6 P. T. CLEVE. THE PLANKTON OF THE NORTH SEA, THE ENGLISH CHANNEL, AND THE SKAGERAK. 



In fact the tripos-plankton seeras ainong the prevailino: mäss of southern origin to 

 contain a certain ainonnt of northern species. 



Two remarkable species, Halosplicera and Microsetella, have disappeared from the 

 North Sea since the spring. 



Species in the southern neritic plankton. The flow of Atlantic water through the 

 English Channel into the North 8ea had already taken place before the month of June. This 

 becomes apparent if we consider the plankton coUected at Helder. Phceocystis Pouchetii 

 was very abundant there in April and the beginning of May. It was seen in the begin- 

 ning of April at Plymouth. When that species disappeared from Helder (before the 12th 

 of May) a number of southern forms, such as Noctiluca, Ceratulina Bergonii, Eucamjna 

 zodiacus, Guinardia flaccida and Rhizosolenia Stolterfothii began to develop. 



In June we find that the typical Atlantic species RJiizosolenia styliformis had spread 

 över the whole area from Holland to Skagen, thus indicating that Atlantic water had 

 arrived from the E. Channel before June and that its plankton had been dispersed through 

 the water with low salinity off the Continental coast. The foUowing species of the southern 

 neritic plankton occur normally in the open Atlantic: 



Äcurtia Clausii, Diplopsalis lenticula, 



Paracalanus porvus, Rhizosolenia styliformis. 



The following species live, as a rule, above the coast banks, or only occur excep- 

 tionally in the ocean: 



Animals. Plants. 



Acaiiia longiremis, Chcetoceros densus, 



Centropages hamatus, C. didyinus, 



Teinora longicornis, C. Villei, 



Fodon Leuckartii, Eucampia zodiacus, 



Noctiluca miliaris. Guinardia flaccida, 



Lauderia annulata, 

 Rhizosolenia Shriibsolei, 

 Rh. Stolterfothii, 

 Stephanopyxis turgida. 



Of these species the Noctiluca is the most important. It seems to be confined to 

 the English Channel and the continental coasts of the North Sea. I liave not seen a 

 single specimen in any of the thousand plankton-samples from the Atlantic, which I have 

 examined. 



3. The North Sea in July— August 1898. 



But few plankton-samples were collected at this period and those only from the 

 eastern and southern parts of the North Sea, so that they cannot aiford us a complete 

 insight into the state of the North Sea. 



