i 4 o THE LIFE OF CRUSTACEA 



that they pass through the finest silk plankton-nets, 

 and have to be sought for by special methods of 

 collection recently devised for the purpose. All 

 these organisms possess the green colouring matter 

 (chlorophyll) that enables them to live, as the higher 

 plants do, on the carbon dioxide and other sub- 

 stances dissolved in the water. The smaller animals 

 of the plankton feed on these vegetable organisms, 

 and in their turn serve as food for larger animals. 

 The Herring, the Mackerel, the gigantic Basking 

 Shark, and the still more gigantic Greenland Whale, 

 all feed directly on the animal plankton, and we 

 have already seen that the animals of the deep sea 

 depend entirely on the same source of food-supply. 

 Further, very many of the bottom-living animals of 

 shallow water swim at the surface in the early stages 

 of their life, and feed on the other plankton animals 

 and plants. Indeed, it is no exaggeration to say 

 that " all fish is diatom " in the same physiological 

 sense as " all flesh is grass," and the study of the 

 plankton is thus of practical importance as well as 

 of scientific interest. 



Of all the minute animals that form the inter- 

 mediate links in the chain between diatom and fish 

 or whale, the Crustacea are the most important and 

 the most numerous both in species and in individuals. 

 The Copepoda are more richly represented than any 

 of the other groups, and it would be difficult to find 

 a sample of marine plankton from which they were 



