Nemertines

It is a well-known fact that many Nemertines break up into pieces when irritated; but although this statement is current in text-books as well as in special memoirs the matter has received very little attention, and I find no account of any internal changes which may take place in the tissues of the body previous to the process of fragmentation. Indeed, it appears that the statement is made rather vaguely, and its application is scarcely so general as might be inferred. There is no doubt but that Carinella, Linens, and other elongated and comparatively slender forms do fragment, and that these fragments can remain alive for a considerable time, as Dalyell, and later M’Intosh have recorded; and further, it appears that the anterior end of such a fragmented Lineus may produce a new posterior end, and M’Intosh figures various stages in the formation of a head to a more posterior piece. As the specimen which he kept under observation lived in very unfavorable conditions of food, &c., he did not observe the completion of the head even after some months, but remarks that in a state of nature such a regeneration most probably takes place frequently.1 Further, we are left somewhat in doubt as to whether a Nemertine does of its own accord, and independently of irritation, normally break up into pieces.

@article{benham1896fission,
    author = "Benham, W. Blaxland",
    title = "Fission in Nemertines",
    year = "1896",
    journal = "Journal of Cell Science",
    abstract = "It is a well-known fact that many Nemertines break up into pieces when irritated; but although this statement is current in text-books as well as in special memoirs the matter has received very little attention, and I find no account of any internal changes which may take place in the tissues of the body previous to the process of fragmentation. Indeed, it appears that the statement is made rather vaguely, and its application is scarcely so general as might be inferred. There is no doubt but that Carinella, Linens, and other elongated and comparatively slender forms do fragment, and that these fragments can remain alive for a considerable time, as Dalyell, and later M’Intosh have recorded; and further, it appears that the anterior end of such a fragmented Lineus may produce a new posterior end, and M’Intosh figures various stages in the formation of a head to a more posterior piece. As the specimen which he kept under observation lived in very unfavorable conditions of food, \&c., he did not observe the completion of the head even after some months, but remarks that in a state of nature such a regeneration most probably takes place frequently.1 Further, we are left somewhat in doubt as to whether a Nemertine does of its own accord, and independently of irritation, normally break up into pieces.",
    url = "https://doi.org/10.1242/jcs.s2-39.153.19",
    doi = "10.1242/jcs.s2-39.153.19",
    number = "153",
    pages = "19-31",
    volume = "S2-39"
}

@article{crossref1918deepsea,
    title = "Deep-Sea Nemertines",
    year = "1918",
    journal = "Nature",
    url = "https://doi.org/10.1038/101353b0",
    doi = "10.1038/101353b0",
    number = "2540",
    pages = "353-353",
    volume = "101"
}

Summary 1. Primitive vertebrates have many characters which do not appear to be immediately represented in any obvious precursor. 2. The hypothesis is put forward that creeping, or bottom‐living worms, classifiable as nemertines, attempted to recolonize the water by acquiring buoyancy, by swimming or both. 3. Some of those that succeeded were able to do so by adopting a method of filter‐feeding made adequate by the development of gill slits, and then converting the proboscis rudiment into a semi‐rigid structure (the notochord) essential for oscillatory swimming movements. These were the precursors of the protochordates and the vertebrates. 4. The acquisition of the ability to swim efficiently depended on the simultaneous development of distance receptors and the co‐ordination of the information that they provided with that provided by proprioceptors on the one hand and with the changing motor systems on the other. This entailed the development of a far more extensive nervous system, probably by incorporating into it much more of the dorsal ectoderm, and superimposing a primarily sensori‐motor system on to the more vegetative system already present in nemertines. This was achieved by elaboration of the placodal folding of the ectoderm which is characteristic of many nemertine embryos of the present day. In this manner a new central nervous system was combined and integrated with the existing more primitive ‘autonomic’ system and the cephalic ganglia of the nemertines became incorporated in the hypothalamus. 5. The hypothesis suggests likely precursor tissues from which the following vertebrate structures could plausibly have developed: olfactory organ, lateral line system, anterior and posterior pituitary, thyroid, pineal organ, chloride‐secreting cells, oxyntic cells, notochord, urinogenital system, liver, and several others. 6. The possible origins of the vertebrate eye, the somitic muscles, and some other fundamental features of the vertebrates are discussed. The evidence is still equivocal. 7. The acquisition of filter‐feeding and swimming by nemertine worms would be expected to bring into operation a whole new system for the selection of appropriate genes and thus inaugurate a period of very rapid and probably divergent evolution, made even more rapid by the enormous advantages gained by animals acquiring efficient form‐vision together with rapid and well co‐ordinated movements. It is thought likely that the rapid evolution of this group of animals, which possessed few features conducive to clear fossil remains, accounts for the apparent break in the continuity of the story of animal evolution which has until now made the origin of the vertebrates such an enigma. 8. It is suggested that studies of the proteins and secretions of the various organs of the nemertines and their comparison with those of the organs of the more primitive vertebrates might lead to the establishment of some important homologies.

