What Is Segmental?

The definition of “segmentation” in biology is not straightforward, but its conceptual grounds have been thoroughly discussed in the literature (Beklemishev 1969c; New'man 1993; Budd 2001; Scholtz 2002,2010; Minelli and Fusco 2004; Tautz 2004; Fusco 2008; Couso 2009; Hannibal and Patel 2013; also see Chapter 1). Because of the historical load and ambiguity of the word “segmentation,” I favor the term

“segmental” and adopt a definition that is strictly based on the structure of individual traits rather than whole organisms (Budd 2001; Scholtz 2002, 2010). Thus, throughout this chapter, I use “segmental trait” to refer to any repetitive structures or organs that are arranged in a regular series along the anteroposterior body axis.

To fit this definition, two main conditions must be satisfied: the structures should have a repetitive morphology and be evenly distributed (i.e., in a regular series). For example, a series of transverse commissures between longitudinal nerve cords are segmental when their arrangement and orientation are the same, and they are equidistant to each other. This approach is flexible enough to be applied to individual cells (e.g., neurons), cell bundles (e.g., muscles), tissue boundaries (e.g., epithelial folds), or even to full organs (e.g., nephridia), and each structure or organ system can be accessed individually.

Putting the focus on individual traits bypasses the need to define what a “segment” is and the juggling between what is “true” or “pseudo” segmentation. In this manner, it is more straightforward to identify and compare segmental traits across bilaterians without running into various conceptual dilemmas.

It is also important to note that a trait is “segmental” regardless of how the structures became arranged in a segmental manner during ontogeny. This focus on the structure avoids conflating pattern and process (Scholtz 2010), and helps to explicitly recognize that a segmental organization can be achieved by different cellular and developmental processes even within the same organism.

As a final disclaimer, the presence of a segmental trait is not, by any means, evidence for a hypothetical fully “segmented” ancestor. Evolutionary hypotheses are only discussed trait-by-trait when sufficient comparative and phylogenetic data are available.

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