196 THE FLOATING ANIMALS 



which Plankton becomes concentrated into swarms or 

 streaks ; it is by no means universally distributed, even 

 over a small district. Such streaks should be surveyed, 

 if possible for dimensions both horizontally and 

 vertically. Particular attention should be paid to 

 the comparative abundance of plant food inside and 

 outside the area, as well as to their position as regards 

 current and tide, for it has been suggested that they 

 are formed exactly as a river throws raffle on its 

 banks. 



All special adaptations of a planktonic organism to 

 its conditions of life should be noted. A striking 

 series of these adaptations is naturally directed towards 

 preventing weak swimmers from sinking. This object 

 is attempted in various ways, of which the chief are — 

 either (a) a diminution of weight by the formation 

 inside the animal of light fluids (in the bubbles of 

 Foraminifera and Radiolaria), fats (Copepoda), jelly 

 (jelly-fish and salps) or gas (Siphonophores) ; or (b) an 

 increase of surface in order to increase the friction in 

 falling through the water, produced either by flattening 

 the body (Phyllosoma, Sapphirina) or the growth of 

 long spines (Foraminifera, Crustacean larvse) . 



Coloured sketches from life are always valuable away 

 from home waters. Most of the larger invertebrate 

 Plankton is nearly colourless and transparent, and any 

 exceptions to this should be noted, and, if possible, 

 sketched. 



Of late years the Plankton has been used, in con- 

 nection with the " International Investigation of the 

 North Sea and Adjacent Waters," as a means of 

 tracing out the derivation of the body of water in 

 which it occurs, and as an indication of the amount of 

 available fish-food ; for this purpose most attention has 

 been paid to the minuter animals and plants (micro- 



