Segments and Series

The term “series” is used in numerous contexts and with very different meanings (Toepfer 2017). If “series” is used to refer to the repetition of similar yet somewhat different structures or units along a temporal or spatial axis, then segments in animals are prime examples of series. Segmentation in this sense is a relational concept. Segments never appear in isolation; there is no such thing as a body comprised of just one segment. Thus segments only assume the quality of segments when occurring in serial repetition. This reveals a difference to the use of the term in other contexts. A circular segment or arc, for example, is simply cut out of a whole and can exist singularly, without being dependent on other segments.

Because animal segments are formed through a set of serially concordant internal and external substructures that occur together, they represent multiple series that are spatially and temporally interrelated. An implicit basic assumption of this concept is that originally, all segments of a body are organized in the same way. Over the course of evolution, this originally homonomous pattern undergoes a diversification, and heteronomous segmentation results. The body is subdivided in this way into functional and morphologically distinct units (tagmata) such as head, thorax, and abdomen (Figure 1.3). Segments thus form a theme with variations and are viewed as elements or modules that can be consolidated into larger units, which in turn can be interpreted as modular (Toepfer 2017).

Animal segments are therefore assigned a special quality that essentially distinguishes them from other serial structures. As always, in biology such a comparison

Diversification of segments and formation of spatial-functional units along the body axis

FIGURE 1.3 Diversification of segments and formation of spatial-functional units along the body axis. A. Exemplified by a crayfish as a representative of the arthropods (exploded exoskeleton, dorsal view). In the crayfish, the head and the thorax are fused to an anterior tagma called cephalothorax (dorsally, the segmentation is concealed due to a fusion; segmentation is only visible at the ventral side indicated by the series of appendages) and a posterior tagma, the pleon with obvious segmentation. B. A kangaroo as an example of a vertebrate (skeleton, lateral view) showing the head (skull), the neck, the thorax (indicated by the rib cage), the abdomen, and the tail. (Photographs: Eberle and Eisfeld, © Humboldt-Universitat zu Berlin.) and the resulting categorization concern two interrelated aspects. For one thing, this is a typological, classificatory problem: What properties define a segment and distinguish it from other serial structures? Is that which is defined as a segment always the same?

For another thing, segments do not only make up a class of structures; they are also a product of the historical, genealogical process of evolution. Consequently, the ensuing questions assume a different form: What precursor structures led to the formation of segments? How often and where in the genealogy of the animals did segmentation emerge? From this perspective, segments and other serial structures can be understood as steps in a transformation process. The boundary between segments and non-segments becomes fluid.

 
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