26 
since even in this stage the organic material plays a great part in the economy 
of the sea. 
1. Historical summary. It was Johannes Miller, who first remarked 
that a quantity of small organisms, plants and animals, occurred floating in the 
sea; he fished them up by means of gauze nets and called them »Auftrieb«. 
Hensen (1882 p. 1) introduced the expression plankton; with this he understands: 
»alles was im Wasser treibt, ob hoch oder tief, ob todt oder lebendig.« Through 
Hensen numerous investigations of the plankton have been carried out. The 
method in these investigations is, as known, that the planktou is fished with the 
plankton net of silk gauze with different, but always very small width of mesh. 
The plankton fished in these nets consists almost exclusively of organisms, plants 
and animals mixed. Of dead material, which according to the definition should 
also belong to the plankton, there is very little. Planktology is thus practically 
the study of the microscopic animals and plants which float in the water. 
Ås time went on, however, the view was gained, that the plankton which 
could be fished in the plankton nets, the »net plankton«, as it is now called, only 
represented a part of the plankton found in the water. Kofoid filtered the water 
through hardened filters and thus found, that the ordinary plankton nets of 
millers” gauze only caught 2,2—50 9%/, of the total plankton. Almost about the 
same time Lohmann (1902, p. 1) found in the gut of Appendiculariae numerous 
small organisms of which nothing was seen in the net plankton. Now the Appen- 
diculariae are themselves a kind of plankton net. By means of a sieve apparatus, 
which is simply many times finer than the plankton nets, they are able to filter 
the sea-water which passes through them. Thus, organisms must occur in the 
sea-water which cannot be detected in the net plankton because they are too 
small and pass through the nets. 
It was necessary, therefore, to seek for other methods which could help 
in the study of the plankton. Such a method was found in the centrifugal apparatus 
(cf. especially Lohmann, 1908). The specific gravity of the plankton is greater 
than that of the water, and the plankton can thus be almost completely precipi- 
tated by means of such a centrifugal apparatus. In this way Lohmann showed, 
that in addition to the plankton organisms occurring in the nets, there is a large 
number of small organisms, which are so small that they pass through the nets; 
and he found, further (1. c., p. 347), that large quantities of dust-fine detritus 
occurred as well as the plankton organisms. 
Recently Pitter (»Die Ernåhrung der Wassertiere«, 1909) has emphasized 
the presence of dissolved organic matter in seawater. Piutter relies here on 
analyses from the Mediterranean made by Natterer (Polaexpedition Iste u. 2te 
Reihe 1893—1894). By means of the permanganate method, to be mentioned 
later, Natterer determined how much oxygen was required for the oxidation of 
the organic material in 1 litre of seawater. He found that from 1.90—2.s5 mgm. 
were required, in one case even 6 mgm. of oxygen (the figures from Piitter). 
Further, Raben (Wiss. Meeresunters. N. F. Bd. 11) has investigated the quantity 
of organic material in the Kiel Fjord and Baltic by means of a modification of 
Messinger's method. He found that 3.—13.» mgm. of organic combined carbon 
occurred in I litre of water. The water was filtered before analysis. 
