PHYLUM ARTHROPODA

The arthropods are the most numerous of the animal phyla, including nearly one million known species (and more are being discovered all of the time!) Arthropods are found in almost all conceivable habitats, and feed on a great variety of materials. The arthropods are the only group of invertebrates to fully adapt to terrestrial life. All arthropods are characterized by the presence of a rigid, chitinous body covering called an exoskeleton. Primitively, the arthropod body plan consisted of a number of segments (as in annelids), each segment probably bearing one pair of jointed appendages. The segments and the jointed appendages have become modified for many different purposes. The classification of arthropods is complex, but is based largely on the patterns of loss or fusion of segments and the way in which the appendages are modified. All arthropods also share an open circulatory system. Arthropods are divided into three major subphyla, each of which is subdivided into a number of classes. The major categories we will consider are:

• Subphylum Trilobitomorpha: an extinct group of primitive marine arthropods. They were characterized by a distinct head with a segmented body, each segment bearng jointed appendages.
• Subphylum Chelicerata: The chelicerates are relatively simple, mainly terrestrial arthropods that lack antennae and mandibles, but have the first pair of appendages modified to form specialized, fang-like feeding appendages called chelicerae. There are two body regions, a cephalothorax (fused head and thorax) and an abdomen. We will examine two classes in this group:
  • Class Merostomata: a group of aquatic arthropods, the horseshoe crabs, with compound eyes, gills and five or six pairs of appendages in addition to the chelicerae.
  • Class Arachnida: terrestrial arthropods with no gills, simple eyes and six pairs of jointed appendages.
• Subphylum Crustacea: The crustaceans are characterized by the presence of jaw-like mandibles (chewing mouth parts), two pairs of antennae, and biramous (two-branched) appendages. There are two body regions, a cephalothorax and an abdomen. There is a single class.
  • Class Malacostraca: These common aquatic arthropods include the lobsters, crayfish, shrimps and a variety of other aquatic organisms. They respire mainly by gills.
• Subphylum Hexapoda:
  • Class Insecta: This class is the largest group of arthropods and includes mainly terrestrial arthropods with a distinct head, thorax and abdomen, one pair of antennae, three pairs of legs, jaws and up to two pairs of wings. This group includes most of the familiar creatures affectionately known as "bugs".
• Subphylum Myriapoda:
  • Class Chilopoda: The centipedes have long, flattened bodies with many segments, each of which has a single pair of legs.
  • Class Diplopoda: The millipedes have a long, relatively cylindrical body with many segments, each bearing two pairs of legs.

     

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