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.
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 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.
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.