Zoology 250 (2002) ARTHROPODA (I) ( Onychophora study images; click on tree to see full cladogram)

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• 1) Are 3 big questions regarding arthropods: i) intermediate forms? ii) one or multiple origins? iii) why so successful (segmented body, rigid exoskeleton, jointed limbs, image-forming eyes)?

• 2) Ph. ONYCHOPHORA (velvet worms, 70 spp.) a small terrestrial phylum discovered in 1826; marine examples are known from lower Cambrian (~ 500 MYA; e.g., Hallucigenia); predators on insects

  • a) 'Missing link' shares many similarities with Annelida & Arthropoda:
    
    - annelid-like: cuticle (soft, not articulated) & body wall muscles (polychaete-like); simple gut; paired, segmental nephridia; ciliated reproductive tract; paired, paddle-like, unjointed limbs

    - arthropod-like: cuticle must be molted; reduced coelom; spacious hemocoel with a muscular heart; gut lined with cuticle; trachea/ spiracle system for respiration; large brain

  • b) unique trait: slime papillae

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• 1) The Arthropoda is the largest animal phylum (more than 750,000 spp)
  
  a) strongly metameric body plan, tagmatization widespread & varied
  b) the acron & telson (=pygidium; bears the anus) are considered homologous to the annelid prostomium & pygidium
  c) lack of motile body cilia throughout phylum supports monophyly
  d) all possess a semi-rigid exoskeleton & jointed limbs

• 2) The semi-rigid, jointed exoskeleton (cuticle) must be molted
  
  a) a body segment may have up to 4 plates (sclerites): dorsal tergum (=notum), two lateral pleura (Mandibulata only), ventral sternum
  b) the cuticle has 3 main layers (epicuticle, exocuticle, endocuticle) formed mostly of chitin & protein, but may include wax, tanned proteins & minerals (e.g., CaCO3); it is thin & flexible at joints
  c) molting is controlled by ecdysone; only epi- & exocuticle is shed (includes lining of foregut, hindgut, trachea, gills, & apodemes)
  d) new cuticle is produced underneath the old one before it is shed
  e) following a molt, water or air inflates the body to its new size
  f) molting fracture planes vary among subphyla

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