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The morphological and cellular plasticity of sponges (phylum Porifera) allows them to adapt to the variations in the environmental conditions. Therefore, they often dominate the benthic communities in the diverse marine and freshwater ecosystems from tropical to polar regions. The ecological success of the sponges is partially a result of their rapid regeneration capacity enabling them to recover from damages. Sponges are known to possess remarkable regenerative and reconstitutive abilities ranging from the re-building of a functional body from dissociated cells to wound healing or body part regeneration. As an ancient animal lineage, sponges are important models in studies aimed at understanding of the evolution of animal regeneration mechanisms. The present study is devoted to the Leucosolenia cf. variabilis (Montagu, 1818) (class Calcarea) regeneration. This is a common species in littoral habitats along the North European coasts from the English Channel to the White Sea. It was showed their quick wound healing and high regeneration capacity after different surgical interventions indicating that Leucosolenia is a convenient model for sponge regeneration investigations. The aim of our study is to discover the mechanism of the whole sponge restoration from the small piece of the L. cf. variabilis body wall using the light, confocal and electron microscopy. Sponge wall consists of two epithelial sheets—exopinacoderm and choanoderm—and the mesohyl inside. The cells possess cell junctions. Our investigation has shown that two main events take part in the regeneration process from the piece of L. cf. variabilis body wall to the well-functioning whole sponge belong to (1) the transdifferentiation of choanocytes into endopinacocytes which build the regenerative membrane and (2) the proliferation of the choanocytes. The epithelial morphogenesis plays the main role during the development of the regenerative membrane. The regenerative membrane is formed due to the convergent spreading and fusion of epithelial layers: the exopinacoderm on the external side of the membrane and the endopinacoderm—on the internal. The formation of the endopinacoderm is most likely occurs due to the transdifferentiation of the choanocytes near the wound edges into endopinacocytes through their flattening and losing of flagellum and microvilli collar. Thus, at this stage, the regenerative membrane consists of two epithelial layers (exo- and endopinacoderm) and the thin mesohyl between them. As a result of the epithelial morphogenesis the piece of the L. cf. variabilis body wall changes its form and size. It becomes larger and acquires the tubular form that is similar to the whole sponge shape. The study was supported by grant from RFBR № 19-04-00563.