Xenacoelomorphs are small free-living bilaterian worms without a through-gut, coeloms, or excretory system, which might hold a key position in the animal tree of life as the sister group to the remaining bilaterians (Ruiz-Trillo et al. 1999; Jondelius et al. 2002; Hejnol et al. 2009; Rouse et al. 2016; Cannon et al. 2016; Giribet 2016; Laumer et al. 2019); but see Marletaz et al. (2019) and Philippe et al. (2019) for an alternative view. For this reason, the presence or absence of specific traits in these worms can be highly informative to understand the evolution of bilaterian morphology (Hejnol and Pang 2016) and to what concerns the contents of this chapter, the evolution of segmental traits.

Xenacoelomorpha is comprised of xenoturbellids, nemertodermatids, and acoels. Even though some species are elongated, there are no obvious repetitive structures in the body wall or internal organs in either of these groups. Nevertheless, some aspects about the organization of the nervous system and musculature are worth mentioning.

The nervous system of xenacoelomorphs is greatly variable (Martinez, Hartenstein, and Sprecher 2017), usually consisting of a nerve net, but some lineages independently evolved longitudinal nerve condensations and cords (Hejnol and Pang 2016). In one of these lineages, the acoels, the longitudinal nerve cords are intercepted by transverse commissures along the anteroposterior axis (Semmler et al.

2010; Вегу et al. 2010; Hejnol and Martindale 2008). Such commissures, however, are neither symmetric nor regularly spaced, and are restricted to the anterior portion of the body (Bery et al. 2010).

The musculature of xenacoelomorphs is organized in an orthogonal grid with circular (transverse), longitudinal, and often diagonal fibers (Tyler and Hyra 1998; Hooge and Tyler 1999; Ladurner and Rieger 2000; Hooge 2001; Meyer-Wachsmuth, Raikova, and Jondelius 2013; Bprve and Hejnol 2014). The circular muscles are usually evenly spaced (Hooge and Tyler 1999), but the density of fibers can change according to the body region (Gschwentner et al. 2003).

Interestingly, the developmental processes that result in a gridlike musculature differ between the Xenacoelomorpha groups. In acoels, muscle progenitors develop progressively from the animal to the vegetal pole, and the emerging fibers are already polarized transversely forming a regular series of circular muscles along the anteroposterior axis (Figure 9.2A) (Ladurner and Rieger 2000; Semmler, Badly, and Wanninger 2008).

In contrast, muscle progenitors in nemertodermatids appear all around the body, and their projections are irregular, not following a specific orientation in relation to the body axes (Borve and Hejnol 2014). Nevertheless, the nemertodermatid juveniles exhibit a neatly organized series of circular muscles (Bprve and Hejnol 2014). Presumably, the musculature grid in nemertodermatids emerges secondarily from the cellular interactions between progenitors, while in acoels a patterning mechanism must impose the polarity of embryonic cells during early development. Which mode reflects the ancestral condition for the group remains to be determined.

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