Kinorhyncha (Mud Dragons)
Mud dragons are minute and elongated marine invertebrates with a spiny appearance (Hyman 195le). Different than other groups covered in this chapter, kino- rhynchs have an evident segmental organization with the trunk divided into eleven segments, known as zonites (Figure 9.2K) (Zelinka 1908; Hyman 195 le). These are demarcated by a series of overlapping cuticular plates connected by thin transverse joints repeated along the anteroposterior axis (Hyman 1951e; Neuhaus and Higgins 2002). This segmental arrangement is not only external but also occurs internally in the musculature and the nervous system.
For instance, the dorsoventral and diagonal muscles are arranged in a series of pairs along the body that match the subdivisions of the body wall (Figure 9.2L) (Muller and Schmidt-Rhaesa 2003; Schmidt-Rhaesa and Rothe 2006; Herranz et al. 2014; Altenburger 2016). Similarly, the nervous system exhibits two transverse commissures per segment between the longitudinal nerve cords (Nebelsick 1993; Neuhaus and Higgins 2002; Herranz et al. 2019) and a series of paired ventral neuronal somata is present in some species (Figure 9.2M) (Herranz, Pardos, and Boyle 2013; Herranz et al. 2019). Thus, in adult kinorhynchs the segmental traits of distinct organ systems are well correlated to a degree comparable with annelids or arthropods, at least in the species studied so far. Nevertheless, additional data is needed to corroborate if this integrated segmental organization is a feature shared by the entire group (Herranz et al. 2019).
The ontogeny of kinorhynch segmental structures remains elusive due to limited developmental data (Hejnol 2015a)—most observations are restricted to the heroic efforts of E.N. Kozloff (1972, 2007) in a species of the Pacific Northwest: Echinoderes kozJoffi. What we know about kinorhynch embryogenesis is that the trunk segments of E. kozloffi first become visible midway through development after the embryo elongates in the anteroposterior axis (Kozloff 2007). The developmental processes involved in the elongation (e.g., cellular rearrangements or growth zone) are not known. Development is direct and the juvenile hatches with at least eight of the future eleven adult segments (Kozloff 1972; Neuhaus and Higgins 2002; Schmidt- Rhaesa and Rothe 2006). Additional segments are added at the posterior end during postembryonic development, suggesting the presence of a subcaudal growth zone in kinorhynchs (Neuhaus 1995; Lemburg 2002; Neuhaus and Higgins 2002; S0rensen, Accogli, and Hansen 2010).
An interesting aspect revealed by kinorhynch developmental studies is that the correspondence between the number of dorsoventral muscles and trunk segments present in adults is not observed in juveniles. At their earliest stage, juveniles have only eight trunk segments but already display their final number of ten pairs of dorsoventral muscles (Schmidt-Rhaesa and Rothe 2006). This suggests that kinorhynch segmental traits might develop with a certain degree of independence during embryogenesis before becoming well integrated in the adult stage.