86 SEA FISHERIES 



Temperature and salinity are not merely the two 

 principal agents of oceanic circulation ; they are the 

 factors which make life itself possible in the depths of 

 the sea. 



We use the term plankton to denote a heterogeneous 

 mixture of living creatures, comprising very minute adult 

 species, both vegetable and animal, the larvae of these 

 and of larger species, and the eggs of all kinds of vege- 

 tables and animals, which, although sometimes having a 

 movement of their own, are carried along by the mass 

 of the water containing them. The collective term 

 plankton, in short, denotes the "living dust" which is 

 scattered over the ocean ; the " living emulsion " of 

 which I spoke on the first page of this book. We shall 

 comprehend the importance of this element if we 

 remember (i) that the majority of pelagic species r re- 

 main pelagic during all stages of their existence ; (2) that 

 nearly all marine species, of whatever kind corals, 

 starfish, sea-urchins, sea-worms, molluscs, fish, &c. are 

 pelagic during the early stages of their youth ; and 

 (3) that the vegetable plankton, which is by far the most 

 abundant, transforms the inorganic substances of the 

 ocean into living substance ; it is the vegetable plankton 

 which, with the salts dissolved in the water, creates life ; 

 and it is plankton which serves to nourish the entire 

 animal population of the seas. 



Vegetable plankton is composed of bacteria and adult 

 microscopic algae, the latter belonging to three primary 

 groups : Diatomaceae, Peridinaceae, Cyanophyceae. 

 The Diatoms are algae possessing a fragile flinty carapace, 

 which constitutes a veritable suit of armour. Their 

 * The terms planktonic and pelagic are synonymous. 



