III — HOEK— INTRODUCTORY 



It need hardly be said, that the knowledge of the plankton-forms living in the 

 northern seas, has been very greatly widened by these investigations of the first 

 two years. Some idea of the wealth of forms may be gained by the help of the 

 following figures: 



the phytoplankton is represented in the hauls by more than 325 species, the 

 Zooplankton by about 600 species; 



of the phytoplankton ^^ 200 species, belong to the diatoms, -4^ 75 to the Peridineje 

 and 4z 50 to the remaining smaller groups ; 



of the Zooplankton, 4r 70 species are Radiolaria, -f; yo species Foraminifera, 4- 70 

 species the remaining Protozoa, 4; 60 species Coelenterata, + 10 different larval 

 forms of Echinodermata, + 50 species Vermes (in the general sense); over 100 

 species Copepoda, 130 species of Crustacea belonging to other groups. Then, some 

 species of Bryozoa, Tunicata, Mollusca and Prochordata occur either as larvse or 

 adult forms; lastly, the eggs and larvse of various species of fishes are also repeat- 

 edly taken. 



To have an exact knowledge of the elements forming the plankton at one's 

 disposal, is indeed the first condition which must be fulfilled if we would form a 

 true picture of the distribution and, in general, of the extraordinarily important 

 role played by the plankton in the economy of the sea. As is well known, two 

 different things are considered when we speak of the great importance of the 

 plankton for the production of the sea. 



a. The drifting (so-called pelagic) eggs and larvse of many fishes as also the 

 young fry of many species belong to the plankton of the sea: we may call this 

 the "active" important part of the plankton. 



b. The larger plankton-forms (Crustacea etc.) feed upon the smaller, which 

 again are dependent for their support on the very smallest of the phytoplankton. 

 Since the larger plankton forms constitute the chief food of very important food- 

 fishes, and the other fishes (the predatory fishes) live upon the smaller, mostly 

 plankton-eating fishes, it is clear that all plankton-forms, either as direct or 

 indirect fish-food, play an extremely important even if "passive" role. 



The plankton investigations, which have been made up to the present on the 

 so-called seasonal cruises, have done more for the expansion of our knowledge 

 regarding the plankton-forms of the second category than for those of the first. 

 In order to study the occurrence and distribution of the pelagic fish-eggs, 

 fish-larvse etc, of the principal food-fishes, it is self-evident that expeditions at 

 intervals of three months are quite insufficient. This part of the investigations 

 had therefore to be carried out independently of the other plankton studies and 

 as a matter of fact, it has been taken along with the fishery investigations: 

 as is shown more closely in Appendix E great advances have also been made in 

 this field. 



With regard to the important plankton forms called passive above, the hauls 

 made during the seasonal cruises have contributed valuable material, not oui}- for 

 the separation of the species but also for the geographical and bath>-metrical 

 distribution of the various forms. A very useful foundation for further investigations 

 has thus undoubtedly been laid. What is still required, for a true picture of 



