— 46 — 



parison of the numerous data given in the tables clearly shows that these ani- 

 mals can be grouped in different classes according to their geographical occur- 

 rence. Of the most common (32) species, the distribution of which has been 

 mapped out in the Plankton Résumé, about half (14) belong to the open ocean, 

 and of these, some few (4) have their special habitat in the northern seas (the 

 Norwegian Sea, etc.), among them the Galanus finmarchicus, which is of im- 

 portance from a fishery-biological point of view, its occurrence apparently stand- 

 ing in relation to the migrations of the herring. The remainder (10) are, in 

 our area, found mainly in the open Atlantic water, and move therefrom north- 

 ward round Scotland into the North Sea; some even into the Skagerak. Scar- 

 cely any penetrate, under normal conditions, as far as the southern part of the 

 North Sea. To the northern oceanic forms may be added two of more arctic cha- 

 racter. The second half (16) of the Copepoda occur mainly in the coastal wa- 

 ters. A few (3) of these species are found especially or exclusively in the inner 

 parts of the Baltic, and are thus brackish water forms, while most of them are 

 wanting in the water of this region, which is only slightly salt, even though their 

 distribution is otherwise very different. They may be divided into northern 

 species (5), temperate (6) and sub-tropiacl (2), the lastnamed occur within our 

 area only in the Channel the temperate species are distributed throughout the 

 whole of the North Sea, including its southern part, where the more northerly 

 species however, are not found. On the whole, the southern part of the North 

 Sea and the inner part of the Channel form a water with fairly well defined bound- 

 aries (the Oithona region) which may also be characterised hydrographically. 



In a similar manner, the most important representatives of the other 

 animal groups and of the phytoplankton have been dealt with: the Tintinnoidea, 

 Radiolaria, Coelenterata, Chœtognatha, Cyphonautes, Rotatoria, Ostracoda, 

 Cladocera, Amphipoda, Schizopoda, Pteropoda, and Tunicata, besides Diato- 

 mea, Peridiniales, Halosphœra and Flagellata. As soon as all this matter is pub- 

 lished, it is intended to sum up the most important results in a general view 

 of the distribution of plankton organisms and their occurrence in the various 

 waters investigated. 



Besides the geographical distribution of species, the investigations have 

 also furnished valuable contributions to our knowledge of their seasonal occur- 

 rence; in this sphere, however much yet remains to be done. The necessary 

 basis for accurate knowledge on this point is acquaintance with the history of 

 developement of the species (their different stages of developement). Here also 

 investigations have been carried out, especially by German and Norwegian in- 

 vestigators. The different] larval stages for instance, which are to be disting- 

 uished in regard to the species of Copepods occurring in the Belt Sea, have been 



