Nemertea (Ribbon Worms)
Nemerteans are slender hunters with a typical unsegmented external appearance, but that can exhibit some internal segmental traits (Hyman 1951c). Examples of nemertean segmental traits are serially arranged ovary pairs, lateral gut diverticula, transverse nerve commissures, and one single case of annular constrictions in the trunk.
A few interstitial nemertean species have a conspicuous segmented appearance where the external body wall is subdivided along the anteroposterior axis into trunk segments (Berg 1985; Norenburg 1988; Chernyshev and Minichev 2004). Each segment is demarcated by epidermal constrictions that coincide with a rather unusual trait for this group—a segmented digestive tract (Berg 1985). In these species, the gut is divided into barrel-shaped segments delimited by an internal membrane with a funnel aperture connecting the compartments (Berg 1985). Both the body wall and gut segmental traits are apomorphies of this particular group of nemerteans, and possibly associated to their interstitial habit (Chernyshev and Minichev 2004; Sundberg and Strand 2007). Given the predictable nature of these traits, a tight developmental control must be involved in patterning these segmental tissue boundaries.
The presence of transverse commissures arranged in regular intervals between the longitudinal nerve cords of nemerteans has been described in classical works (e.g., Hubrecht 1887; Burger 1895). However, this ladder-like appearance varies across species (Beckers, Faller, and Loesel 2011; Beckers, Loesel, and Bartolomaeus 2013; Chernyshev and Magarlamov 2013; Beckers, Kramer, and Bartolomaeus 2018). Serially arranged circular nerves can also be present (Figure 9.4A) (Beckers, Loesel, and Bartolomaeus 2013; Chernyshev and Magarlamov 2013). These neural structures probably develop either in adults or late juveniles, since they are not detected in larval or early juvenile stages of nemerteans (Hay-Schmidt 1990; Chernyshev and Magarlamov 2010; Maslakova 2010; Hindinger, Schwaha, and Wanninger 2013; Dohren 2016; Martfn-Duran et al. 2018). Furthermore, the regular arrangement of the nervous system is likely not part of the nemertean ground pattern, but a derived feature associated with a pelagic habitat (Beckers, Kramer, and Bartolomaeus 2018).
Another and more widespread nemertean segmental trait is the serial arrangement of ovary pairs along the body axis (Figure 9.4B) (Coe 1905; Strieker et al. 2002). The number of pairs ranges from fewer than ten to several thousands, and each ovary can be connected to a correspondent gonoduct and genital opening through the body wall (Strieker et al. 2002). In addition, the ovaries are tightly intercalated by lateral gut diverticula (Figure 9.4C), indicating a certain morphological integration between these traits. Developmental mechanisms responsible for the segmental organization of nemertean ovaries are still unknown.
Finally, nemerteans usually have a single anterior pair of protonephridia, but some groups can have serially arranged protonephridia reaching up to 300 pairs—a condition that probably evolved secondarily (Bartolomaeus and Dohren 2010). Along the body wall and proboscis there are series of densely packed, but not so regularly distributed, circular muscles (Chernyshev 2015, 2010; Chernyshev and Kajihara 2019).