. Principles of modern biology. Biology. The Animal Kingdom - 655 muscles stretch from part to part, bridging the joints. Thus each part of the exoskeleton serves as a lever in effecting muscular move- ment and each joint serves as a fulcrum. Moreover, the arthropod exoskeleton, to which the muscles attach internally, stands in distinct contrast to the vertebrate endo- skeleton, to which the muscles attach ex- ternally. Classes of the Arthropoda. The natural groups into which this large phylum can be divided are: Class 1. The Crustacea (Lobsfers, Shrimps, Barnacles, Crabs, etc.). Typically the
. Principles of modern biology. Biology. The Animal Kingdom - 655 muscles stretch from part to part, bridging the joints. Thus each part of the exoskeleton serves as a lever in effecting muscular move- ment and each joint serves as a fulcrum. Moreover, the arthropod exoskeleton, to which the muscles attach internally, stands in distinct contrast to the vertebrate endo- skeleton, to which the muscles attach ex- ternally. Classes of the Arthropoda. The natural groups into which this large phylum can be divided are: Class 1. The Crustacea (Lobsfers, Shrimps, Barnacles, Crabs, etc.). Typically these are aquatic Arthropoda with gills, two pairs of antennae, and at least five pairs of legs. Usu- ally the body consists of a cephalothorax and abdomen (Fig. 32-30). Class 2. The Arachnida (Spiders, Ticks, Scorpions, Horseshoe Crabs, etc.). These mainly terrestrial Arthropoda have four pairs of legs, no antennae, and a cephalothorax. Book lungs are the usual respiratory mecha- nism, although a few forms have gills and some have tracheae (Fig. 32-30). Class 3. The Insecta (Flies, Bugs, Beetles, Bees, Butterflies, etc.). These terrestrial Arthropoda have three pairs of legs, only one pair of antennae, and (usually) two pairs of wings. A tracheal system of air tubes (p. 657) provides for respiration and typically the body dis- plays a distinct head, thorax, and abdomen (Fig. 32-30). Class 4. The Chilopoda (The Centipedes). These are elongate, flattened, terrestrial Ar- thropoda, with a distinct head, bearing one pair of antennae; there is no thorax and the body consists of at least fifteen segments (usually more), each (except the first) bear- ing one pair of seven-jointed walking legs. The appendage of the first body segment is highly modified, forming a formidable four-. Crayfish CRUSTACEA Grasshopper INSECTA Spider ARACHNIDA Centipede Millipede Peripatus DIPLOPODA CHILOPODA ONYCHOPHORA Fig. 32-30. Phylum Arthropoda: a representative animal from each of the six living
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