KONGL. SV. VET. AKADEMIENS HANDLINGAR. RAND 32. wo 3. 7 
Another organism, the flagellate Phwocystis Pouwchetii, appears in the same time as Che- 
toceros decipiens, frequently in great abundance, and has about the same distribution. 
I therefore include also that organism among the cheto-species. 
IV. Trichoplankton (Sign 7). This type rules in the western Atlantic and 
constitutes in the summer the plankton of the Irminger Sea. Its origin is doubtful. 
Typical trichoplankton was gathered in the Bering Sea during the expedition of the Vega 
and it is an open question whether it spreads from the northern Pacific to the northern 
Atlantic or vice versa. In the winter (1897—98) the characteristic species of the tricho- 
plankton appeared as far down to the south as to the south of the Azores, which possibly 
may be owing to the plankton of the Labrador-current having been conveyed by the 
south-going branch of the Gulf-stream. In the summer it is confined to the western 
and arctic Atlantic, but in the winter it spreads to Scandinavia. 
The temperature of the trichoplankton-water varies between 6° and 12° and the 
salinity amounts to about 34 p. m. 
As the trichoplankton frequently becomes mixed with chetoplankton and with sira- 
plankton it is a difficult matter to make out to what type a number of species really 
belong. I consider the following as chiefly tricho-organisms: 
Animals. Plants. 
Calanus. finmarchicus, Ceratium tripos v. arctica, 
Fritillaria borealis, Chetoceros atlanticus, 
Spadella hamata, Coscinodiscus oculus iridis, 
Cyttarocylis denticulata (with the varie- Rhizosolenia obtusa, 
ties edentula, media and- gigantea), R. senispina, 
Ptychocylis acuta, Thalassiosira gravida, 
Tintinnus minutus. Thalassiothria longissima. 
On the coasts, washed by the trichoplankton-water, there originates a peculiar kind 
of derived trichoplankton, which I have designated as 
Northern neritic plankton (Sign Ns). This somewhat variable type occurs at the 
coast of Iceland, in Skagerack and in the fjords of Sweden during the winter, also in the 
fjords of Norway and on the Norwegian coast-banks, where it becomes in the summer 
slowly replaced by triposplankton. This kind of plankton seems to invade the coasts of 
Scotland and Scandinavia twice a year, viz. in the spring in company or in connection 
with the chetoplankton and in the autumn in connection with the trichoplankton. The 
detailed study of this kind of plankton will doubtless afford very interesting results as 
different species rule on different coasts. Thus for instance the Asterionella spathulifera 
abounds in the south coast of Iceland and becomes from thence transported to Scotland 
and as far southwards as to the coast of Holland. On the coast-banks of Norway 
Ceratium tripos v. longipes is the ruling species. The Limfjord of Denmark affords par- 
ticular advantages for the development of some species, as Skeletonema costatum and Cheto- 
ceros debilis,' which remain there for the greater part of the year. 
1 C. G. T. PETERSEN. Beretning “fra den danske biologiske Station 1898. 
