Gastric pouches and the mucociliary sole: setting the stage for nervous system evolution

In the paper

“In line with the postulated origin of nerve net neurons from ectochoanocytes, nerve net synaptic transmission appears to be largely glutamatergic, as recently postulated for ctenophores [156] and as indicated by many lines of circumstantial evidence in cnidarians.”

What the sources say

• Supported

  In “Parallel evolution of neural organization” (page not available), the paper identifies neural nets in the ctenophore Pleurobrachia and proposes glutamate as a signalling molecule, citing glutamate-receptor expression and transmitter tests. That supports the citing sentence’s attribution of a postulate about glutamatergic signalling in ctenophores; this citation is not offered for the separate statement about cnidarians. (LLM, conf=0.87)

  cites [156] Moroz LL et al. 2014 The ctenophore genome and the evolutionary origins of neural systems. Nature 510, 109– 114. (10.1038/nature13400) 24847885 PMC4337882 source

In the paper

“In these animals, an initial nerve net may have evolved via division of labour from mechanosensory-contractile cells in the lateral body wall, enabling coordinated movement of the growing body that involved both mucociliary creeping and changes of body shape.”

In the paper

“In addition, we can infer that early metazoan cells formed lamellipodia and filopodia, using proteins that reorganize cortical actin filaments such as the Arp2/3 complex, the actin cross-linking protein fascin, and myosin X”

What the sources say

• Not checked yetcites [39] Sebe-Pedros A, Burkhardt P, Sanchez-Pons N, Fairclough SR, Lang BF, King N, Ruiz-Trillo I 2013 Insights into the origin of metazoan filopodia and microvilli. Mol. Biol. Evol. 30, 2013– 2023. (10.1093/molbev/mst110) 23770652 PMC3748353 source
• Not checked yetcites [40] Jekely G 2014 Origin and evolution of the self-organizing cytoskeleton in the network of eukaryotic organelles. Cold Spring Harb. Perspect. Biol. 6, a016030 (10.1101/cshperspect.a016030) 25183829 PMC4142967 source

In the paper

“Such equipped animals may have been close to some simple Ediacaran fossils that have been interpreted as grazing on algal mats with a mucociliary sole for external digestion [3].”

What the sources say

• Not checked yetcites [3] Sperling EA, Vinther J 2010 A placozoan affinity for Dickinsonia and the evolution of Late Proterozoic metazoan feeding modes. Evol. Dev. 12, 201– 209. (10.1111/j.1525-142X.2010.00404.x) 20433459 source

In the paper

“Many of these molecules and associated functions were key to the evolution of the first neural cell types, as will be detailed below.”

In the paper

“Stem metazoans were likely composed of one main characteristic cell type, resembling, structurally and functionally, the choanoflagellates [8–10; see however 11].”

What the sources say

• Not checked yetcites [8] Nielsen C 2008 Six major steps in animal evolution: are we derived sponge larvae? Evol. Dev. 10, 241– 257. (10.1111/j.1525-142X.2008.00231.x) 18315817 source
• Not checked yetcites [9] Haeckel E 1874 Die Gastraea-Theorie, die phylogenetische Classification des Thierreiches und die Homologie der Keimblätter. Jena Z. Naturwiss. 8, 1– 55.
• Not checked yetcites [10] Maldonado M 2004 Choanoflagellates, choanocytes, and animal multicellularity. Invertebr. Biol. 123, 1– 22. (10.1111/j.1744-7410.2004.tb00138.x) source

In the paper

“This supports homology of sponge inner layer with the endomesoderm (i.e. the gastroderm) of other animals.”

In the paper

“further diversification of the cell types making up the nerve net and the mucociliary sole gave rise to the plethora of receptor cells, neuron types and muscular and ciliary effector cells present in today's cnidarians and bilaterians.”

In the paper

“Filopodia are highly dynamic structures that probably played a role in anchoring and stabilizing cells in the blastaea (figure 2a,b).”

