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. [Illustration: 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.)]