@article{willmer1974nemertines,
    author = "WILLMER, E. N.",
    title = "NEMERTINES AS POSSIBLE ANCESTORS OF THE VERTEBRATES",
    year = "1974",
    journal = "Biological Reviews",
    abstract = "Summary 1. Primitive vertebrates have many characters which do not appear to be immediately represented in any obvious precursor. 2. The hypothesis is put forward that creeping, or bottom‐living worms, classifiable as nemertines, attempted to recolonize the water by acquiring buoyancy, by swimming or both. 3. Some of those that succeeded were able to do so by adopting a method of filter‐feeding made adequate by the development of gill slits, and then converting the proboscis rudiment into a semi‐rigid structure (the notochord) essential for oscillatory swimming movements. These were the precursors of the protochordates and the vertebrates. 4. The acquisition of the ability to swim efficiently depended on the simultaneous development of distance receptors and the co‐ordination of the information that they provided with that provided by proprioceptors on the one hand and with the changing motor systems on the other. This entailed the development of a far more extensive nervous system, probably by incorporating into it much more of the dorsal ectoderm, and superimposing a primarily sensori‐motor system on to the more vegetative system already present in nemertines. This was achieved by elaboration of the placodal folding of the ectoderm which is characteristic of many nemertine embryos of the present day. In this manner a new central nervous system was combined and integrated with the existing more primitive ‘autonomic’ system and the cephalic ganglia of the nemertines became incorporated in the hypothalamus. 5. The hypothesis suggests likely precursor tissues from which the following vertebrate structures could plausibly have developed: olfactory organ, lateral line system, anterior and posterior pituitary, thyroid, pineal organ, chloride‐secreting cells, oxyntic cells, notochord, urinogenital system, liver, and several others. 6. The possible origins of the vertebrate eye, the somitic muscles, and some other fundamental features of the vertebrates are discussed. The evidence is still equivocal. 7. The acquisition of filter‐feeding and swimming by nemertine worms would be expected to bring into operation a whole new system for the selection of appropriate genes and thus inaugurate a period of very rapid and probably divergent evolution, made even more rapid by the enormous advantages gained by animals acquiring efficient form‐vision together with rapid and well co‐ordinated movements. It is thought likely that the rapid evolution of this group of animals, which possessed few features conducive to clear fossil remains, accounts for the apparent break in the continuity of the story of animal evolution which has until now made the origin of the vertebrates such an enigma. 8. It is suggested that studies of the proteins and secretions of the various organs of the nemertines and their comparison with those of the organs of the more primitive vertebrates might lead to the establishment of some important homologies.",
    url = "https://doi.org/10.1111/j.1469-185x.1974.tb01083.x",
    doi = "10.1111/j.1469-185x.1974.tb01083.x",
    number = "3",
    pages = "321-363",
    volume = "49"
}

@inproceedings{willner1975the1,
    author = "Willner, E. N",
    title = "The possible contribution of nemertines to the problem of the phylogeny of the protochordates",
    year = "1975",
    booktitle = "Symposium of the Zoological Society, London, v. 36, p. 319-345",
    note = "talkorigins\_source = {true}; raw\_reference = {Willner, E. N., 1975, The possible contribution of nemertines to the problem of the phylogeny of the protochordates: Symposium of the Zoological Society, London, v. 36, p. 319-345.}"
}

@incollection{wolken1975protochordates,
    author = "Wolken, Jerome J.",
    title = "Protochordates",
    year = "1975",
    booktitle = "Photoprocesses, Photoreceptors, and Evolution",
    url = "https://doi.org/10.1016/b978-0-12-762050-3.50014-6",
    doi = "10.1016/b978-0-12-762050-3.50014-6",
    pages = "168-180"
}