In the paper

“a recent study comparing cellular transcriptomes indicates that a hierarchical tree structure indeed dominates cell-type interrelationships [6]”

What the sources say

• Not checked yetcites [6] Liang C, Consortium F, Forrest AR, Wagner GP 2015 The statistical geometry of transcriptome divergence in cell-type evolution and cancer. Nat. Commun. 6, 6066 (10.1038/ncomms7066) 25585899 source

In the paper

“Most probably, nutrients were shared with cells inside the choanoblastaea via the primary body cavity [8].”

What the sources say

• Not checked yetcites [8] Nielsen C 2008 Six major steps in animal evolution: are we derived sponge larvae? Evol. Dev. 10, 241– 257. (10.1111/j.1525-142X.2008.00231.x) 18315817 source

In the paper

“In line with this, some sponge larvae settle with the blastopore facing downwards [41].”

What the sources say

• Not checked yetcites [41] Leys SP, Eerkes-Medrano D 2005 Gastrulation in calcareous sponges: in search of Haeckel's gastraea. Integr. Comp. Biol. 45, 342– 351. (10.1093/icb/45.2.342) 21676779 source

In the paper

“This mode replaced the microphagy of individual cells.”

What the sources say

• Not checked yetcites [3] Sperling EA, Vinther J 2010 A placozoan affinity for Dickinsonia and the evolution of Late Proterozoic metazoan feeding modes. Evol. Dev. 12, 201– 209. (10.1111/j.1525-142X.2010.00404.x) 20433459 source

In the paper

“While the ancient choanocytes probably already used an intercellular mucoid mesh for particle trapping (see above), this mesh now covered the entire epithelium around the gastric opening, facing the substrate.”

In the paper

“This means that, from the very beginning, metazoan feeding was mucociliary—using both motile cilia and mucus for food capture.”

In the paper

“The starting point will be the first metazoan, which most probably resembled a sphere of epithelialized choanoflagellate-like cells, the ‘choanoblastaea’ [8].”

What the sources say

• Not checked yetcites [8] Nielsen C 2008 Six major steps in animal evolution: are we derived sponge larvae? Evol. Dev. 10, 241– 257. (10.1111/j.1525-142X.2008.00231.x) 18315817 source

In the paper

“A larger set of genes specifying endomesoderm has recently been found expressed in the ciliated micromeres of the amphiblastula larva of the calcareous sponge Sycon ciliatum, which later invaginate and transdifferentiate into the choanocytes of the adult sponge [42]”

What the sources say

• Not checked yetcites [42] Leininger S et al. 2014 Developmental gene expression provides clues to relationships between sponge and eumetazoan body plans. Nat. Commun. 5, 3905 (10.1038/ncomms4905) 24844197 source

In the paper

“Additional support comes from the conserved expression of conserved transcription factors such as brachyury, and signalling molecules such as hedgehog, in the blastoporal tissue of various metazoans [43,44].”

What the sources say

• Not checked yetcites [43] Arendt D 2004 Comparative aspects of gastrulation. In Gastrulation (ed. Stern C). New York, NY: Cold Spring Harbor Laboratory Press.
• Not checked yetcites [44] Arendt D, Tosches MA, Marlow H In press From nerve net to nerve ring, nerve cord and brain—evolution of animal nervous systems. Nat. Rev. Neurosci. 26675821

In the paper

“On the effector side, early metazoan cells already possessed a complex machinery for cellular contractions and shape changes based on a dynamic actomyosin cytoskeleton.”

What the sources say

• Not checked yetcites [36] Steinmetz PR et al. 2012 Independent evolution of striated muscles in cnidarians and bilaterians. Nature 487, 231– 234. (10.1038/nature11180) 22763458 PMC3398149 source

In the paper

“Proteins required for action potential generation and propagation were likewise present in early metazoans, such as voltage-gated sodium [28,29], potassium and calcium channels [30].”

What the sources say

• Not checked yetcites [28] Liebeskind BJ, Hillis DM, Zakon HH 2011 Evolution of sodium channels predates the origin of nervous systems in animals. Proc. Natl Acad. Sci. USA 108, 9154– 9159. (10.1073/pnas.1106363108) 21576472 PMC3107268 source
• Not checked yetcites [29] Zakon HH 2012 Adaptive evolution of voltage-gated sodium channels: the first 800 million years. Proc. Natl Acad. Sci. USA 109(Suppl 1), 10 619– 10 625. (10.1073/pnas.1201884109) PMC3386883 22723361 source
• Not checked yetcites [30] Yu FH, Catterall WA 2004 The VGL-chanome: a protein superfamily specialized for electrical signaling and ionic homeostasis. Sci. STKE 2004, re15. 15467096

In the paper

“The inward fold of one-half of the sphere then led to the next important ancestor, the gastraea.”

