Brachiopoda (Lamp Shells)

Brachiopods are bivalved marine invertebrates of sessile habit that use an intricate ciliated tentacular-crown known as the lophophore to capture food (Hyman 1959c). Despite the enclosed adult body and reduced morphology, larval brachiopods exhibit a combination of putative segmental traits in different germ layers, such as transverse body wall folds, repeated coelomic pouches, and serially arranged chaetae sacs.

The body of brachiopod larvae can be subdivided into two, three, or four lobes along the anteroposterior axis, each delimited by transverse epithelial furrows (Figures 9.4G and 9.5C) (Morse 1873; Kowalevsky 1883b; Lacaze-Duthiers 1861). Even though this segmented appearance has stirred much debate in the late 19th century (Balfour 1880; Shipley 1883; Masterman 1899; Conklin 1902), these ectodermal boundaries are not in fact segmental and each furrow has a distinct morphology (Vellutini and Hejnol 2016). Interestingly, the furrow at the head-trunk boundary is delimited by abutting stripes of engrailed and wntl (Figure 9.5C) (Vellutini and Hejnol 2016), a hallmark pattern of the segment polarity cascade in arthropods (Ingham 1991).

On the other hand, the most prominent segmental trait of brachiopods occurs in the mesoderm—a regular series of four paired coelomic pouches distributed along the anteroposterior axis of a larval stage (Nielsen 1991). The three posterior pairs are also associated with correspondent pairs of dorsal chaetae sacs (Figure 9.4H) (Nielsen 1991).

These coelomic pouches are sequentially formed during gastrulation from evagi- nations of the archenteron (Nielsen 1991) with the exception of the anteriormost pair, which might be formed by cell delamination (Freeman 2000). The partitioning, however, is incomplete, and the tissue remains unsegmented on its ventral side (Figure 9.4H) (Vellutini and Hejnol 2016). In this region, a series of transverse muscle fibers aggregate and form three distinct transverse bundles connecting the pair of mediolateral longitudinal muscles (Altenburger and Wanninger 2010). A segmental mesoderm, however, is not a widespread feature among larval brachiopods. The mesoderm morphology varies considerably between species and the tissues can also be unsegmented or simply split into anterior and posterior regions (Hyman 1959c), suggesting the paired coelomic pouches is a derived trait.

Before the morphological boundaries are visible in the embryonic mesoderm, engrailed transcripts are detected in two pairs of stripes at the posterior region of the second and third coelomic pouches (Figure 9.5B) (Vellutini and Hejnol 2016). After the mesodermal boundaries are established, the gene pax2/5/8 is detected in paired stripes between the same pouches, while components of the Hedgehog pathway such as the transcription factor gli become expressed during pouch formation (Figure 9.5B) (Vellutini and Hejnol 2016). However, neither gene exhibited a reiterated pattern throughout the four pairs of coeloms as one would expect of a typical segmental patterning, suggesting that genes relevant for segmenting the brachiopod mesoderm have not yet been identified.

In adult brachiopods, the presence of two pairs of lateral mesenteries and two nephridial pairs have been interpreted as evidence for segmentation in brachiopods (Malakhov and Kuzmina 2006; Temereva and Malakhov 2011), however the segmental nature of these traits remains controversial.

Phoronida (Horseshoe Worms)

Phoronids are sessile marine invertebrates closely related to brachiopods (Kocot et al. 2017). The adult body does not show any external segmental trait and there are only a few internal traits with presumptive segmental organization. The ventral nerve cord is intercepted by a series of transverse commissures (Temereva 2012; Temereva and Wanninger 2012) and circular muscles are distributed along the body (Santagata and Zimmer 2002; Santagata 2004; Temereva and Tsitrin 2013). However, the spatial regularity is not completely evident and further work is required to verify the segmental nature of these neurons and muscles. In addition, despite having a tri- meric coelomic organization in adults (Masterman 1899; Siewing 1973) and larvae (Temereva and Malakhov 2006, 2011), these compartments are not arranged in a segmental manner along the anteroposterior axis.

Bryozoa (Moss Animals)

Bryozoans are discrete but ubiquitous bilaterians in marine and freshwater environments that form colonies with complex and intricate arrangements (Hyman I959d). A colony is formed by functional units named zooids. Their body is reduced to a lopho- phore, gut, and spacious mesoderm accommodating the gonads. There is no trace of segmental traits in individual zooids, but the colony of one abyssal Mediterranean species shows a unique segmental trait. The colony has a root, a peduncle with modified zooids, and tip with regular zooids (d’Hondt 1976). The peduncle is composed by a series of annular zooids forming a peculiar segmented stalk. This stacked arrangement is not common in other bryozoan colonies and is an unusual case of supraorganismal segmental organization in bilaterians.

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