@article{demski1987phylogeny,
    author = "DEMSKI, LEO S.",
    title = "Phylogeny of Luteinizing Hormone‐Releasing Hormone Systems in Protochordates and Vertebrates",
    year = "1987",
    journal = "Annals of the New York Academy of Sciences",
    url = "https://doi.org/10.1111/j.1749-6632.1987.tb36282.x",
    doi = "10.1111/j.1749-6632.1987.tb36282.x",
    number = "1",
    pages = "1-14",
    volume = "519"
}

The deuterostomes are a monophyletic group of multicellular animals that include the Chordata, a phylum that exhibits a unique body plan within the metazoans. Deuterostomes classically contained three phyla, Echinodermata, Hemichordata, and Chordata. Protochordata describes two invertebrate chordate subphyla, the Tunicata (Urochordata) and the Cephalochordata. Tunicate species are key to understanding chordate origins, as they have tadpole larvae with a chordate body plan. However, molecular phylogenies show only weak support for the Tunicata as the sister-group to the rest of the chordates, suggesting that they are highly divergent from the Cephalochordata and Vertebrata. We believe that members of the Tunicata exhibit a unique adult body plan and should be considered a separate phylum rather than a subphylum of Chordata. The molecular phylogeny of the deuterostomes is reviewed and discussed in the context of likely morphological evolutionary scenarios and the possibility is raised that the ancestor of the Tunicata was colonial. In this scenario, the colonial tadpole larva would more resemble an ancestral chordate than the solitary tadpole larva. In contrast, the true chordates (vertebrates and cephalochordates) would have evolved from filter-feeding benthic worms with cartilaginous gill slits, similar to extant enteropneust hemichordates.

@article{zeng2005molecular,
    author = "Zeng, Liyun and Swalla, Billie J",
    title = "Molecular phylogeny of the protochordates: chordate evolution",
    year = "2005",
    journal = "Canadian Journal of Zoology",
    abstract = "The deuterostomes are a monophyletic group of multicellular animals that include the Chordata, a phylum that exhibits a unique body plan within the metazoans. Deuterostomes classically contained three phyla, Echinodermata, Hemichordata, and Chordata. Protochordata describes two invertebrate chordate subphyla, the Tunicata (Urochordata) and the Cephalochordata. Tunicate species are key to understanding chordate origins, as they have tadpole larvae with a chordate body plan. However, molecular phylogenies show only weak support for the Tunicata as the sister-group to the rest of the chordates, suggesting that they are highly divergent from the Cephalochordata and Vertebrata. We believe that members of the Tunicata exhibit a unique adult body plan and should be considered a separate phylum rather than a subphylum of Chordata. The molecular phylogeny of the deuterostomes is reviewed and discussed in the context of likely morphological evolutionary scenarios and the possibility is raised that the ancestor of the Tunicata was colonial. In this scenario, the colonial tadpole larva would more resemble an ancestral chordate than the solitary tadpole larva. In contrast, the true chordates (vertebrates and cephalochordates) would have evolved from filter-feeding benthic worms with cartilaginous gill slits, similar to extant enteropneust hemichordates.",
    url = "https://doi.org/10.1139/z05-010",
    doi = "10.1139/z05-010",
    number = "1",
    pages = "24-33",
    volume = "83"
}

@article{raineri2006are,
    author = "RAINERI, MARGHERITA",
    title = "Are protochordates chordates?",
    year = "2006",
    journal = "Biological Journal of the Linnean Society",
    url = "https://doi.org/10.1111/j.1095-8312.2006.00620.x",
    doi = "10.1111/j.1095-8312.2006.00620.x",
    number = "4",
    pages = "637-637",
    volume = "87"
}

@incollection{holland2008protochordates,
    author = "Holland, Peter W. H. and Wada, Hiroshi",
    title = "Protochordates",
    year = "2008",
    booktitle = "METHODS IN MOLECULAR BIOLOGY™",
    url = "https://doi.org/10.1007/978-1-60327-483-8\_38",
    doi = "10.1007/978-1-60327-483-8\_38",
    pages = "563-566"
}

@incollection{hollandNoneprotochordates,
    author = "Holland, Peter W. H. and Wada, Hiroshi",
    title = "Protochordates",
    year = "None",
    booktitle = "Molecular Embryology",
    url = "https://doi.org/10.1385/1-59259-270-8:513",
    doi = "10.1385/1-59259-270-8:513",
    pages = "513-516"
}