The present issue of the "Guides" is of particular significance, for several reasons. It includes a number of marine species, and therefore – for the first time – deals with a significant fraction of non-continental animals. The Onychopoda are the only group (in addition to Penilia of the Ctenopoda) which evolved a series of species living in the World Ocean. All three families include numerous species which colonized brackish waters, however. The capacity of the Onychopoda to thrive both at oceanic salinities and in brackish waters suggests similarities in their physiology and, being a common feature to these families, provides an additional reason for a combination of these into one higher taxon.
The Onychopoda are comprised of 33 species, including 7 oceanic, 2 fresh-water, and approximately 24 species in the autochthonous Caspian fauna. Only in the Caspian Lake are representatives of all three families to be found. They here produced groups with morphologically bizarre shapes, whose origin is related to the dramatic geological past of this lacustrine sea.
The studies of this group in the Caspian Lake were begun by a remarkable carcinologist, Georg Ossian Sars, and greatly advanced by an outstanding zoogeographer and hydrobiologist, Philaret Dmitrievich Mordukhai-Boltovskoi.