Structural and Ontogenetic Segment Definitions Lead to Paradoxes

Numerous attempts have been undertaken to define segments generally or for individual groups of animals by means of a combination of developmental characteristics and/or a set of repeated substructures that occur together (Scholtz 2002). Even if such definitions are taken rather loosely, this approach leads immediately to several paradoxes. Contradictions often result between morphological, structural criteria and the criteria of developmental biology. This is the case if a body section is considered a segment if it is formed from a budding zone yet morphologically does not exhibit the necessary substructures such as extremities, coelom, or nerve ganglia, and vice versa.

Segments Do Not Form Spatial and Differential Units

The left and right halves of a segment evidently display a certain time-space independence of its subdivisions. This is already apparent in the different speeds of cell division on the right and left sides in crustacean embryos (Scholtz 1990). The difference is consistent throughout the entire length of the embryo. In lancelets, the larvae develop a conspicuous asymmetry in the two body halves in that the segments of the left and right halves are shifted relative to one another about the length of half a segment and the mouth develops on the left side (Hatschek 1882). The ventral and the dorsal sides do not necessarily exhibit the same series of their segmental substructures. In Notostraca among the branchiopods, the posterior segments develop not one but up to six pairs of legs per segment or body ring (Linder 1952). In the diplopods, among the myriapods (Figure 1.1) each segment or body ring is associated with two pairs of legs and nerve ganglia. Embryological studies have shown that even at the molecular level, the subdivision of the ventral side proceeds differently than on the dorsal side (Janssen et al. 2006). Furthermore, it has been shown that among insects and some crustaceans, the initial subdivision of the body through segment polarity genes does not deal with segments, but with serial structures shifted half a segment and determined by cell clones. These are called parasegments and are interpreted as the essential serially developmental units (Lawrence 1992).

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