• 1) Although tremendously diverse (>50,000 spp.), crustaceans share three distinctive traits: i) 2-pair antennae, ii) terminal moveable spine(s) off the telson, and iii) a nauplius larval stage

• 2) The head has 5 segments, each bearing a pair of limbs (2 antennae, 1 mandible, 2 maxillae); additional thoracic segments may be fused with the head and their limbs modified as maxillipeds for feeding

• 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 (<20mm) in most classes/subclasses, but large size (>100mm) occurs in three subclasses

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  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, presence/absence of movable spines (rami) on the telson

  f) primitively, crustaceans were probably particle feeders; however the most successful class (Malacostraca) is mainly raptorial

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

• 5) Particle feeding in Crustacea is accomplished in many ways

  a) filter feeding at small size is difficult because of high viscosity: water has no momentum, 'wall effects' are large, flow is reversible

  b) in fairy shrimp a filter box generates high pressures that force water through a sieve of setae; other small spp use similar methods

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