In the paper

“Enterocytes are optimized towards the uptake of nutrients; either directly from the mucus or from the nutrient-enriched water of the gastric cavity.”

In the paper

“Another member of the Fox family, foxi, specifies the osmoregulatory ionocytes, another characteristic cell type of vertebrate mucociliary epithelia [116];”

What the sources say

• Not checked yetcites [116] Dubaissi E, Papalopulu N 2011 Embryonic frog epidermis: a model for the study of cell–cell interactions in the development of mucociliary disease. Dis. Model Mech. 4, 179– 192. (10.1242/dmm.006494) 21183475 PMC3046089 source

In the paper

“It is thus plausible that the benthic gastraea evolved longitudinal muscle fibres innervated by an ectodermal nerve net, by division of labour as illustrated in figures 4f and 5a,b.”

In the paper

“With the invention of the mucociliary sole around the gastric opening, the endochoanocytes were no longer needed for mucoid particle trapping. In consequence, they diversified into more specialized cell types—enterocytes, excretory cells and contractile cells (figure 4e).”

In the paper

“moreover, foxA has been found to specify small 5HT-secreting cells in the mucociliary epithelium that regulate ciliary beating [117,118].”

What the sources say

• Not checked yetcites [117] Dubaissi E, Rousseau K, Lea R, Soto X, Nardeosingh S, Schweickert A, Amaya E, Thornton DJ, Papalopulu N 2014 A secretory cell type develops alongside multiciliated cells, ionocytes and goblet cells, and provides a protective, anti-infective function in the frog embryonic mucociliary epidermis. Development 141, 1514– 1525. (10.1242/dev.102426) 24598166 PMC3957375 source
• Not checked yetcites [118] Walentek P, Bogusch S, Thumberger T, Vick P, Dubaissi E, Beyer T, Blum M, Schweickert A 2014 A novel serotonin-secreting cell type regulates ciliary motility in the mucociliary epidermis of Xenopus tadpoles. Development 141, 1526– 1533. (10.1242/dev.102343) 24598162 source

In the paper

“A digestive ventral sole, used for external digestion and movements, also exists in today's placozoans [104] (figure 4b).”

What the sources say

• Not checked yetcites [104] Ruthmann A, Behrendt G, Wahl R 1986 The ventral epithelium of Trichoplax adhaerens (Placozoa): cytoskeletal structures, cell contacts and endocytosis. Zoomorphology 106, 115– 122. (10.1007/BF00312113) source

In the paper

“Therefore, the evolutionary origin of the nerve net appears to be closely linked to that of the musculature [44].”

What the sources say

• Not checked yetcites [44] Arendt D, Tosches MA, Marlow H In press From nerve net to nerve ring, nerve cord and brain—evolution of animal nervous systems. Nat. Rev. Neurosci. 26675821

In the paper

“In the cerianthids, a basal cnidarian lineage, the nerve net appears to be of purely ectodermal nature [128], which suggests that the evolution of inner layer neurons occurred independently in ctenophore, cnidarian and bilaterian lineages [136].”

What the sources say

• Not checked yetcites [128] Peteya DJ 1973 A light and electron microscope study of the nervous system of Ceriantheopsis americanus (Cnidaria, Ceriantharia). Z. Zellforsch. 141, 301– 317. (10.1007/BF00307408) 4147590 source
• Not checked yetcites [136] Watanabe H, Fujisawa T, Holstein TW 2009 Cnidarians and the evolutionary origin of the nervous system. Dev. Growth Differ. 51, 167– 183. (10.1111/j.1440-169X.2009.01103.x) 19379274 source

In the paper

“Finally, mucociliary epithelia are frequently found in foregut and neural midline tissue in bilaterians (that can be derived from tissue surrounding the gastric opening, see below).”

