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An arthropod is an animal with a segmented body, jointed appendages and an exoskeleton that is periodically shed during growth. Insects, crustaceans and Arachnida illustrate different organizations of this shared structural plan.

A crustacean is a member of the arthropod group that includes crabs, shrimps and many small aquatic forms. Different appendages can support swimming, feeding and respiration. Familiar large-bodied examples represent only part of the group’s diversity.

Dracunculus medinensis is the Guinea worm, a parasitic roundworm. Its life cycle involves aquatic crustaceans carrying larvae and a vertebrate host in which the adult female develops. Release of larvae into water connects the host-associated and aquatic stages.^[ref]

A small bivalved crustacean. Sexually dimorphic fossil carapaces can provide indirect evidence about reproductive investment.