a) head has 5 segments with limbs: 2 antennae, 1 mandible, 2 maxillae

b) head covered by a shield: a carapace (a structure) may extend over some or all thoracic segments, yielding a cephalothorax(body region)

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)

e) primitively had an elongate body and many similar biramous legs

a) habitats: most classes are marine, but one class (Branchiopoda) is primarily freshwater and three others have freshwater members

b) lifestyles: most are free living; 3 classes have a number of parasitic species, and one subclass (Cirripidea) is entirely sessile as adults

c) body size: generally small (<20 mm) in most classes/subclasses, but large size (>100 mm) occurs in three subclasses

d) classes are distinguished by: number of tagmata, number of segments per tagmatum, number of thoracic segments covered by a carapace, number of segments fused with the carapace, and presence/absence of movable spines (rami) on the telson

e) most diverse class (Malacostraca) share 3 characters: i) 21 somites, ii) stalked eyes in adults, iii) limbs on abdominal segments; 3 general body forms: shrimp-like, lobster-like, crab-like (multiple origins)

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 totally 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

For some spectacular computer animations of the form, and of the swimming, escape, and prey capture behaviors of the predatory copepod Euchaeta, see the Virtual Copepod Page.