• SubPh. CRUSTACEA (crabs, shrimps & relatives; >50,000 marine, freshwater & terrestrial spp.); share 3 traits: i) 2-pair of antennae, ii) terminal moveable spine(s) on telson, & iii) a nauplius larval stage

• 2) The body plan is surprisingly stereotyped for such a diverse group:
  
  a) head has 5 segments with limbs: 2 antennae, 1 mandible, 2 maxillae
  b) head is typically covered by a carapace; the carapace may extend over a few or all thoracic segments (-> cephalothorax)
  c) some thoracic segments may fuse with the head and their limbs modified as maxillipeds for feeding; walking legs= periopods
  d) abdominal segments may or may not have limbs (pleopods)

• 3) Crustacean diversity
  
  a) 5 classes: 3 major (Branchiopoda, Maxillopoda, Malacostraca) & 2 small, phylogenetically important (Remipedia, Cephalocarida)
  b) most are marine, but one class (Branchiopoda) is primarily freshwater and three others have freshwater members
  c) most are free living; 2 classes have a number of parasitic species, and one subclass (Cirripidea) is entirely sessile as adults
  d) body size is generally small (<20 mm) in most classes/subclasses, but large size (>100 mm) occurs in three subclasses
  e) classes are distinguished by: number of tagmata, number of segments per tagmatum, number of anterior segments covered by a carapace, number of segments fused with the carapace, and presence/absence of movable spines (rami) on the telson
  f) most diverse class (Malacostraca) share 3 characters: i) all have 21 somites, ii) stalked eyes in adults, iii) limbs on abdominal segments

• 4) Primitive Crustacea had an elongate body with many similar biramous limbs; most have a carapace that covers the head & part/all of thorax

• 5) Particle feeding in Crustacea is accomplished in many ways
  
  a) primitively, crustaceans were probably particle feeders
  b) filter feeding at small size is difficult because of high viscosity: water has no momentum, 'wall effects' are large, flow is reversible
  c) in fairy shrimp a filter box generates high pressure that forces water through a sieve of setae; other small species use similar methods

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