Developmental genetics and arthropod evolution: on body regions of Crustacea

The evolution of arthropod body plans remains an irksome issue for the field of arthropod systematics. For example, Gould believed it was illusory to use patterns of tagmosis as a basis for higher-level classification of arthropods and that “… all modern creatures called Crustacea…” displayed the “… stereotyped tagmosis of this class…” regardless of their “… variety in size and ecological specialization” ( Gould 1991 : 414). Yet, the distinctness of the various arthropod body plans has been an important basis for the higher classification of arthropod subphyla and classes, e.g., Snodgrass (1952) . For most arthropod groups above the class level we can clearly assign taxonomic ranks based on anatomical characteristics of the head, location of gonopores, numbers and arrangements of trunk limbs, and so forth. The Crustacea stand as a sole exception to this in that they exhibit no single body plan but actually possess several Baupläne (structural or archetypal body plans indicative of higher taxa), some more or less distinguished from the others ( Schram 1986 ).