What the sources say

• Not checked yetcites [107] Tzetlin A, Purschke G 2005 Pharynx and intestine. In Hydrobiologia: morphology, molecules, evolution and phylogeny in polychaeta and related taxa, vol. 535/536 (eds Bartolomaeus T, Purschke G), pp. 199– 225. Berlin, Germany: Springer. source
• Not checked yetcites [108] Kristensen RM, Funch P 2000 Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida. J. Morphol. 246, 1– 49. (10.1002/1097-4687(200010)246:1<1::AID-JMOR1>3.0.CO;2-D) 11015715 source
• Not checked yetcites [109] Nielsen C 2004 Trochophora larvae: cell-lineages, ciliary bands, and body regions. 1. Annelida and mollusca. J. Exp. Zool. 302B, 35– 68. (10.1002/jez.b.20001) 14760653 source
• Not checked yetcites [110] Nielsen C, Hay-Schmidt A 2007 Development of the enteropneust Ptychodera flava: ciliary bands and nervous system. J. Morphol. 268, 551– 570. (10.1002/jmor.10533) 17469131 source

In the paper

“Nerve net-like arrangements of neurons cover the almost entire ectoderm in cnidarians and ctenophores [61,129,130–136].”

What the sources say

• Not checked yetcites [61] Hertwig O, Hertwig R 1879 Die Actinien. In Studien zur Blättertheorie. Jena, Germany: Gustav Fischer.
• Not checked yetcites [129] Jager M, Chiori R, Alie A, Dayraud C, Queinnec E, Manuel M 2010 New insights on ctenophore neural anatomy: immunofluorescence study in Pleurobrachia pileus (Muller, 1776). J. Exp. Zool. B Mol. Dev. Evol. 316B, 171– 187. (10.1002/jez.b.21386) 21462312 source
• Not checked yetcites [130] McFarlane ID 1984 Nerve nets and conducting systems in sea anemones: two pathways excite tentacle contractions in Calliactis parasitica. J. Exp. Biol. 108, 137– 149. source
• Not checked yetcites [131] Westfall JA 1996 Ultrastructure of synapses in the first-evolved nervous systems. J. Neurocyt. 25, 735– 746. (10.1007/BF02284838) 9023721 source
• Not checked yetcites [132] Westfall JA, Elliott CE 2002 Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones. Invert. Biol. 121, 202– 211. (10.1111/j.1744-7410.2002.tb00060.x) source
• Not checked yetcites [133] Parker GH 1919 The elementary nervous system. Philadelphia, PA: Lippincott. source
• Not checked yetcites [134] Hernandez-Nicaise M-L 1973 Le Systeme Nerveux des Ctenaires. I. Structure et Ultrastructure des Reseaux Epitheliaux. Z. Zellforsch. 137, 223– 250. (10.1007/BF00307432) 4144272 source
• Not checked yetcites [135] Bullock TH, Horridge GA 1965 Structure and function in the nervous system of invertebrates. San Francisco, CA: Freeman and company.
• Not checked yetcites [136] Watanabe H, Fujisawa T, Holstein TW 2009 Cnidarians and the evolutionary origin of the nervous system. Dev. Growth Differ. 51, 167– 183. (10.1111/j.1440-169X.2009.01103.x) 19379274 source

In the paper

“At the molecular level, the evolutionary divergence of mucociliary cell types in metazoans appears to be closely linked to the diversification of the Fox gene family.”

What the sources say

• Not checked yetcites [72] Vij S et al. 2012 Evolutionarily ancient association of the FoxJ1 transcription factor with the motile ciliogenic program. PLoS Genet. 8, e1003019 (10.1371/journal.pgen.1003019) 23144623 PMC3493443 source
• Not checked yetcites [112] Stubbs JL, Oishi I, Izpisua Belmonte JC, Kintner C 2008 The forkhead protein Foxj1 specifies node-like cilia in Xenopus and zebrafish embryos. Nat. Genet. 40, 1454– 1460. (10.1038/ng.267) 19011629 PMC4648715 source
• Not checked yetcites [113] Yu X, Ng CP, Habacher H, Roy S 2008 Foxj1 transcription factors are master regulators of the motile ciliogenic program. Nat. Genet. 40, 1445– 1453. (10.1038/ng.263) 19011630 source

In the paper

“Such hypothetical animal, which may have been close to the Eumetazoan ancestor, is depicted in figure 4c.”

