Nematomorpha (Horsehair Worms)
Horsehair worms are filiform bilaterians with a complex life cycle, where the adult stages are free-living inhabiting water streams, and the larval stages are parasitic on arthropod hosts (Hyman 195le). The body of adult nematomorphs is covered by a thick cuticle ornamented by areole structures, but there are no annulations (see Bolek et al. 2010).
In contrast, the cuticle of the parasitic larva is markedly annulated (Figure 9.21) (Montgomery 1904; May 1919; Marchiori, Pereira, and Castro 2009; Bolek et al. 2010; Szmygiel et al. 2014). Similar to nematodes, the annulation encompasses the cuticle and the hypodermal cell layer beneath. However, the rings in nematomorph larvae are somewhat irregular, and the number of folds in each side does not correspond exactly (Figure 9.2J) (Montgomery 1904; Zapotosky 1975). It is also unlikely that these cuticle rings are anchored to actin bundles since no circumferential fibers are visible in the larva (Muller, Jochmann, and Schmidt-Rhaesa 2004).
Overall, the annulation of larval nematomorphs is incipient compared to the cutic- ular rings of nematodes, but the group is still lacking developmental data (Hejnol 2015a), and earlier embryonic stages need to be investigated further to clarify the morphogenesis of these annulations.
Priapulida (Penis Worms)
Priapulids are marine mud-dwelling scavengers fittingly known as penis worms due to their body shape. Adult priapulids have three well-defined body regions, an anterior proboscis (introvert), an elongated trunk, and a posterior region with caudal appendages (Hyman 195le). The trunk has a characteristic annulation pattern formed by a series of transverse cuticular rings—between 40 and 180, depending on the species (Figure 9.2N) (McCoy 1845; Hyman 1951e; Sanders and Hessler 1962; Hammond 1970; Morse 1981; Shirley and Storch 1999).
The trunk cuticle in adults consists of a thin outer layer and a thick inner layer with regularly spaced circumferential projections named apodemes (Figure 9.20). These structures delve deep into the tissue and are tightly intercalated with the rings of circular muscles around the body wall (Oeschger and Janssen 1991). Each cuticular ring corresponds to the position of a circular muscle, which appears to bulge the inner cuticle layer (Oeschger and Janssen 1991). Therefore, the cuticle annulation of priapulids is likely the result of a tight association between the circular musculature and the cuticle of the trunk.
The circular muscles first appear in the embryos of Priapulus caudatus during gastrulation with a few transversely oriented fibers located at the introvert-trunk boundary (Figure 9.2P) (Martln-Duran and Hejnol 2015). Additional circular muscle cells get distributed along the whole trunk region, and form a grid pattern with the longitudinal musculature (Figure 9.2P). The circular musculature is well developed in the hatching larvae (Figure 9.2P) (Martfn-Duran and Hejnol 2015). but no obvious annulation is visible in this first larval stage and, unfortunately, the trunk is hidden beneath a lorica in later stages (Higgins, Storch, and Shirley 1993; Wennberg, Janssen, and Budd 2009). Nevertheless, juvenile stages can already display cuticular rings (Sorensen et al. 2012), and further developmental studies might corroborate the idea that priapulid trunk annulation emerges from the tight association between the circular musculature and the cuticle.
In addition, the peripheral nervous system of adult priapulids displays circular neurite bundles arranged regularly along the anteroposterior axis (Figure 9.2Q) (Rothe and Schmidt-Rhaesa 2010). However, this orthogonal pattern must develop during postembryonic stages and juvenile growth, since it is not present in the larval stages (Marti'n-Duran et al. 2016).