CH. VIl] THE DISTRIBUTION OF THE PLANKTON 151 



with varying latitude. If this were so then the plankton would 

 change uniformly from north to south. But the floating pelagic 

 life of the Atlantic is distributed much more irregularly than this, 

 and its nature in different parts of the whole area is determined 

 by the existence of the two great current systems — the Equatorial 

 and Polar Stream circulations. 



P. T. Cleve^ was the first to attempt to give a general account 

 of the distribution of the plankton over the whole area of the 

 North- Western Ocean. Dealing mainly with the diatom forms as 

 constituting the most characteristic organisms of the pelagic life 

 of the sea he shewed that there were six main types of oceanic 

 plankton in this area. Including the Peridinians and Oscillatoria 

 these were : 



Tripos-Plankton : diatoms are relatively scarce in this type, 

 the characteristic organisms being Peridinians and micro-crustacea. 

 It is a form of plankton which is characteristic of the sea on the 

 north-east coast of Scotland, round the Shetlands, and off the 

 coasts of Norway. 



Styli-Plankton : diatoms are abundant and the principal species 

 are Rhizosolenia styliformis and R. alata. Chaetoceros lorenzianus 

 (in the south) and C. gracillima (in the north) are also present. 

 Styli-plankton is characteristic of western Europe down to N.E. 

 off Bermuda. 



Chaeto- Plankton : the characteristic diatoms are Chaetoceros 

 borealis, C. decipietis, and C. constrictus. It is a N. Atlantic 

 plankton. 



Desmo- Plankton: the principal organism is the alga Tricho- 

 desmium. This is the prevalent plankton of the Antilles and 

 Brazil currents. 



Tricho-Plankton : the principal diatoms are Thalassiothrix 

 longissitna and Rhizosolenia semispiaa. This is an Arctic plankton 

 and is found in the sea round Iceland, in the Irminger Sea and in 

 Davis Straits. 



Sira-Plankton : this type is characterised by the prevalence of 

 the diatom Thalassiosira nordenskioldii. It is found in the 

 Arctic Ocean. 



1 '' Phyto-Plankton of the Atlantic and its Tributaries,'' Upsala, 1897. 



