As for species, it can be said generally that the different oceanic areas and even different coasts present different species, but we know practically nothing of variation amongst Polychaeta, and many so called species may be mere local varieties, for frequently the descriptions of "new species" are scarcely intelligible. At any rate we know that certain species occur at widely separated localities, for two or three species of Polynoids occur in Japan, and again at Dinard on the French coast. A considerable number of species are common to both sides of the North Atlantic ocean, having been obtained off Norway and in the Gulf of the St. Lawrence. A few of these which are common on our coasts may be enumerated:—_Nereis pelagica_, _Nicomache lumbricalis_, _Glycera capitata_, _Thelepus cincinnatus_, _Scoloplos armiger_, _Sabella pavonia_, _Ophelia limacina_, _Aphrodite aculeata_, _Trophonia plumosa_, _Polynoë squamata_, _Capitella capitata_, _Sthenelais limicola_. {300}As for _bathymetrical_ distribution,[360] many genera occur at all depths, though Polychaetes appear to be most abundant, as far as we know at present, in "shallow water"—that is, down to twenty fathoms or so; but this may be due to the greater facility of collection on shore and in these slight depths, for the "Challenger" obtained considerable numbers of new species at greater depths. The "deep-sea" forms are chiefly tubicolous, and since these tubes are fixed and partially embedded in the bottom, probably comparatively few are brought up. Some genera occur at very great depths; thus the Terebellid _Leaena abyssorum_ and the Serpulid _Placostegus benthalianus_ were brought up from 3125 fathoms—the greatest depth from which Polychaetes were obtained by H.M.S. "Challenger"; and it is interesting to note that species of each of these two genera occur in shallow water, the Serpulid being represented in our own coast fauna by _P. tricuspidata_. Amongst our own fauna, a few examples may be given of the "replacement of species."[361] The littoral _Sthenelais boa_ is represented by _S. limicola_ in deeper water; _Sabellaria alveolata_ by _S. spinulosa_; _Polynoë imbricata_ by several deep-water species. Similarly with genera: the littoral _Pomatoceros_ is replaced by _Serpula_ in deeper water; and the Hesionid _Psamathe_ by _Castalia_.