On the Origin of the Nervous System Based on Studying Sensillae in Acoel Turbellarians (Acoela)

Ya.I. Zabotin*, A.I. Golubev**

Kazan Federal University, Kazan, 420008 Russia

E-mail: *yaroslav_zabotin@rambler.ru, **anatolii.golubev_1937@mail.ru

Received May 17, 2017

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Abstract

Acoel turbellarians (Acoela) are of great interest in discussions on the origin of the nervous system, because they are among the most primitive multicellular animals. In particular, we can trace that they undergo all the main stages, which are characteristic of Bilateria in general, in formation of the nervous and sensory systems. The relatively simple organization of the nervous apparatus is, however, often accompanied in Acoela with the diversity of their epidermal sensory structures. During the ultrastructural investigation of Convoluta convoluta, the unusual sensillae containing myofibrils have been found. Each cell is clearly divided in height into the apical sensitive and basal contractile parts. The sensitive part bears one cilium with a thin transversely striated spindle-shaped rootlet; the basal part contains a bundle of myofibrils. The presence of sensory-muscular cells in Acoela allows further discussion on the origin of the nervous system in Bilateria. Along with N. Kleinenberg and O. and R. Hertwig's classical concepts, it is possible to suggest that the third scenario took place in this evolutionary process. Apparently, the sensitive and contractile parts of the sensory-muscular cell diverged into sensillae and muscle cells, respectively. Subsequently, both types of cells independently came into contact with the processes of neurons. This feature of acoel sensillae may be a consequence of still weak cell differentiation of these peculiar invertebrates and, at the same time, is a morphological basis for further divergence of two different tissues in higher bilaterally symmetrical animals. The revision of the literature data on the early periods of evolution of nervous-sensory apparatus in various taxa of lower invertebrates (sponges, coelenterates and acoel turbellarians) is presented.

Keywords: Acoela, nervous system, sensillae, sensory-muscular cells, evolution, phylogeny

Figure Captions

Fig. 1. Sensory muscular cells of Convoluta convoluta: a – single sensillum, b – group of sensillae. Scale: 1.0 ?m. Designations: C – cilium, CR – cilium rootlet, EС – epidermal cell, EMC – epithelial-muscular cell, MC – muscle cell, MF – myofibrils, NC – neural cell, SC – sensillum cell, SMC – sensory-muscular cell.

Fig. 2. Schematic reconstruction of the origin and relation of nervous, sensory, and muscular systems in metazoans: a – according to N. Kleinenberg's “neuromuscular” theory [1], b – according to O. and R. Hertwig's “sensory” theory [2, 3], c – according to the original “sensory-muscular” hypothesis. For designations see Fig. 1.

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For citation: Zabotin Ya.I., Golubev A.I. On the origin of the nervous system based on  studying sensillae in acoel turbellarians (Acoela). Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2017, vol. 159, no. 3, pp. 421–432. (In Russian)


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