144 THE LIFE OF CRUSTACEA 



istic 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 Halocypridse (Fig. 48), in which the shell 

 is thin, uncalcified, and almost membranous. Many 

 species, particularly of the Copepoda, are seen, 



Fig. ^8—ConcJicecia curta, an Ostracod of the Plankton, x 40. 

 (Partly after G. W. Mliller.) 



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 



