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Arthropod

2007 Schools Wikipedia Selection. Related subjects: General Biology

             How to read a taxoboxArthropoda
   Mexican redknee tarantulaBrachypelma smithi
   Mexican redknee tarantula
   Brachypelma smithi
                Scientific classification

   Kingdom:     Animalia
   Superphylum: Ecdysozoa
   Phylum:      Arthropoda
                Latreille, 1829

                            Subphyla and Classes

     * Subphylum Trilobitomorpha
          + Trilobita - trilobites (extinct)
     * Subphylum Chelicerata
          + Arachnida - spiders, scorpions, etc.
          + Merostomata - horseshoe crabs, etc.
          + Pycnogonida - sea spiders
     * Subphylum Myriapoda
          + Chilopoda - centipedes
          + Diplopoda - millipedes
          + Pauropoda
          + Symphyla
     * Subphylum Hexapoda
          + Insecta - insects
          + Collembola - springtails
          + Diplura
          + Protura
     * Subphylum Crustacea
          + Branchiopoda – brine shrimp etc.
          + Remipedia
          + Cephalocarida – horseshoe shrimp
          + Maxillopoda - barnacles, fish lice, etc.
          + Ostracoda – seed shrimp
          + Malacostraca - lobsters, crabs, shrimp, etc.

   Arthropods (Phylum Arthropoda, from the Greek ἄρθρον, meaning joint and
   ποδός, meaning foot) are the largest phylum of animals and include the
   insects, arachnids, crustaceans, and others. More than 80% of described
   living animal species are arthropods , with over a million modern
   species described and a fossil record reaching back to the late
   proterozoic era. Arthropods 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 (~¼  mm) up to forms several metres long. The largest living
   arthropod is the Japanese spider crab, with a leg span up to 3½  m (12
   ft), and some prehistoric arthropods were even larger, such as
   Pterygotus and Arthropleura.

   Arthropods are characterised by the possession of a segmented body with
   appendages on each segment. They have a dorsal heart and a ventral
   nervous system. All arthropods are covered by a hard exoskeleton made
   of chitin, a polysaccharide, which provides physical protection and
   resistance to desiccation. Periodically, an arthropod sheds this
   covering when it moults.

Basic arthropod structure

   Blue crab (Callinectes sapidus), a crustacean
   Blue crab (Callinectes sapidus), a crustacean

   The success of arthropods is related to their hard exoskeleton,
   segmentation, and jointed appendages. The appendages are used for
   feeding, sensory reception, defense, and locomotion. The muscle system
   is more or less assisted by hydraulics originated from the blood
   pressure created by the heart . The hydraulic system in spiders is
   especially well developed.
   Harpaphe haydeniana, a myriapod
   Harpaphe haydeniana, a myriapod

   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 and most other terrestrial species have tracheal
   systems: air sacs leading into the body from pores called spiracles in
   the epidermis cuticle. Others use book lungs, or gills modified for
   breathing air as seen in species like the coconut crab. Some areas of
   the legs of soldier crabs are covered with an oxygen absorbing
   membrane. The gill chambers in terrestrial crabs sometimes have two
   different structures: one that is gilled and used for breathing
   underwater, and another specially adapted to take up oxygen from the
   air (a pseudolung). Arthropods also have a complete digestive system
   with both a mouth and anus.

   Arthropods have an open circulatory system. Haemolymph containing
   haemocyanin, a copper-based oxygen-carrying protein (the copper makes
   the blood blue, unlike humans that use hemoglobin which uses iron that
   makes it red), 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, but it is reduced to a tiny cavity
   around the reproductive and excretory organs, and the dominant body
   cavity is a haemocoel, filled with haemolymph which bathes the organs
   directly. The arthropod body is divided into a series of distinct
   segments, plus a pre-segmental acron which usually supports compound
   and simple eyes and a post-segmental telson. These are grouped into
   distinct, specialised body regions called tagmata. Each segment, at
   least primitively, supports a pair of appendages.

   The cuticle 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 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 — jointed feet — and separates
   them from their relatives, the Onychophora and Tardigrada, also called
   Lobopoda (and which is sometimes included in a group called
   Panarthropoda that also includes arthropods). The exoskeletons 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, ecdysis, is expensive
   in terms of energy, and during the moulting period, an arthropod is
   vulnerable.

Classification of arthropods



   Arthropoda
              Paradoxopoda

                            Myriapoda

                            Chelicerata
              Pancrustacea

                               Cirripedia

                               Pemipedia

                               Collembola


                                  Branchiopda

                                  Cephalocarida

                                  Malacostraca

                                  Insecta

         Phylogenetic relationships of the major extant arthropod groups,
       derived from mitochondrial DNA sequences . Taxa in pink are parts of
                             the subphylum Crustacea.

   Arthropods are typically classified into five subphyla, of which one is
   extinct :
    1. Trilobites are a group of formerly numerous marine animals that
       died in the mass extinction at the end of the Permian-Triassic
       extinction event.
    2. Chelicerates include spiders, mites, scorpions and related
       organisms. They are characterised by the presence of chelicerae.
    3. 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.
    4. 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, though genetic evidence
       tends to support a closer relationship between hexapods and
       crustaceans.
    5. 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
   including anomalocarids and euthycarcinoids , 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.

   The phylogeny of the arthropods has been an area of considerable
   interest and dispute. The validity of many of the arthropod groups
   suggested by earlier authors is being questioned by recent studies;
   these include Mandibulata, Uniramia and Atelocerata. The most recent
   studies tend to suggest a paraphyletic Crustacea with different hexapod
   groups nested within it . The remaining clade of Myriapoda and
   Chelicerata is referred to as Paradoxopoda or Myriochelata.

   Since the International Code of Zoological Nomenclature recognises no
   priority above the rank of family, many of the higher groups can be
   referred to by a variety of different names .

Evolution

   A velvet worm
   A velvet worm



               Sipuncula
   Articulata

                             Mollusca
               Euarticulata

                                            Annelida
                             Panarthropoda

                                            Onychophora

                                            Tardigrada

                                            Arthropoda

                   A phylogeny of the arthropods after Nielsen.

   Arthropods are thought to have branched from an ancestor of the
   segmented worms during the Pre-Cambrian era . Velvet worms
   (Onychophora) are a good example of what it is imagined that their
   ancestors looked like, and their similarity to caterpillars and
   millipedes is thought to be not entirely coincidental . The common
   ancestral arthropod, though, apparently happened to be one that had
   evolved not just chitinous mouthparts like other segmented worms, but
   also a chitinous structure all over its body; with all arthropods, the
   segments have become distinct (at least in larvae), each covered with
   one or more plate, and with legs, or limbs, one pair per segment.

   At one point, it was believed 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 contradicted by genetic studies and is now rejected by
   most biologists.

   Arthropods are grouped together with two similar phyla ( Tardigrada and
   Onychophora) to form the monophyletic group Panarthropoda.

   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.

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