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Tube-dwelling Bivalvia, which enclose themselves in secondary calcareous envelopes (crypts), evolved polyphyletically within the Gastrochaenacea, Pholadacea and Clavagellacea. The adaptive strategies for maintaining a suitable life position within a soft sediment were the main factor affecting the evolution of these forms. These basic strategies (boring in or cementation to bioclasts; construction of long vertical crypts anchored within the sediment by their own length; elongation and bending of the siphonal extremity of the crypt to reach to the surface after disturbances; passive reorientation by water currents; neo-formation of motile structures) were adopted convergently by representatives of the three superfami-lies. □Mollusca, Pelecypoda, tube dwelling, functional morphology, ecology.