Onychophora (Velvet Worms)

Onychophorans are multilegged predators with velvety skin and nocturnal habit that live in humid forest soil (Ruppert, Fox, and Barnes 2004c). Because they exhibit segmental traits in several organ systems—with noteworthy exceptions—and are the sister group to arthropods (Giribet and Edgecombe 2017), velvet worms are a key group for understanding the evolution of the canonical arthropod segmental traits (Mayer and Whitington 2009; Whitington and Mayer 2011; Martin, Gross, Hering, et al. 2017a; Martin, Gross, Pfliiger, et al. 2017b; Smith and Goldstein 2017).

There are no folds demarcating external trunk boundaries and, even though minute segmental papillae can be present in the cuticle (Franke and Mayer 2014), the trunk segments are only evident from the series of paired appendages used for locomotion (Ruppert, Fox, and Barnes 2004c). Internally, onychophorans exhibit a mix of segmental and non-segmental traits in the nervous, circulatory, and muscle systems (Franke and Mayer 2014; Martin, Gross, Pfliiger, et al. 2017).

In the peripheral nervous system, for example, the leg and nephridial nerves branching from the ventrolateral nerve cords are distributed in a segmental manner in register with the pair of appendages per segment (Mayer 2015). However, the nerve cords themselves lack segmental ganglia (i.e., the neuronal bodies are distributed along the cord) and the numerous ring and median commissures are neither evenly spaced nor match the leg innervations (Mayer and Harzsch 2007, 2008; Whitington and Mayer 2011; Mayer. Martin, et al. 2013; Mayer 2015; Martin, Gross, Hering, et al. 2017; Martin, Gross, Pfliiger, et al. 2017). In the same manner, the musculature does not show any obvious segmental arrangement, except for the muscles associated with the legs (Hoyle and Williams 1980; Mayer, Franke, et al. 2015) and gonads are paired non-segmental structures (Storch and Ruhberg 1990; Brockmann et al. 1999). Additional segmental traits include one pair of nephridia per trunk segment (Mayer 2006) and serially repeated valves (i.e., ostia) in the circulatory system through which the blood enters the heart (Ruppert, Fox, and Barnes 2004c).

During embryonic development onychophorans also produce somites in the mesoderm (Mayer, Franke, et al. 2015). They are formed after gastrulation from bilateral bands of mesoderm that elongate and subdivide into segmental blocks of solid tissue, which progressively hollow out (Mayer, Franke, et al. 2015). The somitogenesis process occurs without a posterior growth zone (Mayer et al. 2010). The nervous system also develops from head to tail, but without any sign of segmental organization. First, the neuronal precursors giving rise to the ventrolateral nerve cords delaminate from the ectoderm, and only later in development the leg nerves and commissures are established (Mayer and Whitington 2009; Mayer, Franke, et al. 2015; Martin, Gross, Hering, et al. 2017). Onychophoran embryos have in addition a conspicuous but transient segmental trait made from thickenings of the ventral ectoderm, known as the ventral organs (Whitington and Mayer 2011), which serve as attachment points for the developing leg muscles (Oliveira et al. 2013).

The molecular patterning of onychophoran embryos has been extensively investigated, particularly regarding the expression of known arthropod segmentation genes (Mayer, Franke, et al. 2015). The segment polarity genes engrailed, wingless, and hedgehog are expressed in a segmental manner with a pattern similar to arthropods, but are only detected after the morphological furrows have formed (Eriksson et al. 2009; Janssen and Budd 2013; Franke and Mayer 2014). The upstream factors regulating onychophoran somitogenesis remain unclear (Janssen 2017).

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