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Spiralia

Chaetognatha (Arrow Worms)

Chaetognaths are small torpedo-like marine predators, known as arrow worms, with a well-defined head region and a slender trunk showing no marked subdivisions (Hyman 1959a). Even so, some traits of the chaetognath nervous system are organized in a segmental manner. For example, the central neuropil—a dense synaptic area of the ventral ganglion of chaetognaths—comprises an ordered series of transverse fibers that form around 80 microcompartments along the anteroposterior axis (Figure 9.3A) (Bone and Pulsford 1984; Harzsch and Muller 2007). At the flanks of the neuropil, there are neuronal cell bodies, which are also organized in a serial manner, forming a highly organized grid pattern (Perez et al. 2013). Experimental data from cell proliferation assays suggest the organized pattern emerges from the iterated asymmetric divisions of neuronal progenitors (Perez et al. 2013).

Alongside the neuropil lies another distinct segmental trait of chaetognaths: a repetitive series of paired neurons with RFamide-like immunoreactive somata (Figure 9.3B) (Bone et al. 1987; Goto et al. 1992; Harzsch and Muller 2007; Harzsch et al. 2009; Rieger et al. 2011). The arrangement is widely conserved to the point that these neurons can be homologized between the different species, indicating that such a segmental trait is likely part of the chaetognath ground pattern (Harzsch et al. 2009). It is unclear, however, how they become arranged in a segmental manner during embryonic development. Hatchlings already exhibit the four anteriormost pairs of neurons (Rieger et al. 2011), suggesting that the differentiation progresses from anterior to posterior. But the embryonic origin of these neurons and the cellular processes responsible for their segmental arrangement remain unknown.

Rotifera (Wheel Worms)

Wheel worms are aquatic bilaterians characterized by a prominent ciliary organ, hence their common name, and a highly specialized masticatory apparatus known as the mastax (Hyman 195le). The body is divided into head, trunk, and foot, all of which can be segmented along the anteroposterior axis by skeletal plates and folds in the tegument body wall (Segers 2004; Fontaneto and Smet 2014). The plates are often articulated forming conspicuous telescopic rings that can be retracted-extended at will by the animals (Figure 9.3C) (Fontaneto and Smet 2014). Unlike kinorhynchs, the skeletal plates of rotifers are intracytoplasmic and formed by a dense lamina within the syncytial integument layer (Fontaneto and Smet 2014).

Rotifers also exhibit a transverse series of circular muscles along the anteroposterior axis (Figure 9.3D) (Hochberg and Litvaitis 2000; Sprensen 2005; Leasi and Ricci 2010; Fontaneto and Smet 2014). The number, width, and configuration of these muscles—which are often incomplete, forming semicircular paired lateral bands—varies between species, although the circular arrangement is considered the ancestral condition (Leasi and Ricci 2010). Regardless, the number and position of these circular muscles do not seem to correspond with the organization of the skeletal plates, when present (e.g., Sprensen 2005; Hochberg and Lilley 2010; Leasi and Ricci 2010). The ontogeny of the circular musculature has not yet been described

Selected segmental traits in Spiralia

FIGURE 9.3 Selected segmental traits in Spiralia (Chaetognatha, Rotifera, Micrognathozoa, Gastrotricha, and Platyhelminthes). Arrows indicate serially repeated structures unless otherwise noted. A. The ventral nervous center (vnc) in the chaetognath Spadella cephaloptera (left) and the segmental microcompartments in the central neuropil of Sagitta setosa revealed by synapsin (right). Scale bars = 100 pm (left) and 50 pm (right). Left image reprinted from Rieger et al. (2011) with permission from John Wiley & Sons. Right image by Harzsch and Muller (2007) licensed under CC-BY. B. Repetitive series of paired neurons with RFamide- positive somata (Dl-5) in the chaetognath Sagitta enflata. Scale bar = 25 pm. Image reprinted from Harzsch et al. (2009) with permission from Springer Nature. C. Telescopic rings in the foot (f) of the bdelloid rotifer Rotaria macrura. Scale bar = 20 pm. Image by Diego Fontaneto from Gross (2007) licensed under CC-BY. D. Circular musculature in the rotifer Philodina sp. Magnified x630. Image reprinted from Hochberg and Litvaitis (2000) with permission from Springer Nature. E. Body wall annulations in the acanthocephalan Mediorhynchus afri- canus. Scale bars = 500 pm. Image reprinted from Amin et al. (2013) with permission from Springer Nature. F. The accordion-like thorax in the micrognathozoan Limnognathia maer- ski. Scale bar = 20 pm. Image reprinted from Giribet et al. (2004) with permission from John Wiley & Sons. G. Serially repeated dorsoventral musculature in L. maerski. Scale bar = 10 pm. Image by Bekkouche et al. (2014) licensed under CC-BY. H. Series of lateral spines in the

