The plastic nervous system of Nemertodermatida

Abstract

Nemertodermatida are microscopic marine worms likely to be the sister group to acoels, forming with them the earliest extant branch of bilaterian animals, although their phylogenetic position is debated. The nervous system of Flagellophora cf. apelti, Sterreria spp. and Nemertoderma cf. westbladi has been investigated by immunohistochemistry and confocal microscopy using anti-tubulin, anti-5-HT and anti-FMRFamide antibodies. The nervous system of F. cf. apelti is composed of a large neuropile and a loose brain at the level of the statocysts with several nerve fibres surrounding them and innervating the broom organ. Sterreria spp. shows a commissural-like brain and several neurite bundles going frontad and caudad from this. At the level of the statocysts there is also a thicker aggregation of immunoreactive fibres. The nervous system of N. cf. westbladi consists of a nerve ring lying outside the body wall musculature at the level of the statocyst and a pair of ventro-lateral neurite bundles, from which extend thinner fibres innervating the ventral side of the animal. Numerous bottle-shaped glands were observed, innervated by fibres starting both from the brain and the neurite bundles. The nervous system of the nemertodermatids studied to date displays no common pattern; instead, there is considerable plasticity in the general morphology of the nervous system. In addition, the musculature of Sterreria spp. has been studied by phalloidin staining. It shows diagonal muscles in the anterior quarter of the body and a simple orthogonal grid in the posterior three quarters, being simpler than that of the other nemertodermatids. High-resolution differential interference contrast microscopy permitted to better visualize some morphological characters of the species studied, such as statocysts, sperm and glands and, in combination with anti-tubulin staining, describe in detail the broom organ in F. cf. apelti. Finally, we note an apparent absence of innervation of the gut in Nemertodermatida similar to the condition in Xenoturbella.

Lateral neural borders as precursors of peripheral nervous systems: A comparative view across bilaterians

“…One plausible explanation is that the neural patterning of xenacoelomorphs is intermediate between cnidarians and nephrozoas. Like many protostomes, xenacoelomorphs have multiple paired longitudinal nerve cords in various dorsoventral positions (Achatz & Martinez, ; Raikova, Meyer‐Wachsmuth, & Jondelius, ; Raikova, Reuter, Jondelius, & Gustafsson, ). And similar to cnidarians, xenacoelomorphs have a uniformly distributed, diffuse basiepidermal nerve net and Xenoturbella species even have no nerve cord (Achatz & Martinez, ; Hejnol & Pang, ; Raikova et al., , ).…”

“…Like many protostomes, xenacoelomorphs have multiple paired longitudinal nerve cords in various dorsoventral positions (Achatz & Martinez, ; Raikova, Meyer‐Wachsmuth, & Jondelius, ; Raikova, Reuter, Jondelius, & Gustafsson, ). And similar to cnidarians, xenacoelomorphs have a uniformly distributed, diffuse basiepidermal nerve net and Xenoturbella species even have no nerve cord (Achatz & Martinez, ; Hejnol & Pang, ; Raikova et al., , ). But the alternative hypothesis of the urnephrozoan origin of BMP‐repression of neuropotency is challenged by comparative studies on various species in Spiralia, e.g., mollusk Ilyanassa (Lambert et al., ) and Ambulacraria [Echinodermata, e.g., sea star Patiria miniata (Yankura, Koechlein, Cryan, Cheatle, & Hinman, ) and Hemichordata, e.g., acorn worm Saccoglossus (Lowe et al., )].…”

“…One plausible explanation is that the neural patterning of xenacoelomorphs is intermediate between cnidarians and nephrozoas. Like many protostomes, xenacoelomorphs have multiple paired longitudinal nerve cords in various dorsoventral positions (Achatz & Martinez, 2012;Raikova, Meyer-Wachsmuth, & Jondelius, 2016;Raikova, Reuter, Jondelius, & Gustafsson, 2000).…”

“…And similar to cnidarians, xenacoelomorphs have a uniformly distributed, diffuse basiepidermal nerve net and Xenoturbella species even have no nerve cord (Achatz & Martinez, 2012;Hejnol & Pang, 2016;Raikova et al, 2000Raikova et al, , 2016. But the alternative hypothesis of the urnephrozoan origin of BMP-repression of neuropotency is challenged by comparative studies on various species in Spiralia, e.g., mollusk…”

Xenacoelomorpha's significance for understanding bilaterian evolution

“…Some acoel groups of the Bursaria have evolved a new opening for the extrusion of fertilized oocytes and can possess specialized copulatory organs [22]. A basiepidermal nerve net can be reconstructed for the ground pattern of the Xenacoelomorpha since xenoturbellids lack any nervous system internalizations, as do most nemertodermatids [13,16 ], but all xenacoelomorphs investigated so far possess a basiepidermal nerve net. Explicit excretory organs that conduct ultrafiltration are absent in cnidarians as well as in xenacoelomorphs, which renders nephridia -protonephridia and metanephridia -a novelty for the Nephrozoa.…”