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A phylum that includes the well-known insects, spiders, ticks, and crustaceans, as well as many smaller groups, some of which are known only as fossils. Arthropodous animals make up about 75% of all animals that have been described. The estimated number of known species exceeds 780,000. Of this number the class Insecta alone contains about 700,000 described species. Arthropods vary in size from the microscopic mites to the giant decapod crustaceans, such as the Japanese crab with an appendage span of 5 ft (1.5 m) or more.

The adult arthropod typically has a body composed of a series of ringlike segments, muscularly movable on each other. The integument is sclerotized by the formation of hardening substances in the cuticle, and the segmental limbs are many-jointed. These characteristics, taken together, distinguish the arthropods from all other animals. Young stages may be quite different from the adults, and some parasitic species differ very radically from their relatives.

Arthropod evolution is no longer the clear-cut subdivision of a single phylum, Arthropoda, into three structurally divergent subphyla. Advances in functional morphology, comparative embryology, spermatology, serology, and paleontology have brought an array of new hypotheses about relationships of arthropodous animals. At the center of debate is the question of monophyly versus polyphyly: Did all arthropodous animals evolve from a common ancestor or did several distinct lineages evolve along similar pathways? Two opposing classification schemes are presented; numerous variations on these schemes can be found in the literature. The first pair of classifications is as follows:

  • Phylum Uniramia

    • Subphylum: Onychophora

    •      Myriapoda

    •         Hexapoda (Insecta)

  • Phylum Trilobita (Trilobitomorpha)

  • Phylum Crustacea

  • Phylum Chelicerata

    • Versus

  • Phylum Arthropoda

    • Subphylum Arachnata

      • Superclass: Trilobita

      •      Chelicerata

    • Subphylum Mandibulata

      • Superclass: Crustacea

      •     Myriapoda

      •       Insecta

  • Phylum Onychophora

Alternatively, a slightly different and expanded pair of classifications is as follows:

  • Phylum Uniramia

    • Subphylum Onychophora

    • Subphylum Myriapoda

      • Class: Chilopoda

      •     Diplopoda

      •     Symphyla

      •     Pauropoda

      •     Arthropleurida

    • Subphylum Hexapoda

      • Class: Protura

      •     Collembola

      •     Diplura

      •     Thysanura

      •        Pterygota (Insecta)

  • Phylum Crustacea

    • Class: Cephalocarida

    •     Remipedia

    •     Branchiopoda

    •     Ostracoda

    •     Tantulocarida

    •     Maxillopoda

    •        Malacostraca

  • Phylum Cheliceriformes

    • Subphylum Pycnogonida

    • Subphylum Chelicerata

      • Class: Merostomata

      •      Arachnida

  • Phylum Trilobitomorpha

    • Class: Trilobitoidea

    •     Trilobita

      • Versus

  • Phylum Onychophora

  • Phylum Arthropoda

    • Subphylum Cheliceromorpha

      • Infraphylum: Pycnogonida

      •       Chelicerata

        • Superclass: Xiphosurida

        •        Cryptopneustida

          • Class: Eurypterida

          •       Archnida

    • Subphylum Gnathomorpha

      • Infraphylum: Trilobitomorpha

          • Class: Trilobita

          •      Trilobitodea

      • Infraphylum: Mandibulata

          • Class: Cheloniellida

          •     Crustacea

          •     Myriapoda

          •          Insecta

  • Body segmentation, or metamerism, is the most fundamental character of the arthropods, but it is shared by the annelid worms, so there can be little doubt that these two groups of animals are related. The limbs of all modern arthropods develop in the embryo from small lateroventral outgrowths of the body segments that lengthen and become jointed. Hence it may be inferred that the arthropods originated from some segmented worm that acquired similar lobelike limb rudiments and thus, as a crawling or walking animal, became distinguished from its swimming relatives. Then, with sclerotization of the integument, the limbs could lengthen and finally become jointed, providing greater locomotor efficiency. In their later evolution, some of these limbs became modified for many other purposes, such as feeding, grasping, swimming, respiration, silk spinning, egg laying, and sperm transfer. The body segments, corresponding to specialized sets of appendages, tend to become consolidated or united in groups, or tagmata, forming differentiated body regions, such as head, thorax, and abdomen. Annelida; Metameres.

    Sclerotization of the cuticle may be continuous around the segments. More usually, it forms discrete segmental plates, or sclerites. A back plate of a segment is a tergum, or notum; a ventral plate is a sternum; and lateral plates are pleura. The consecutive tergal and sternal plates, unless secondarily united, are connected by infolded membranes, and are thus movable on each other by longitudinal muscles attached on anterior marginal ridges of the plates. Since nearly all the body and limb muscles are attached on integumental sclerites, there is little limit to the development of skeletomuscular mechanisms.

    All arthropods have all the internal organs essential to any complex animal. An alimentary canal extends either straight or coiled from the subapical ventral mouth to the terminal anus. Its primary part is the endodermal stomach, or mesenteron, but there are added ectodermal ingrowths that form a stomodeum anteriorly and a proctodeum posteriorly. The nervous system includes a brain and a subesophageal ganglion in the head, united by connectives around the stomodeum, and a ventral nerve cord of interconnected ganglia. Some of the successive ganglia, however, may be condensed into composite ganglionic masses. Nerves proceed from the ganglia. Internal proprioceptors and surface sense organs of numerous kinds are present, chiefly tactile, olfactory, and optic. A usually tubular pulsatory heart lies along the dorsal side of the body and keeps the blood in circulation. In some arthropods arteries distribute the blood from the heart; in others it is discharged from the anterior end of the tube directly into the body cavity. The blood reenters the heart through openings along its sides.

    Aquatic arthropods breathe by means of gills. Most terrestrial species have either flat air pouches or tubular tracheae opening from the outside surface; some have both. A few small, soft-bodied forms respire through the skin. Excretory organs open either at the bases of some of the appendages or into the alimentary canal. Most arthropods have separate sexes, but some are hermaphroditic, and parthenogenesis is of common occurrence. The genital openings differ in position in different groups and are not always on the same body segment in the two sexes.Insecta

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    From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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