In the paper

“The invention of the mucociliary sole was likely paralleled by cell-type diversification (figure 4d), specializing in the production of mucus, the secretion of digestive enzymes, the transport of mucus and the uptake of dissolved nutrients [45].”

What the sources say

• Not checked yetcites [45] Takashima S, Gold D, Hartenstein V 2012 Stem cells and lineages of the intestine: a developmental and evolutionary perspective. Dev. Genes Evol. 223, 85– 102. (10.1007/s00427-012-0422-8) 23179635 PMC3873164 source

In the paper

“Excitingly, the pharynx can be everted and used as a mucociliary sole in benthic ctenophores [62].”

What the sources say

• Not checked yetcites [62] Hyman LH 1940 The invertebrates: protozoa through Ctenophora. New York, NY: McGraw-Hill Book Company.

In the paper

“We will elaborate here that the first neurons emerged in this ancestor in the context of an ectodermal nerve net (figures 4f and 5).”

In the paper

“in Nematostella, foxB [122] and foxA [123] are expressed in the ectodermal ciliated pharynx and foxC [122] in pharyngeal endoderm; in Hydra, foxA/budhead [124] is likewise expressed in the endoderm around the oral opening, suggesting that the mucociliary epithelium around the gastric opening represents the ancient site of activity of the foxA, B, C, D, E superfamily.”

What the sources say

• Not checked yetcites [122] Magie CR, Pang K, Martindale MQ 2005 Genomic inventory and expression of Sox and Fox genes in the cnidarian Nematostella vectensis. Dev. Genes Evol. 215, 618– 630. (10.1007/s00427-005-0022-y) 16193320 source
• Not checked yetcites [123] Fritzenwanker JH, Saina M, Technau U 2004 Analysis of forkhead and snail expression reveals epithelial–mesenchymal transitions during embryonic and larval development of Nematostella vectensis. Dev Biol 275, 389– 402. (10.1016/j.ydbio.2004.08.014) 15501226 source
• Not checked yetcites [124] Martinez DE, Dirksen ML, Bode PM, Jamrich M, Steele RE, Bode HR 1997 Budhead, a fork head/HNF-3 homologue, is expressed during axis formation and head specification in hydra. Dev. Biol. 192, 523– 536. (10.1006/dbio.1997.8715) 9441686 source

In the paper

“The presence of a well-developed nerve net correlates with the presence of basiepithelial muscle fibres directly innervated by nerve net neurons, in cnidarians and in ctenophores [61,67,128,132,133,135,136,138–142].”

What the sources say

• Not checked yetcites [61] Hertwig O, Hertwig R 1879 Die Actinien. In Studien zur Blättertheorie. Jena, Germany: Gustav Fischer.
• Not checked yetcites [67] Mackie GO 1970 Neuroid conduction and the evolution of conducting tissues. Q. Rev. Biol. 45, 319– 332. (10.1086/406645) 4395914 source
• Not checked yetcites [128] Peteya DJ 1973 A light and electron microscope study of the nervous system of Ceriantheopsis americanus (Cnidaria, Ceriantharia). Z. Zellforsch. 141, 301– 317. (10.1007/BF00307408) 4147590 source
• Not checked yetcites [132] Westfall JA, Elliott CE 2002 Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones. Invert. Biol. 121, 202– 211. (10.1111/j.1744-7410.2002.tb00060.x) source
• Not checked yetcites [133] Parker GH 1919 The elementary nervous system. Philadelphia, PA: Lippincott. source
• Not checked yetcites [135] Bullock TH, Horridge GA 1965 Structure and function in the nervous system of invertebrates. San Francisco, CA: Freeman and company.
• Not checked yetcites [136] Watanabe H, Fujisawa T, Holstein TW 2009 Cnidarians and the evolutionary origin of the nervous system. Dev. Growth Differ. 51, 167– 183. (10.1111/j.1440-169X.2009.01103.x) 19379274 source
• Not checked yetcites [138] Westfall JA, Elliott CF, Carlin RW 2002 Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida. J. Morphol. 251, 83– 92. (10.1002/jmor.1075) 11746469 source
• Not checked yetcites [139] Grimmelikhuijzen CJP, Westfall JA 1995 The nervous systems of Cnidarians. In The nervous systems of invertebrates: an evolutionary and comparative approach (eds Breidbach O, Kutsch W), pp. 7– 24. Basel, Switzerland: Birkhäuser Verlag. source
• Not checked yetcites [140] Satterlie RA 2002 Neuronal control of swimming in jellyfish: a comparative story. Can. J. Zool. 80, 1654– 1669. (10.1139/z02-132) source

