178 BRITISH FISHERIES 



plankton are so small as to be invisible to the 

 naked eye, but sometimes they are so numerous 

 as to discolour the water. They have an im- 

 portance in the natural economy of the sea which 

 is quite incommensurate with their size as indi- 

 viduals. Every class in the animal kingdom is 

 represented in the plankton, and whole groups 

 of plants have no other method of occurrence. 

 Amongst the plants are the diatoms, which are 

 to be regarded as minute sea-weeds or alga? 

 composed each of a single organic unit or cell, 

 the desmids and other unicellular plants, spores 

 of sea-weeds, etc. The animals are extremely 

 varied in character. At the bottom of the scale 

 of life are the protozoa, or animals of microscopic 

 size, each of which, like a diatom, consists of 

 one cell. These organisms, amongst which are 

 the foraminifera and radiolaria, protozoa provided 

 with limy and flinty skeletons respectively, are 

 the most numerous, form the great bulk of the 

 animal plankton. Then we have the "jelly-fishes," 

 by which popular term is meant a multitude of 

 larger organisms medusae, ctenophores, siphono- 

 phora, and the like. Crustacea are very abundant, 

 particularly the copepoda, a very large group of 

 minute animals belonging to the same class as the 

 shrimp and crab. Among the molluscs, a group 

 called the pteropods (" winged-shells " or " sea- 

 butterflies ") are so abundant in the plankton as 

 to form the major portion of the food of the whale. 



