The variation encountered in the caudal tagma, or posterior-most body region, within crustaceans is striking such that Makarov (1978), so taken by it, suggested that this region be given its own descriptor, the urosome. In the classic interpretation, the so-called telson of arthropods is homologized with the last body unit in Annelida, the pygidium (Westheide and Rieger, 1996; Grüner, 1993; Hennig, 1986). Within that view, the telson and pygidium are said to not be true segments because both structures supposedly lack coelomic sacs and ganglia. Likewise, the telson of adult arthropods has been defined as the structure, or region, arising from the posterior-most component of the germ band behind the segment segregating growth zone as demarcated by the teloblasts (Calman, 1909), and such a definition also applies to the pygidium of annelids (Anderson, 1973).
Debate about the nature of the caudal tagma has not been frequent, but when it has occurred it has been contentious. Sharov (1966) and Bowman (1971) argued from the viewpoint of gross anatomy that the definition of a telson in crustaceans depends on the presence and location of the anus. According to this line of reasoning two types of non-homologous urosomes can be distinguished: one type, possessing caudal rami and a terminal anus, is not a telson but rather a true body segment, or anal somite; the other type, exhibiting no caudal rami and with the anus opening on the antero-ventral surface, is a true telson. Schminke (1976) forcefully argued against this interpretation and made detailed reference to the complex caudal region of the bathynellacean syncarids, which exhibit both uropods and relatively elaborate caudal rami.
A parallel issue concerns the presence, location, and numbers of teloblasts vis-a-vis the urosome. True segments are said to occur anterior to this zone of active cell proliferation. However, Amphipoda lack ectoteloblasts entirely (Scholtz and Dohle, 1996), and the absence of these cells makes it difficult to evaluate what is or is not segmental ectoderm in the group, at least when applying the traditional definition. All the ectodermal cells in amphipods arise out of a process of cell segregation into bands on the germinal disc (Wolff and Scholtz, 2002). In Artemia, an active growth zone of cells rather than teloblasts generates a mass of tissue for subsequent differentiation without employing any regular cleavage or segregation pattern (Gilbert, 1997).
The advent of developmental genetics has provided other criteria to assess cell status. Segments can now be located based on expression patterns of segmentation genes such as engrailed (Patel, 1994; Scholtz, 1993, 1995), rather than relying on the location of teloblasts. However, this too must be used with caution. For example, Scholtz (1995) described the patterns of engrailed striping in the crayfish Cherax destructor. The pleon in this species develops anterior to the broad, long, bilobed mass of the presumptive telson. The formation of engrailed stripes appears in stages as the germ band grows posteriorly. At 60% of development, the first six pleomeres are delineated by stripes of engrailed across the entire width of the pleon. However, two additional stripes, a seventh and an eighth, also are present in the center part of the presumptive telson. At 65% development a further ninth stripe briefly appears. Pleomeres 1 through 7 also express engrailed in the Anlagen of their ganglia, while the eighth and ninth ectodermal engrailed stripes disappear by the time of 70-75% development. The first seven ganglia of the Cherax pleon display the characteristic CNS pattern of double commissures. An Anlage of the eighth ganglion forms but possesses only a single commissure. Nevertheless, out of this ganglion two nerves extend posteriorly into the presumptive telson. The ninth stripe completely disappears, while the Anlagen of ganglia 6 through 8 eventually fuse to form a single unit. One might ask whether the ‘telson’ of Cherax is segmental in nature, i.e., composed of fused segments along with the telsonic portion.
Thus the pattern of segmentation vis-a-vis the posterior pleon and the presumptive telson are not nearly as easy to characterize in fact as the classic anatomical definitions of the eumalacostracan caudal region would lead us to believe. We believe that the issues raised during the debates of the 1970s about the crustacean body terminus are not settled and are worthy of an across the board study of all crustacean urosomes.