Loricifera (Girdle Wearers)
Loriciferans are microscopic interstitial bilaterians related to priapulids and kinorhynchs that were only discovered in the 1980s (Kristensen 1983). Their body is divided into an introvert, thorax, and abdomen, and partially enclosed by a sclero- tized cuticle—the lorica (Kristensen 1983).
The adult thorax is divided into two segments, one anterior portion with appendages and a posterior portion without which are covered by accordion-like rows of plates in the larval stages (Kristensen 1983; Higgins and Kristensen 1986). There are no internal structures that match the arrangement of these external thorax traits.
In the abdomen, however, a variable number of transverse circular or paired lateral muscle bundles are serially arranged in different species (Gad 2005; Kristensen, Neves, and Gad 2013; Neves et al. 2013), even though the morphology of these transverse muscle bands is not perfectly repeated (Neves et al. 2013). There is no evidence so far of segmental traits in the nervous system of loriciferans (e.g., serial ganglia), but their neural structures have not yet been studied by immunohistochemistry (Herranz et al. 2019).
The development of loriciferans is barely known (Hejnol 2015a). Embryos have only been observed in one aberrant species with a viviparous paedogenetic life cycle and develop inside a cystlike mega-larva (Heiner and Kristensen 2009). Nevertheless, it is interesting to note that the transverse musculature present in adult loriciferans is not present in the larval stages (Neves et al. 2013).
Tardigrada (Water Bears)
Tardigrades are common dwellers in aquatic and moist terrestrial habitats, widely recognized for their bearlike appearance and for the ability to survive harsh conditions (Ruppert, Fox, and Barnes 2004c; Mpbjerg et al. 2011). The body has a segmented appearance with a defined head and four trunk segments, each bearing a pair of jointless legs.
A cuticle secreted by the epidermis covers the whole body and can exhibit transverse folds, which correspond to the flexion zones of the body where the cuticle is thinner (Greven 1984). The cuticle folding does not correspond to the trunk segments (Ruppert, Fox, and Barnes 2004c). Due to the reduced body size, tardigrades lack coelomic cavities, respiratory or excretory organs, and thus show no obvious internal segmental organs (Beklemishev 1969b).
For instance, the musculature of tardigrades is rather complex (e.g., Marchioro et al. 2013) but only a few muscle sets have a segmental pattern (Schmidt-Rhaesa and Kulessa 2007; Halberg et al. 2009; Smith and Jockusch 2014). Leg muscles in segments two and three can be nearly identical (Smith and Jockusch 2014; Smith and Goldstein 2017) but the morphology is usually distinct for each pair of legs (Schmidt- Rhaesa and Kulessa 2007; Halberg et al. 2009). In addition, even though the muscle attachment points are conserved among tardigrades (Marchioro et al. 2013) they do not correspond to segment borders (Schmidt-Rhaesa and Kulessa 2007).
On the other hand, the ventral nervous system of tardigrades has a rope ladder organization with a series of four trunk ganglia linked by a longitudinal pair of connectives (Zantke, Wolff, and Scholtz 2008; Persson et al. 2012; Mayer, Kauschke, et al. 2013a; Mayer, Martin, et al. 2013b; Schulze and Schmidt-Rhaesa 2013; Schulze, Neves, and Schmidt-Rhaesa 2014; Smith and Jockusch 2014; Smith and Goldstein 2017). Upon a closer inspection, however, it is evident that each ventral ganglion has a unique (non-repeated) morphology (Mayer, Kauschke, et al. 2013; Smith and Jockusch 2014; Smith and Goldstein 2017).
During embryogenesis, the trunk ganglia differentiate already at their final adult positions relative to the anteroposterior axis from individual clusters of four neural progenitors (Hejnol and Schnabel 2005). Depending on the species, these four ganglia develop either simultaneously (Hejnol and Schnabel 2005) or following an anteroposterior progression (Gross and Mayer 2015). The longitudinal connectives are only established later in development (Gross and Mayer 2015).
The mesoderm of developing tardigrade embryos is subdivided into repeated somites that correspond to the trunk segments (Gross, Treffkorn, and Mayer 2015). Interestingly, the developmental processes generating the segmental organization in the mesoderm differ between species. In one species, after mesodermal progenitors enter the blastocoel, they migrate and proliferate with no apparent anteroposterior polarity or growth zone, and the resulting mesodermal bands later subdivide into four paired groups of cells composing each somite (Hejnol and Schnabel 2005). In contrast, somites can form by sequentially pinching off the presumptive endomeso- dermal tube (Gabriel et al. 2007).
At the molecular level, Engrailed protein is expressed in rows of dorsolateral ectodermal cells before any morphological boundary is visible on the ectoderm, but after somite boundaries have been established (Gabriel and Goldstein 2007). The expression matches the posterior border of each somite suggesting that engrailed could be involved in setting up these ectodermal boundaries (Gabriel and Goldstein 2007). In addition, anterior Hox genes have been shown to be expressed in a segment-specific manner and could have a role in establishing the segment identities (Smith et al. 2016). Nevertheless, gene expression data on tardigrade development remains limited (Gross, Treffkorn, and Mayer 2015).