Nemertodermatida

The length of the body of representatives of Nemertodermatida ranges from 150 microns (young individuals) to 1 cm. The body is cylindrical, drop-shaped or filiform, as a rule, with rounded ends. Most of the representatives are colorless, but some species are entirely red or have a red stripe on the dorsal side of the body ^[7] . Endosymbiotic bacteria live in the epidermis of some representatives (for example, Meara stichopi and Nemertoderma westbladi ) ^[8] .

Like in Acoela, in Nemertodermatida under the epidermis lies a characteristic network of the roots of the cilia . At the base of the cilia have a typical composition of "9 × 2 + 2" (two microtubules in the center and nine microtubule doublets around the circumference), but closer to the end of the axon the microtubule 8 and 9 doublets become singlets, due to which the end of the cilium narrows. The epidermis is multilayered, and the nuclei of the epidermis cells are not placed under the muscle layer. Dying cells of the epidermis are replaced by neoplasts ( stem cells ) from the parenchyma , which fills the body cavity. On top of the epidermis lies well-pronounced glycocalyx . Under the epidermis is a layer of extracellular matrix . The basement membrane is poorly expressed. At the front end of the body, the frontal gland and several cervical glands open. Spermatozoa Nemertodermatida, unlike Acoela, has one flagellate , like the rest of Metazoa , but they are somewhat modified, probably due to internal fertilization . The two families of Nemertodermatida, Nemertodermatidae and Ascopariidae , are distinguished by their sperm ultrastructure. Like the intestinal turbellarians, the Nemertodermatida have a statocysts equilibrium organ, however, unlike them, it contains two statolites, not one. The nervous system of Nemertodermatida lies in the epidermis and has no general plan and is arranged differently in different species ^[9] . For example, in primitive representatives of the class, the nervous system is represented by two annular connections, longitudinal nerve cords and basepithelial nerve plexus . Nemertodermatida has an intestine lined with epithelium and supplied with digestive glands, but the lumen of the intestine is rather narrow ^{[7] [6]} .

Like in Acoela, in Nemertodermatida duet crushing takes place, however it begins as radial, inherent in secondary rotary. Further, the micromeres are slightly shifted clockwise, giving a spiral pattern ^[6] .

Exclusively marine microscopic worms that inhabit the benthos or epibenthos in sandy or muddy bottom sediments ^[7] . One species, Meara stichopi , lives in the intestines of holothurians ^[8] .

Nemertodermatida family ties have been a subject of controversy since the discovery of the first species described by the German helminthologist Ott Steinböck in 1930. He attributed Nemertodermatida to flatworms and, based on morphology, concluded that the new group of worms may be close to the ancestors of flatworms. In 1937, Einar Vestblad attributed the Nemertodermatida to the group of enteric turbellarians, which at that time had the rank of a detachment as part of the type of flatworms. However, enteric turbellarians and Nemertodermatida have many morphological differences, and in 1940 Karling separated Nemertodermatida from Acoela. Ultrastructural analysis showed, however, that the intestinal turbellarians and Nemertodermatida have an equally organized system of ciliary roots, on the basis of which in 1985 Ehlers proposed combining them into the Acoelomorpha type ^[5] .

Molecular analysis of Nemertodermatida has led to controversial results. An analysis of the rRNA genes of the Nemertinoides elongatus species carried in 1987 attributed Nemertodermatida to . However, the analysis of p- DNA sequences of a larger number of species identified the Nemertodermatida into an early distinguished clade of two-sidedly symmetrical animals, isolated before the separation of the primary and secondary ones. According to this analysis, Nemertodermatida are neither flatworms nor the Acoela sister group . The detached position of Nemertodermatida was later confirmed by analysis of several nuclear protein -coding genes. However, the first analysis conducted in 2009 confirmed the isolation of the Acoelomorpha type, consisting of enteric-turbellarians and Nemertodermatida and sister to Nephrozoa , a group that includes all the rest of Bilateria. However, the second phylogenomic analysis (2011) again abolished the Acoelomorpha and attributed Nemertodermatida to secondary-torn animals. Subsequent studies have shown that the Nemertodermatida, Acoela and Xenoturbellida constitute a type of Xenacoelomorpha, sister to the clade, including all other two-sidedly symmetrical animals (Nephrozoa) ^{[5] [6]} .

Below is the classification of the Nemertodermatida class as of 2018 according to the WoRMS database ^[1] .

• Family Ascopariidae Sterrer, 1998
  • Genus Ascoparia Sterrer, 1998
    • Ascoparia neglecta Sterrer, 1998
    • Ascoparia secunda Sterrer, 1998
  • Genus Flagellophora Faubel & Dörjes, 1978
    • Flagellophora apelti Faubel & Dörjes, 1978
• Nemertodermatidae Steinbock family , 1930
  • Genus Meara Westblad, 1950
    • Meara stichopi Westblad 1949
  • Genus Nemertinoides Riser, 1987
    • Nemertinoides elongatus Riser, 1987
    • Nemertinoides glandulosum Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Nemertinoides wolfgangi Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
  • Genus Nemertoderma Steinboeck, 1930
    • Nemertoderma bathycola Steinböck, 1930
    • Nemertoderma westbladi (Westblad) Steinbock, 1938
  • Genus Sterreria Lundin, 2000
    • Sterreria boucheti Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria lundini Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria martindalei Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria monolithes Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria papuensis Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria psammicola Sterrer, 1970
    • Sterreria rubra Faubel, 1976
    • Sterreria variabilis Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014
    • Sterreria ylvae Meyer-Wachsmuth, Curini Galletti & Jondelius, 2014