerae is difficult to determine in many Lower Palaeozoic fossils and the presence of median eyes (or their tubercle) and some degree of differentiation in the opisthosoma into a preabdomen and postabdomen are better characters for recognising a fossil chelicerate.
		Aglaspidids represent a poorly defined group which warrants revision and it is difficult to resolve the position of aglaspidids relative to chelicerates and the Paleomerus/Strabops lineage.
	members.tripod.com /~DrJasonDunlop/cp4.html (367 words)

	Palaeontology - Research - What are Trilobites?
		Their main subgroups are the crustaceans (like prawns and crabs), the chelicerates (including spiders, scorpions, and mites), and the wholly terrestrial groups Insecta and Myriapoda (the latter including millipedes and centipedes).
		Early workers took the geological antiquity of trilobites as evidence that they were the most primitive kind of arthropod, and may have included the ancestors of crustaceans and chelicerates.
		A single pair of antennae is likely a primitive feature for all arthropods, and the similarity of leg structure along the trilobite body (e.g., without the specialised leg-derived mouthparts of crustaceans or insects) can also be interpreted as primitive.
	www.amonline.net.au /palaeontology/research/trilobites02.htm (970 words)

	BioMed Central \| Full text \| The colonization of land by animals: molecular phylogeny and divergence times among ...
		The consistent support for a close relationship between myriapods and chelicerates, using mitochondrial and nuclear genes and different methods of analysis, suggests that this unexpected result is not an artefact of analysis.
		Myriapods (centipedes, millipedes) and chelicerates (e.g., arachnids, horseshoe crabs) have figured prominently in the earliest evidence of terrestrial animals.
		For example, the first taxonomically identifiable body fossils of terrestrial animals are arachnids and chilopods from the late Silurian (~419 Ma) of England [6], and the oldest unambiguous evidence of sub-aerial animal activity is of arthropod (diplopod-like) trackways from ~450 Ma sediments [16].
	www.biomedcentral.com /1741-7007/2/1 (4687 words)

	childofthecornx: BIOLOGY notes: Unit 3 Lesson 3
		In chelicerates, the head and thorax are fused into a structure known as the cephalothorax, so their body has two segments--the cephalothorax and the abdomen.
		Chelicerates have six pairs of unbranched appendages, all of which are attached to the cephalothorax:
		As you'll see, a significant number of chelicerates are predators, and their appendages play significant roles in their ability to capture prey.
	childofthecornx.livejournal.com /25387.html (4808 words)

	Subphylum Chelicerata (Spiders and their kin)
		The Chelicerata, which I have called a subphylum here for convenience, but which is just as well called a phylum in some texts, is an extremely ancient group of arthropods, including the extinct Eurypterida.
		No chelicerates possess jaws for biting and chewing, but suck up their food in liquid or semi-liquid form.
		Meaning the secrete digestive juices onto the food item as it is held close to the mouth, or inject digestive juices into the preys body, and suck up the half digested soup that results.
	www.earthlife.net /chelicerata/chelicerata.html (433 words)

	Michael's Biodiversiy Project
		Trilobite fossils show that they were segmented (had repeating units), but the segments gradually became fewer in number as they became specialized for specific functions (Dunlop 1999).
		Chelicerates diverged from the trilobites and continued to exist after the extinction of the trilobites.
		The most primitive form of chelicerates was Class Eurypterida or water scorpions.
	www.personal.psu.edu /users/m/d/mdk225/evolution.html (207 words)

	M28.htm
		The Chelicerates are an ancient group, originating from at least early Cambrian times, and are well represented in the fossil record.
		The most familiar living chelicerates are the spiders, scorpions, ticks, and mites (subclass Arachnida).
		Pycnogonids are currently thought to probably represent an early divergence from the evolutionary line leading to the other Chelicerates.
	www.meer.org /M28.htm (712 words)

