382 MICROSCOPIC MARINE ORGANISMS IN HYDROGRAPHY. 



company with l*rof. G. Theel, and with the aid of his net, which could 

 be closed and opened below the water, I made in July an attempt to 

 get plankton from diiferent depths of the fjord. We found in the cold 

 bottom - water very little plankton, some few specimens of a large 

 Sagitta and of Calanus finmarchicus only. At about 30-40 metres the 

 ciliollagellates (among them Ceratium divergens) were abundant, and on 

 the surface the entomostraca. This examination was repeated during 

 the first days of August, when I and Dr. Aurivillius had the oppor- 

 tunity of accompanying Prof. Pettersson and Mr. G. Ekman on 

 the hydrographical expedition which went out at the time. The 

 result was the same as before ; but from the determination of the 

 temperature and the salinity of the water, it became clear that the 

 plankton had been collected in water differing in those respects, and 

 consequently that the different strata of water were characterised by 

 different amounts of plankton, and by different species. Samples of 

 plankton were afterwards collected by the Swedish hydrographical 

 expeditions at the same time as samples of water for physical and 

 chemical research. The examination of the plankton was carried out 

 by Dr. Aurivillius, who took charge of the animal plankton, and by 

 myself, who undertook the vegetable. 



Having examined a large number of samples, I have lately found that 

 the plankton of the Skagerack and Kattegat can be classed according to 

 the prevailing species, and in this way 1 distinguished four types, 

 namely : (1) Tripoa-'pkm'kton, (2) Didymus-planUon, (3) Tricho-planUon, 

 and (4) Sira-planJcton. 



(1) The IVipos-planJitoJi is characterised by its scarcity in diatoms, 

 and its abundance in cilioHagellates and entomostraca, which give to 

 the spirit, in which the samples are preserved, an orange or yellow 

 colour, all the other kinds of plankton colouring it more or less deep 

 green. Among the entomostraca, according to the publications of 

 Dr. Aurivillius, Faraccdanus parvus, Pseudocalanus elongatus, and Ecadne 

 spinifera are the most abundant. Among the cilioflagellates Ceratium 

 trii^os, with the variety macroceros, is the most common. C. divergens, 

 C. furca, and C. fusus occur in less numbers. Diatoms are, as I have 

 said, scarce, the most abundant being Coscinodiscus concinnus and 

 Iihizosolenia gracillima. In winter and early spring the unicellular alga, 

 Halosphaera viridis, is found in abundance. This kind of plankton 

 characterises the water of the Pialtic current, and prevails in the 

 summer in the Kattegat and Skagerack. The organisms consist chiefly 

 of euryheline and eurythismic species, which can withstand the dilution 

 of the Salter North Sea water by the slightly saline Baltic water. 



It seems very probable that this first type of plankton may by future 

 researches be split up into different kinds. We may thus, perhaps, 