FIGURE 9.3 (CONTINUED)

gastrotrich Xenodasys riedli. Scale bar = 50 pm. Image reprinted from Schuster et al. (2018) with permission from Springer Nature. I. Transverse commissures (arrows) intercepting the dorsal nerve cords in the catenulid flatworm Promonotus schultzei. Scale bar = 10 pm. Image reprinted from Reuter et al. (1995) with permission from Springer Nature. J. Lattice-like musculature in the proseriate flatworm Monocelis sp. (left) and in the polyclad flatworm Melloplana ferruginea (right) showing the circular muscles (cm), diagonal muscles (dm), longitudinal muscles (lm), oral muscles (om), and rhabdite glands (rg). Scale bars = 100 pm (left) and 25 pm (right). Left image by Girstmair et al. (2014) licensed under CC-BY. Right image reprinted from Bolanos and Litvaitis (2009) with permission from John Wiley & Sons. K. Early circular myoblast processes in the polyclad flatworm embryos Maritigrella crozieri with few developing longitudinal fibers (lm). Scale bar = 50 pm. Image reprinted from Bolanos and Litvaitis (2009) with permission from John Wiley & Sons. L. Anterior region (left) and body wall infoldings (right) in the tapeworm Taenia taeniaeformis and Hymenolepis папа showing the proglottids (p) and their overlapping margins (ol), neck region (n), scolex (sc), suckers (s), and the rostellum (r). Magnified x 120 and x80, respectively. Images reprinted from Mehlhorn et al. (1981) with permission from Springer Nature. M. Longitudinal section of the body wall in the tapeworm H. папа showing the infoldings (if) in the syncytial tegument layer (tg) of the proglottids (p) with transverse canals of the excretory system (e). Magnified x200. Image reprinted from Mehlhorn et al. (1981) with permission from Springer Nature. N. Transverse commissures (trc) intercepting the lateral nerve cords (Inc) and median nerves (mn) in the nervous system of the tapeworm Hymenolepis diminuta. Scale bar = 100 pm. Image reprinted from Rozario and Newmark (2015) with permission from Elsevier. O. Transverse muscle fibers (arrowheads) and terminal genitalia (asterisks) in the proglottids of tapeworm H. diminuta. Scale bar = 100 pm. Image reprinted from Rozario and Newmark (2015) with permission from Elsevier. R Transverse (tv) and longitudinal (lg) excretory canals in the tapeworm H. diminuta. Scale bar = 100 pm. Image reprinted from Rozario and Newmark (2015) with permission from Elsevier.

in detail. Muscle bands are visible in the embryo before hatching (Boschetti et al. 2005), but the mechanisms and processes that lead to the serial arrangement of these circular muscle cells in rotifers still need to be elucidated.

Acanthocephalans are parasitic worms affiliated to rotifers (Hejnol 2015c). They have a hooked proboscis and no gut (Hyman 195Id; Nicholas 1967). Several species have a trunk segmented by regular annular constrictions (Figure 9.3E) (Southwell and Macfie 1925; Amin et al. 2013). The epidermis at each constriction is attached to the longitudinal muscles isolating the outer circular muscles into rings, which correspond to the annuli (Hyman 1951d). The trunk can also exhibit circular girdles of spines along the body. Internally, a fluid-filled lacunar system can be organized in an orthogonal pattern with a series of circular channels along the anteroposterior axis of the body (Hyman 195Id).

 
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