142 THE LIFE OF CRUSTACEA 



activity. Besides all these more or less temporary 

 visitors, however, there are numerous species, even 

 in the inshore waters, which are adapted to a 

 floating life, and pass their whole existence as 

 members of the plankton. Copepoda of many kinds, 

 some Mysidse, Amphipods like Hyperia which is 

 commonly found sheltering under large jellyfish 

 some species of actively swimming Isopods, and 

 many other forms, are only to be captured by the 

 tow-net ; and now and then, in certain localities, 

 winds and currents may drive into coastal waters 

 shoals of species whose proper home is the open 

 ocean. 



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 be- 

 tween 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 alto- 

 gether, 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) 