In the paper

“In contrast with anthozoan cnidarians, the gastric opening of the CBA was most probably facing the substrate, grazing the algal mats with a mucociliary sole (see above).”

In the paper

“An apical collar of microvilli surrounds a single cilium [49,153], identifying these cells as ectochoanocyte descendants (see above).”

What the sources say

• Not checked yetcites [49] Lyons KM 1973 Collar cells in planula and adult tentacle ectoderm of the solitary coral Balanophyllia regia (Anthozoa: Eupsammidae). Z. Zellforsch. 145, 57– 74. (10.1007/BF00307189) 4149850 source
• Not checked yetcites [153] Chia FS, Koss R 1979 Fine structural studies of the nervous system and the apical organ in the planula larva of the sea-anemone Anthopleura elegantissima. J. Morph. 160, 275– 298. (10.1002/jmor.1051600303) 30200683 source

In the paper

“Also, a slit-like blastopore would have allowed defecation of the digested and nutrient-deprived mucus with larger non-digestible particles at the new rear end.”

In the paper

“Most striking is the spatially restricted expression of gbx genes in all but the first pair of pouches, as similarly observed in sea anemone and amphioxus (figure 6c).”

What the sources say

• Not checked yetcites [44] Arendt D, Tosches MA, Marlow H In press From nerve net to nerve ring, nerve cord and brain—evolution of animal nervous systems. Nat. Rev. Neurosci. 26675821

In the paper

“In these animals, a first net of interconnected sensory neurons and contractile effector cells may have evolved that sped up and steered grazing on algal mats.”

In the paper

“Most probably, the nerve net first thus coordinated some very primitive form of amoeboid locomotion, which required alternating and antagonistic contraction of perpendicular myofibres, in order to elongate the body towards the desired direction and/or to change body shape in reaction to mechanical stimuli that were met on the locomotor trajectory.”

In the paper

“This ultimately led to convergent centralization events in ctenophores, cnidarians and bilaterians.”

In the paper

“In consequence, mucus is no longer transported towards the new ventral midline but towards mouth and anus (white arrows in figure 7b,c, lower panels).”

In the paper

“This closure of the gastric opening along the new ventral midline would have firmly joined together the two bilateral halves of the mucociliary sole.”

In the paper

“Moreover, cnidarian gastric pouches and bilaterian somites share the staggered and collinear expression of hox genes with expression boundaries coinciding with boundaries between pouches/somites [44];”

What the sources say

• Not checked yetcites [44] Arendt D, Tosches MA, Marlow H In press From nerve net to nerve ring, nerve cord and brain—evolution of animal nervous systems. Nat. Rev. Neurosci. 26675821

In the paper

“Key to our hypothesis is the presumed homology of cnidarian gastric pouches with the bilaterian coelomic cavities [164].”

What the sources say

• Not checked yetcites [164] Sedgwick A 1884 On the origin of metameric segmentation and some other morphological questions. Q. J. Microsc. Sci. 24, 43– 82.
• Not checked yetcites [44] Arendt D, Tosches MA, Marlow H In press From nerve net to nerve ring, nerve cord and brain—evolution of animal nervous systems. Nat. Rev. Neurosci. 26675821

In the paper

“The proposed closure of the slit-like opening of the gastric cavity necessarily impacted on mucociliary feeding, as trapped particles could no longer be internalized alongside the entire gastric slit, but only through the new mouth (and, presumably) anus.”

In the paper

“The last important step in the evolutionary scenario developed here is the median closure of the gastric opening, leaving mouth and anus at opposite ends (figure 7).”