	Natural History & Education > Life in Freshwater > Freshwater Animals and Plants > Animals > ...
		There are around 75000 species of Chelicerates divided into three classes: the horseshoe crabs (Merostomata), sea spiders (Pycnogonida) and spiders, scorpions, mites, ticks, chiggers, and harvestmen (Arachnida).
		Most Chelicerates belong to the class Arachnida which counts more than 60 000 species, half of which are spiders.
		Prosoma and abdomen are distinct in spiders and fused in ticks and mites.
	www.freshwaterlife.org /cds_static/en/chelicerates_en_5063_all_1.html (375 words)

	Neurogenesis in myriapods and chelicerates and its importance for understanding arthropod relationships Integrative and ... (Site not responding. Last check: 2007-11-03)
		Second, neural cells arise by stem-cell-like divisions of neuroblasts in insects and crustaceans, whereas groups of mainly postmitotic neural precursors are recruited for the neural fate in chelicerates and myriapods.
		We discuss whether these characteristics represent a sympleisiomorphy of myriapods and chelicerates that has been lost in the more derived Pancrustacea or whether these characteristics are a synapomorphy of myriapods and chelicerates, providing the first morphological support for the Myriochelata group.
		However, recent comparative studies on neurogenesis in the diplopod Glomeris marginata and the chilopod Lithobius forficatus have shown that the myriapods and the chelicerates share several features that cannot be found in homologous form in insects and crustaceans.
	www.findarticles.com /p/articles/mi_qa4054/is_200604/ai_n16123374 (1010 words)

	Arthropod Fossils and PhylogenyEdited by Gregory D.
		The relationships between major arthropod groups (chelicerates, crustaceans, hexapods, myriapods, trilobites) and other players (onychophorans, tardigrades) have preoccupied workers for decades, with further fuel added by the explosion of interest in molecular systematics.
		) have united the chelicerates with the myriapods, or with the crustaceans and insects to the exclusion of the myriapods.
		In part, these conflicting results are influenced by the fundamentally different classes of evidence on which they are based, by the relative infancy of the field of molecular systematics, and by the need for more extensive sampling; both genetic and taxonomic.
	palaeo-electronica.org /1999_1/books/arthropo.htm (1812 words)

	Lander University \| Anoplodactylus
		Pycnogonid tagmosis differs somewhat from that of other chelicerates and three tagmata, cephalon, trunk, and abdomen, can be identified (Fig 1, 18-45) rather than the cephalothorax and abdomen of other chelicerates.
		The typical chelicerate cephalothorax consists of a two-segmented head and four or five segmented thorax.
		In contrast, the pycnogonid cephalon consists of the two segmented chelicerate head fused with two anterior thoracic segments leaving the trunk with three (or more) thoracic segments.
	webs.lander.edu /rsfox/invertebrates/anoplodactylus.html (1563 words)

	Amphipods by Ronald L. Shimek, Ph.D. - Reefkeeping.com
		Crustaceans are predominantly animals of the marine realms, and that group is moderately successful on both the land and in fresh water.
		Insects and chelicerates are predominantly terrestrial or fresh water.
		Although some chelicerates are found in marine habitats, few insects are found there.
	www.reefkeeping.com /issues/2004-09/rs/index.php (3830 words)

	Neurogenesis in myriapods and chelicerates and its importance for understanding arthropod relationships -- Stollewerk ...
		Neurogenesis in myriapods and chelicerates and its importance for understanding arthropod relationships
		whereas in chelicerates and myriapods the central area of the
		precursors are recruited for the neural fate in chelicerates
	icb.oxfordjournals.org /cgi/content/abstract/46/2/195 (303 words)

	Xiphosura (Site not responding. Last check: 2007-11-03)
		Horseshoe crabs (Class Xiphosura) are an ancient group of primarily marine chelicerates.
		Despite their common name, they are actually more closely related to spiders, scorpions and other arachnids rather than true crabs (Crustacea) but it's easy to see how the two have been confused.
		My research at the University of Bristol and subsequently the University of Kansas has focused on the evolution of fossil and living horseshoe crabs, and more generally how they are related to other chelicerates.
	www.people.ku.edu /~ramoore (162 words)

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