In a similar way the strictly neritic forms may sometimes be carried far out to sea, so that it is nowhere possible to draw a hard-and-fast line between the regions occupied by the neritic and the oceanic plankton. With increasing distance from land, however, the larval stages of bottom-living species become fewer, and finally disappear altogether, and there is left an assemblage of animals whose whole existence is passed floating at the surface or at the intermediate depths. How far down from the surface this floating fauna actually descends is a question which has been much debated. It appears now to be certain that there is no stratum of water between the surface and the bottom of the ocean which is devoid of life, although the upper layers (not at, but some distance below, the surface) are probably much more densely populated than those of the abyss. Many of the species appear to undertake more or less extensive migrations in a vertical direction, coming nearer the surface at certain stages of their life-history, and sinking into deeper water at others. Further, some species at least seem to rise to the surface at night, and to sink again during the day. Apart from these vertical movements, which are as yet only imperfectly understood, it is desirable to distinguish between the "epiplankton," comprising the organisms which inhabit the superficial strata of the ocean down to about 100 fathoms, and the "mesoplankton," found at greater depths. The plant-life which is dependent on sunlight belongs to the epiplankton, while the animals of the mesoplankton are dependent, like the bottom animals of the deep sea, on the supply of dead food material falling from above. A third division, the "hypoplankton," has been established for those animals which live immediately above the bottom, but its distinctness from the mesoplankton has not yet been satisfactorily established. Indeed, many of the swimming forms which have already been mentioned in dealing with the Crustacea of the deep sea are probably rather to be considered as belonging to the deep mesoplankton—at least, where their size and swimming powers do not entitle them to be ranked with the "necton."
Fig. 48—Conchœcia curta, an Ostracod of the Plankton. × 40. (Partly after G. W. Müller.)
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Many of the modifications in structure characteristic of pelagic animals may be traced to the necessity for keeping continuously afloat with a minimum of exertion. The Crustacea of the plankton never carry the heavy armour found in bottom-living species. Thus, the thick-shelled Ostracoda of the bottom are represented in the plankton chiefly by the family Halocypridæ ([Fig. 48]), in which the shell is thin, uncalcified, and almost membranous. Many species, particularly of the Copepoda, are seen, under the microscope, to have large globules of oil distributed through the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal. The same purpose is probably served, in many cases, by having large spaces, filled with fluid, within the body. This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively small part of the interior of the animals, the intervening spaces being filled with colourless transparent fluid. Many of the Hyperid Amphipoda show this peculiarity—for example, the relatively gigantic Cystisoma, which is mesoplanktonic in deep water; and it reaches its extreme in Mimonectes ([Fig. 49]), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod.
Fig. 49—Mimonectes loveni. A Female Specimen seen from the Side and from Below, showing the Distended-balloon-like Form of the Anterior Part of the Body. × 3. (After Bovallius.)
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