232. _Vertical Distribution._--The most important general result of the discoveries of the Challenger, as regards the vertical or bathymetrical distribution of the Radiolaria, is the interesting fact that numerous species of this class are found living at the most various depths of the sea, and that certain species are limited to particular bathymetrical zones, _i.e._, are adapted to the conditions which obtain there. In this respect three different Radiolarian faunas may be distinguished, which may be shortly termed "pelagic," "zonarial," and "abyssal." The _pelagic_ Radiolaria swim at the surface, and when they sink (_e.g._, in a stormy sea), only descend to a small depth, probably not more than from {cliii}20 to 30 fathoms (§ 233). The complicated conditions of existence created by the keen struggle for existence at the surface of the sea, give rise to the formation of very numerous pelagic species, especially of Porulosa (SPUMELLARIA and ACANTHARIA). The _abyssal_ Radiolaria are very different from those just mentioned; they live at the bottom of the deep-sea, not resting upon nor attached to it, but probably floating at a little distance above it, and are adapted to the conditions of existence which obtain there (§ 235). Here the Osculosa (NASSELLARIA and PHÆODARIA) seem to predominate. The _zonarial_ Radiolaria live floating at various depths between the pelagic and abyssal species (§ 234). In their morphological characters they gradually approach the pelagic forms upwards and the abyssal downwards.