Micrognathozoa (Jaw Animals)
Limnognathia maerski—the sole representative species of the Micrognathozoa—is a small bilaterian inhabiting mossy environments in Greenland with morphological features that resemble rotifers and other gnathiferans (Kristensen and Funch 2000). The adult body is regionalized into a head, an accordion-like thorax, and an ovoid abdomen
(Figure 9.3F). The thorax is subdivided into five flexible annulations supported by dorsal and lateral plates, which are composed of an intracellular matrix layer (Kristensen and Funch 2000). L. maerski exhibits 13 pairs of serially repeated dorsoventral muscles along the anteroposterior axis (Figure 9.3G) (Bekkouche et al. 2014). The muscle bundles occur in the thorax and abdomen, but their position does not correspond to the arrangement of the thorax annulations (see Bekkouche et al. 2014). There are two pairs of lateral protonephridia in the trunk (Kristensen and Funch 2000) and no segmental traits in the nervous system (Bekkouche and Worsaae 2016b).
The minute hairybacks are elongated free-living bilaterians covered with scales, spines, and bristles (Hyman 1951e). The arrangement of the gastrotrich scales on the body surface is highly ordered, but a segmental organization is more evident in the distribution of spines, bristles, and adhesive tube pairs along the lateral of the body (Figure 9.3H) (Hyman 195le). Internally, there are no obvious segmental structures in the gastrotrich nervous (Rothe and Schmidt-Rhaesa 2009; Rothe, Schmidt-Rhaesa, and Kieneke 2011; Bekkouche and Worsaae 2016a) or muscular systems (Kieneke, Martinez Arbizu, and Riemann 2008b; Bekkouche and Worsaae 2016a). The only exception is the excretory system, where some species can have between two and eleven pairs of protonephridia along the body (Teuchert 1967; Bekkouche and Worsaae 2016a). Nonetheless, because many gastrotrich species have only a single pair of protonephridia, it is not yet resolved if the segmental organization of the excretory system is an ancestral feature (Kieneke, Arbizu, and Ahlrichs 2007; Kieneke et al. 2008a). One other segmental trait that is likely part of the ground pattern of Gastrotricha is the lateral pairs of adhesive tubes (Kieneke, Riemann, and Ahlrichs 2008c).
Developmental studies on gastrotrichs are scarce (Hejnol 2015b). However, Teuchert (1968) provides important insights about the ontogeny of the aforementioned segmental traits. Adults of Turbanella cornuta have several pairs of lateral adhesive tubes and tactile bristles, and four pairs of protonephridia that are serially arranged and evenly spaced along the anteroposterior axis of the animal (Teuchert 1967, 1968). The tactile bristles and adhesive tubes develop at the same locations and almost in concert during embryogenesis, and the juveniles hatch with four lateral pairs of each (Teuchert 1968). Additional pairs of bristles and adhesive tubes are added during postembryonic development by intercalation between the initial four pairs, and not at the posterior end as one might expect (Teuchert 1968). In contrast, the formation of new protonephridia pairs follows an anterior-to-posterior progression. T cornuta juveniles only have a single protonephridia pair located anteriorly, and additional pairs are sequentially added until the fourth pair is formed near the posterior end (Teuchert 1968). This suggests once more that the segmental organization of different traits is achieved by distinct, partly independent developmental mechanisms.