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 classes. 



