Difference between revisions of "Arthropod" - New World Encyclopedia
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**[[Cephalocarida]]* - horseshoe shrimps | **[[Cephalocarida]]* - horseshoe shrimps | ||
**[[Branchiopoda]] - brime shrimp | **[[Branchiopoda]] - brime shrimp | ||
| − | **[[Maxillopoda]] - | + | **[[Maxillopoda]] - [[barnacle]]s, fish lice, [[copepod]]s, etc. |
**[[Ostracoda]]* - seed shrimp | **[[Ostracoda]]* - seed shrimp | ||
**[[Malacostraca]]* - [[lobster]]s, [[crab]]s, [[shrimp]], etc. | **[[Malacostraca]]* - [[lobster]]s, [[crab]]s, [[shrimp]], etc. | ||
{{Taxobox_end}} | {{Taxobox_end}} | ||
| + | '''Arthropods''' ([[Taxonomy#Scientific or biological classification|phylum]] '''Arthropoda''') are the largest [[phylum]]* of [[animal]]s and include the [[insect]]s, [[arachnid]]s, and [[crustacean]]s, as well as [[milliped]]es, [[centiped]]es, among others. Approximately eighty percent of extant (living) animal species are classified as arthropods, with over a million modern species described and a [[fossil]] record reaching back to the early [[Cambrian]]. | ||
| − | + | Arthropods are [[invertebrate]]s that are characterized by the possession of a segmented body, a pair of jointed appendages on each segment, and an [[exoskeleton]]. The jointed appendages, which include both legs and antennae, are the source for the name arthropod, which comes from the [[Greek language|Greek]] for ''jointed foot''. In some species, appendages are not present, but are assumed to have been lost during the course of evolution. All arthropods are covered by a hard [[exoskeleton]], made out of protein and a tough polysaccharide called [[chitin]]. The exoskeleton provides support and protection. Periodically, an arthropod sheds this covering when it [[ecdysis|moults]]. | |
| − | Arthropods | + | Arthropods also have a dorsal [[heart]], a ventral [[nervous system]], an open [[circulatory system]], and a [[digestive system]]. |
| − | Arthropods | + | Arthropods play a major role in [[human]] life, whether as food, [[religion|religious]] symbols, pollinators of flowers, and major links in food chains, or as vectors of [[disease]] and [[agriculture|agricultural]] pests. |
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Arthropods are adapted to almost all environments on earth. They are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various [[symbiosis|symbiotic]] and [[parasite|parasitic]] forms. They range in size from microscopic [[plankton]] (~0.25 [[millimetre|mm]]) up to forms several meters long. | Arthropods are adapted to almost all environments on earth. They are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various [[symbiosis|symbiotic]] and [[parasite|parasitic]] forms. They range in size from microscopic [[plankton]] (~0.25 [[millimetre|mm]]) up to forms several meters long. | ||
| − | == | + | == Anatomy and physiology== |
[[Image:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg|left|thumb|250px|[[Blue crab]], ''Callinectes sapidus'']] | [[Image:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg|left|thumb|250px|[[Blue crab]], ''Callinectes sapidus'']] | ||
The success of the arthropods is related to their hard '''[[exoskeleton]]''', segmentation, and jointed [[appendage]]s. The appendages are used for feeding, sensory reception, defense, and locomotion. | The success of the arthropods is related to their hard '''[[exoskeleton]]''', segmentation, and jointed [[appendage]]s. The appendages are used for feeding, sensory reception, defense, and locomotion. | ||
| − | Most arthropods respire (breathe) through a [[trachea]]l system (exceptions are | + | Most arthropods respire (breathe) through a [[trachea]]l system (exceptions are Pauropoda (centiped-like arthropods), some thysanurans (an order of insects) and some [[arachnid]]s like many [[mite]]s). This creates a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use [[gills]] to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialised for gas exchange. Insects have tracheal systems: air sacs leading into the body from pores, called spiracles, in the epidermis [[cuticle]]*. |
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| + | Arthropods have an [[Circulatory system#Open circulatory system|open circulatory system]]. Haemolymph, a [[copper]]-based [[blood]] analogue, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are [[protostome]]s. There is a coelom (body cavity), but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a hemocoel, filled with hemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental ''acron'' which usually supports compound and simple [[eyes]] and a postsegmental ''[[telson]]*'' (the last body division in crustaceans, but not a true segment). These are grouped into distinct, specialized body regions called ''tagmata''. Each segment at least primitively supports a pair of appendages. | ||
| + | The cuticle (noncellular protective covering) in arthropods forms a rigid exoskeleton, composed mainly of [[chitin]], which is periodically shed as the animal grows. They contain an inner zone (procuticle) which is made of protein and chitin (a polysaccharide) and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of [[proteins]] and [[lipids]]. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called ''sclerites'' on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name—joint feet—and separates them from their very close relatives, the [[Onychophora]] and [[Tardigrada]], also called [[Lobopoda]] (and which is sometimes included in a group called [[Panarthropoda]], that also includes true arthropods). The skeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, [[moult]]ing, is expensive in terms of energy consumption, and during the moulting period, an arthropod is vulnerable. | ||
==Classification of arthropods== | ==Classification of arthropods== | ||
Revision as of 22:23, 17 March 2006
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 Brachypelma smithi | ||||
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Arthropods (phylum Arthropoda) are the largest phylum of animals and include the insects, arachnids, and crustaceans, as well as millipedes, centipedes, among others. Approximately eighty percent of extant (living) animal species are classified as arthropods, with over a million modern species described and a fossil record reaching back to the early Cambrian.
