• 1) Jointed limbs exhibit a tremendous diversity of form
  
  a) may be uniramous (1 branch) or biramous (2 branches)
  b) crustacean limbs have 3 components (protopodite, exite, endite)
  c) the terminal exite= exopodite, the terminal endite= endopodite
  d) non-terminal branches include epipodites (an exite) & gnathobases (an endite)
  e) limbs may be phyllopodous (flattened) or stenopodous (tubular)
  f) movement is controlled by muscles intrinsic to the skeleton that attach at joint margins or to infoldings of the cuticle (apodemes)
  g) limb joints often have alternating ball & socket (di-condylic) hinges in adjacent segments

• 2) Most arthropods have paired, compound eyes (the most spectacular of animal eyes except for vertebrates & cephalopods):
  
  a) composed of multiple ommatidia (units with separate cuticular lenses, light-sensitive cells & nerves)
  b) the image is a composite of many spots of light

• 3) Respiration can only take place across very thin cuticle; respiratory adaptations depend upon the type of environment
  
  a) aquatic:
  
  - book gills occur in primitive aquatic chelicerates (horseshoe crabs); gills in crustaceans are typically exopodites or epipodites, and may be thoracic (most common) or abdominal
  - gills depend on some method for pumping water
  
  b) terrestrial:
  
  - 1 or 2 pair of abdominal 'book lungs' occur in many terrestrial arachnids
  - most uniramians, some arachnids and a few crustacea have spiracle/trachea systems
  - 'lungs' & trachea depend upon diffusion that may be augmented by inspiration/exhalation

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