Zabotin Ya.I. 2017. The ultrastructural organization of Acoela and their phylogenetic relationships // Invertebrate Zoology. Vol.14. No.2: 217–225 [in English].

Kazan (Volga region) Federal University, Kazan, 420008, Russia. E-mail:

ABSTRACT: Acoela represent one of the most spectacular taxa in animal kingdom. Their phylogenetic position as well as a taxonomic rank widely varies in zoological literature from the order of flatworms to the separate phylum within the deuterostomes or the sister taxon of all other bilaterians. One of the reasons of the absence of consensus among morphologists and molecular biologists is the insufficient amount of fine structural data on Acoela. In the present work the new ultrastructural features of four species of Acoela (Archaphanostoma agile, Otocelis rubropunctata, Symsagittifera japonica and Amphiscolops sp.) from different families are described. Along with the archaic characteristics the new apomorphic morphological features of epidermis, body wall musculature and central syncytial parenchyma of species studied were found. The cellular organization of acoels is characterized by the wide morphological diversity, however no significance of the secondarily simplification was found. The alternative views on phylogenetic affinities of Acoela are discussed, and the conclusion of progressive but not regressive evolution of this invertebrate group is proposed.

doi: 10.15298/invertzool.14.2.17

KEY WORDS: Acoela, morphology, ultrastructure, systematics, phylogeny.

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