Morphology of statoliths and anatomy of statocysts were studied in five species of Gonatidae, Moroteuthis robusta (Onychoteuthidae) and Galiteuthis phyllura (Cranchiidae) from the northern Bering Sea, and Todarodes pacificus (Ommastrephidae) from the Pacific waters near Japan. A special experiment was carried out in order to observe statolith mobility on the macula statica and possible endolymph flows within the statocyst in freshly caught gonatid squid Berryteuthis magister. It was found that the statolith may deviate at small acute angles around three axes running through its centre of rotation (located near the spur) without any visual damage of its attachment area to the macula. This finding enabled us to re-consider previous theories and to create a new model of the squid statocyst functioning with the statolith as detector of multidimensional movement. Two types of statolith morphology are distinguished: demersal type statoliths characteristic of near-bottom decapods, and pelagic type statoliths characteristic of all pelagic squids and not dependent on their systematic position. Phylogenetic and ecological features of the statocyst and statolith structure are established. Possible evolutionary trends in development of different types of statocysts in decapods are discussed.