154 MARVELS OF FISH LIFE 



which is carried hither and thither by tides and currents, 

 and is to be found in every ounce of sea water. To 

 this third form of life is applied the term " plankton,' 1 

 and of this plankton, diatoms constitute one of the most 

 important elements. 



The reason why men of exceptional marine biological 

 knowledge and ability, such as Henson and Herdman, 

 have devoted, and are devoting, so much of their time to 

 the study of plankton, is because everything ultimately 

 depends upon it, and in order to understand the con- 

 ditions most favourable to fish life, it is necessary first 

 to have a thorough knowledge of plankton. 



Plankton consists, firstly, of those minute creatures 

 which spend their whole existence drifting about in the 

 sea, as for example, the copepod, and the lowest forms 

 of animal and vegetable life, such as protozoa, infusoria, 

 and diatoms ; and, secondly, of what is known as 

 " transitory plankton." 



The latter mainly consists of the larvae of various 

 marine animals which pass only the early part of 

 their existence drifting in the sea, but in adult life 

 settle on the bottom, or roam the ocean. 



The spat of the oyster already mentioned, while 

 it drifts about, is transitory plankton. Likewise the 

 minute larvae of crabs, lobsters, mussels, whelks, star- 

 fishes, urchins and many others. All these larvae are 

 entirely different in shape and appearance from the 

 ultimate form which they will take in adult life, and most 

 of them are ciliated, like the oyster spat. To transitory 