In the paper

“The prevalence of mechanosensory neurons with actin collar and cilium, multipolar ganglion cells and myoepithelial cells in extant nerve nets supports the idea that the first neurons arose by division of labour from mechanosensory-contractile ectochoanocytes, as hypothesized in the diversification scheme in figure 4f.”

What the sources say

• Not checked yetcites [5] Arendt D 2008 The evolution of cell types in animals: emerging principles from molecular studies. Nature reviews. Genetics 9, 868– 882. (10.1038/nrg2416) 18927580
• Not checked yetcites [66] Kleinenberg N 1872 Hydra. Eine anatomisch-entwicklungsgeschichtliche Untersuchung. Leipzig, Germany: Verlag von Wilhelm Engelmann. source
• Not checked yetcites [67] Mackie GO 1970 Neuroid conduction and the evolution of conducting tissues. Q. Rev. Biol. 45, 319– 332. (10.1086/406645) 4395914 source

In the paper

“Core units of the basal metazoan nerve nets appear to be local two- to three-cell sensory-effector reflex arcs [138] that directly innervate the effector cells (myoepithelial cells or cnidocytes in cnidarians [138], muscle cells and ciliated cells in ctenophores [129]).”

What the sources say

• Not checked yetcites [138] Westfall JA, Elliott CF, Carlin RW 2002 Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida. J. Morphol. 251, 83– 92. (10.1002/jmor.1075) 11746469 source

In the paper

“and finally, cnidarian gastric pouches express genes that specify somitic musculature in bilaterians, such as mox and twist [168].”

What the sources say

• Not checked yetcites [168] Martindale MQ, Pang K, Finnerty JR 2004 Investigating the origins of triploblasty: ‘mesodermal’ gene expression in a diploblastic animal, the sea anemone Nematostella vectensis (phylum, Cnidaria; class, Anthozoa). Development 131, 2463– 2474. (10.1242/dev.01119) 15128674 source

In the paper

“Given that the mucus sticks to the substrate, the anterior-to-posterior movement of the mucus on the ventral body side would have pushed the animals forward.”

In the paper

“Strikingly, an anterior-to-posterior movement of a mucoid thread propagated by motile cilia, the so-called Reissner's fibre, is observed until today in virtually all chordates (vertebrates and amphioxus) [175].”

What the sources say

• Not checked yetcites [175] Vigh-Teichmann I, Vigh B 1983 The system of cerebrospinal fluid-contacting neurons. Arch. Histol. Jpn 46, 427– 468. (10.1679/aohc.46.427) 6362609 source

In the paper

“During this transition, specialized subregions of the former nerve net would have given rise to the bilaterian nerve cord.”

In the paper

“In addition, two types of ganglion cells are commonly observed, in cerianthids [128], sea anemone [132], hydrozoans [148] and ctenophores [129,134].”

What the sources say

• Not checked yetcites [128] Peteya DJ 1973 A light and electron microscope study of the nervous system of Ceriantheopsis americanus (Cnidaria, Ceriantharia). Z. Zellforsch. 141, 301– 317. (10.1007/BF00307408) 4147590 source
• Not checked yetcites [132] Westfall JA, Elliott CE 2002 Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones. Invert. Biol. 121, 202– 211. (10.1111/j.1744-7410.2002.tb00060.x) source
• Not checked yetcites [148] Davis LE 1974 Ultrastructural studies of development of nerves in Hydra. Am. Zool. 14, 551– 573. (10.1093/icb/14.2.551) source

In the paper

“It is reasoned here that one decisive step in the evolution of animals was the formation of a paired fold in the inner lining of the gastraea, the so-called primary mesentery (figure 6a). This mesentery then duplicated into two metameric, bilateral series, which enclosed gastric pouches in each half of the body (figure 6b).”

In the paper

“Beyond that, a plethora of molecular data from sea anemone suggest that bilateral symmetry indeed evolved prior to the cnidarian–bilaterian divergence [165].”

What the sources say

• Not checked yetcites [165] Finnerty JR 2005 Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? Bioessays 27, 1174– 1180. (10.1002/bies.20299) 16237677 source