178 THE FLOATING ANIMALS 



belong the Crustacea are probably more plentiful 

 than any other in Plankton. Of the " water-fleas " 

 (Cladocera), minute animals with two shells, a large 

 pair of rowing antennae, and one large eye, some are 

 not infrequently taken (Evadne, Fig. 107). Another 

 subdivision of Crustacea, also with two shells, is termed 

 the Ostracoda, one family of which is sometimes 

 very abundant (Conchoecia, Figs. 108, 109). 



Probably the most widely spread and prolific group 

 of the whole Animal Kingdom, so far as Plankton is 

 concerned, is formed by the " oar- footed " Copepoda 

 (Figs, no to 112, 114), which are only very rarely, if ever, 

 absent from a haul ; generally minute, they are active 

 swimmers ; they have been met with in such abundance 

 as to give the sea a reddish tinge. When magnified, 

 many show brilliant colourings, due either to irides- 

 cence (Sapphirina, Fig. 114) or to the presence of drops 

 of coloured fat. Many of the " fish-lice " belong to 

 this group ; some are only temporary parasites, and 

 can swim (Fig. no) ; others embed themselves in the 

 flesh of fish and other creatures, and become so altered, 

 in adaptation to a parasitic existence, that only a 

 study of their development shows them to be Copepods, 

 or, indeed, Crustaceans at all. The Barnacles, or 

 Acorn-shells (Cirrhipedia) , are not properly planktonic, 

 but may be found attached to floating weed, drift- 

 wood, etc. These have free-swimming larval stages, 

 enclosed in two shells, like an Ostracod. 



The subdivision of Crustacea, which includes the 

 " sand-hopper " and the so-called " fresh - water 

 shrimp " (the Amphipoda), although far more plentiful 

 on the bottom, has representatives (Fig. 113) in the 

 Plankton. Some of them haunt the bells of jelly-fish, 

 presumably for protection, while others (Phronima, 

 Fig. 115) establish themselves in the hollow skin of 



