Why are arthropods segmented?

Segmentation as an attribute of organisms is being increasingly discussed i n the recent literature because (1) new phylogenies suggest that organisms classically considered to be segmented may lie in separate clades; (2) the molecular basis of segmental development has been much studied; (3) various theories of bilaterian origins place weight on segmentation as a primitive character; (4) there has been recent stress on the importance of modularit y as an evolutionary topic. However, the definition and extent of segmentat ion are highly ambiguous and usually typological. Here, segmentation is reg arded as an attribute of organs, not organisms. The evolution of just one s ystem, the arthropod epidermis, is examined on the basis of the fossil reco rd and the extant euarthropods, tardigrades, and onychophorans. It may be s een to have become segmented in a complex pathway that necessitated shifts in function, redundancy, and changes in associated organs. This complexity must inevitably reflect on, and to an extent have primacy over, the genetic basis for the changes involved. Evolutionary functional morphology has bee n relatively little considered in the context of the evolution of developme nt, but may play an important role in defining the framework within which t his evolution occurs.