Arthropods are invertebrates that are characterized by the possession of a segmented body, a pair of jointed appendages on each segment, and an exoskeleton. The jointed appendages, which include both legs and antennae, are the source for the name arthropod, which comes from the Greek for jointed foot. In some species, appendages are not present, but are assumed to have been lost during the course of evolution. All arthropods are covered by a hard exoskeleton, made out of protein and a tough polysaccharide called chitin. The exoskeleton provides support and protection. Periodically, an arthropod sheds this covering when it moults.
Arthropods also have a dorsal heart, a ventral nervous system, an open circulatory system, and a digestive system.
Arthropods play a major role in human life, whether as food, religious symbols, pollinators of flowers, and major links in food chains, or as vectors of disease and agricultural pests.
Arthropods are adapted to almost all environments on earth. They are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~0.25 mm) up to forms several meters long.
Anatomy and physiology
The success of the arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion.
Most arthropods respire (breathe) through a tracheal system (exceptions are Pauropoda (centiped-like arthropods), some thysanurans (an order of insects) and some arachnids like many mites). This creates a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialised for gas exchange. Insects have tracheal systems: air sacs leading into the body from pores, called spiracles, in the epidermis cuticle.
Arthropods have an open circulatory system. Haemolymph, a copper-based blood analogue, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom (body cavity), but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a hemocoel, filled with hemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental acron which usually supports compound and simple eyes and a postsegmental telson (the last body division in crustaceans, but not a true segment). These are grouped into distinct, specialized body regions called tagmata. Each segment at least primitively supports a pair of appendages.
The cuticle (noncellular protective covering) in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain an inner zone (procuticle) which is made of protein and chitin (a polysaccharide) and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name—joint feet—and separates them from their very close relatives, the Onychophora and Tardigrada, also called Lobopoda (and which is sometimes included in a group called Panarthropoda, that also includes true arthropods). The skeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, moulting, is expensive in terms of energy consumption, and during the moulting period, an arthropod is vulnerable.
Classification of arthropods
Arthropods are typically classified into five subphyla:
- Trilobites are a group of formerly numerous marine animals that died in the mass extinction at the end of the Permian.
- Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae.
- Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
- Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia.
- Crustaceans are primarily marine (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, and many others.
Aside from these major groups, there are also a number of fossil forms as Cambropodus, Anomalocarida and Euthycarcinoidea, mostly from the lower Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.
Related groups
At one point, it was considered that the different subphyla of arthropods had separate origins from segmented worms, and in particular that the Uniramia were closer to the Onychophora than to other arthropods. However, this is rejected by most workers, and is contradicted by genetic studies.
Traditionally the Annelida have been considered the closest relatives of these three phyla, on account of their common segmentation. More recently, however, this has been considered convergent evolution, and the arthropods and allies may be more closely related to certain pseudocoelomates such as roundworms that share with them growth by moulting, or ecdysis. These two possible lineages have been termed the Articulata and Ecdysozoa.
Arthropods and humans
". The larger crustaceans—shrimps, lobsters, and crabs—are used as food throughout the world. Small planktonic crustaceans, such as copepods, water fleas, and krill, are a major link in the food chain between the photosynthetic phytoplankton and the larger carnivores, such as many fish and whales. Although many species of insects and mites attack food crops and timber, arthropods are of enormous benefit to human agriculture. Approximately two-thirds of all flowering plants (angiosperm) are pollinated by insects, and soil and leaf-mold arthropods, which include insects, mites, myriapods, and some crustaceans (pill bugs), play an important role in the formation of humus from decomposed leaf litter and wood.
The stings and bites of arthropods may be irritating of painful, but very few inject dangerous toxins. Medically, arthropods are more significant as carriers of disease such as malaria, yellow fever, dengue fever, and elephantiasis (via mosquitos), African sleeping sickness (via tsetse flies), typhus fever (via lice), bubonic plague (via fleas), and Rocky Mountain spotted fever and Lyme disease (via ticks). Many diseases of domesticated animals are also transmitted by arthropods. "
External links and references
- Common North American Arthropods: 4,000 diagnostic photos, descriptions, taxonomy
- The Arthropod Story (from Understanding evolution @ Berkeley.edu)
- http://www.itis.usda.gov ITIS TSN: 82696
- BugGuide Photographs, life history information, and identification of North American terrestrial arthropods
- http://www.peripatus.gen.nz/Taxa/Arthropoda/Index.html Campbell, Reece and Mitchell. Biology. 1999
- Do spiders have hydraulic legs? (from The Straight Dope)
- Euthycarcinoida
- Euthycarcinoids
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