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ARTHROPODA

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(Latreille, 1829 )

Sections
  • Background
  • Morphology
  • Movement and Moulting
  • Background

    Arthropods are the most species rich group of animals on Earth and are one of the most abundant groups of organisms. They have a great adaptive diversity that has enabled them to thrive in practically every habitat from the deep oceans to high mountains and from hot springs to polar icecaps. Common examples of living arthropods are crabs, shrimps, lobsters, barnacles, insects, ticks and scorpions. There is an emerging consensus, based largely on molecular evidence, that the arthropods belong to a group, the Ecdysozoa, comprising the phyla that share the possession of a cuticular exoskeleton and undergo periodic moulting of that cuticle.

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    Morphology

    The bodies of arthropods typically show metameric segmentation, which is evident in the embryonic development of all arthropods and is a conspicuous feature of many adults, especially the more primitive species. Secondary loss of segmentation may occur in derived forms. Primitively each arthropod segment bears a pair of appendages, the expression of which is controlled by a cascading system of Hox genes. The basic plan of the nervous system consists of a dorsal anterior brain, followed by a ventral nerve cord with marked ganglia in each segment.  The distinguishing feature of arthropods is the chitinous exoskeleton or cuticle that covers their entire body and the jointed appendages. The exoskeleton of arthropods is multifunctional, serving as armour against enemies, a protection against water loss, support for the softer internal organs of the body, and as an attachment for musculature. The cuticle consists of three layers: the innermost endocuticle is flexible and composed of chitin, the intermediate exocuticle is a rigid layer and high in protein, and the thin external epicuticle contains waxy material. The epicuticle serves to prevent loss of water and other substances. Movement is possible by the division of the cuticle into distinct plates separated by regions of flexible arthrodial membrane. Limbs tend to be specialised for functions such as locomotion, feeding, respiration, sensory reception and mating (Boxshall, 2004). Arthropod antennules are primitively uniramous, comprising a single segmented branch whereas all postantennulary limbs are primitively biramous, with an inner endopod and an outer exopod. In extant arachnids, hexapods and myriapods one ramus (the exopod) is lost giving rise to a uniramous condition.

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    Movement and Moulting

    Arthropods move by means of their jointed appendages. The exoskeleton and the extrinsic and intrinsic muscles contained within the body operate together as a lever system. The possession of a rigid external cuticle would cause problems for growing animals. The solution has been periodic shedding of the exoskeleton (moulting). Some arthropods such as lobsters and some crabs moult throughout their lives, but other such as copepods, insects and spiders have a more or less fixed numbers of instars with the final moult coinciding with the attainment of sexual maturity.

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    [ARTHROPODA on the taxonomic tree]

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