PELAGIC FLOATING CRUSTACEA 143 



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 under- 

 stood, 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," 



Many of the modifications in structure character- 



