Criticism of the Term “Segment”
In summary, segments evidently do not form the assumed morphological, structural, or ontogenetically integrated units. The structures that are referred to as segments do not display much correspondence other than their seriality. This leads to the failure of any attempt at a definition. The serial coeloms also cannot be seen as essential characteristics of segments, but as one serial element among others. In terms of classification, therefore, this eliminates the distinction between segment and nonsegment. Taking a look at the development does not necessarily help us any further. On the one hand, segments are formed differently, and on the other hand, it might not be the segments themselves that are laid out, but only the serial morphological boundaries. The different body halves and the ventral and dorsal sides show a certain independence in their serial differentiation. Anomalies that lead to asymmetries or helicomery break up the segmentation pattern and even the serial arrangement. Independent of one another, segmental substructures can be transformed in evolution or even completely lost. Segments probably did not evolve as a w'hole, but through individual steps as the eventual combination of different serially arranged substructures. From the genealogical perspective, segments and certain other serial structures make up a historical continuum. The distinction between segments and other serial body structures is evidently more a question of quantity than quality. This also blurs the boundary to unsegmented serial structures. Does this deconstruction lead the concept of “segment” to vanish? Do segments not exist, or to put it another way, are there no segmented organisms?
Some authors have in fact suggested speaking only of segmented structures or organs and no longer of segmented organisms (Budd 2001; Minelli and Fusco 2004). This choice of terminology does not resolve the problem, however, since the w'ord “segment” was introduced in biology and in colloquial speech to mean a subdivision of a whole. This does not even apply to most of the serially arranged structures. The serially repeated nephridia of an annelid do not show any subdivisions, that is, segments of an entire kidney. Instead, they are kidneys sui generis, which together aim to have a greater impact, but each one can also operate on its owm. Similarly, the legs of a myriapod do not constitute segments of any sort of “super leg”; instead they are individual repeated units.
At this point the concept of “series” makes sense. Structures and organs along the longitudinal axis of the body are serial but not segmented. The seriality can in fact vary between the body halves and between the dorsal and ventral sides. The concept of serial structures also does justice to a more evolutionary view, since, on the one hand, it poses the question as to the origin of segments differently, namely as an opportunity to gradually add serial structures. On the other hand, the concept of a series creates greater conceptual clarity for the controversial question as to whether the last common ancestor of the bilaterians was segmented. This question might just be worded improperly; instead it should be: Did the last common ancestor have internal or external serial structures and, if so, which ones? Were these serial structures then the point of departure for the different combinations of serial structures in the lines that led to the annelids, arthropods, and chordates? With this, the distinction between seriality and segmentation is no longer purely semantic. All in all the concept of serial structures offers greater flexibility in dealing with the observed differences and common ground between individual segments of an organism as well as between the segments of different animal groups.
Precisely by eliminating the term “segment” it might be possible to gain greater insight into what comprises segmentation as seriality and how it emerged through evolution.