The limitation of species to certain regions, or to certain depths of an ocean, may appear at first sight peculiar, in view of the unrestricted communication between all its parts; but there are as efficient "barriers" there as on land, for generally a particular worm can live only in a certain temperature and at a certain pressure, and is dependent for its food on particular organisms, which in their turn depend on the depth and its accompaniments. It is, in fact, so much the more peculiar that certain species are more or less cosmopolitan, or occur at widely distant points. It is less peculiar, of course, to find different species of the same genus at different depths or in different areas, for any slight variation in a species advantageous to new conditions would readily be fixed, and give rise to a new species. The distribution of the Polychaeta depends probably on the pelagic larvae, which are carried by currents from one part of an ocean to another. There can be little doubt that many {301}Polychaetes are very "plastic," and can adapt themselves to changed conditions of life with considerable ease; for _Nereis diversicolor_, _Arenicola marina_, and others live equally well in water of very different densities, and with a different food supply. The great variety in the "habitats," and presumably therefore in their food supply, etc., exhibited by many Polychaetes, as well as the great variation observable in some species of Polynoina, and the close affinity of the species and genera of this sub-family, lead us to the same conclusion. EXTINCT POLYCHAETES.—The most numerous fossil records of the Polychaetes are calcareous tubes of various shapes and sizes; they are irregularly or spirally curved, and are very usually attached at one end, or by one surface, to stones or to fossils. These tubes belong to the Serpulidae, and are referred to the genera _Serpula_, _Spirorbis_, _Ditrupa_, and others.[362] _Spirorbis_ is the oldest unequivocal representative of the Polychaetes, as its tubes are found more or less abundantly in the Silurian and other Palaeozoic strata. In Palaeozoic times _Serpula_ was rare, as it was too in the Trias and Lias, but in the Jurassic strata it becomes abundant. In the chalk, _S. socialis_ may occur in masses like _S. uncinata_ of the present day, forming "Serpulite chalk." In the older tertiaries the genus is represented by _Spirulaea_. _Terebella lapilloides_ occurs in the Lias as a cylindrical, more or less curved tube of sand-grains. [Illustration: FIG. 159.—_Eunicites avitus_ Ehl. A fossil worm from the lithographic slate of Solenhofen: the jaws are seen in front, and the acicula along each side. (From Ehlers.) Natural size.]