36 
1908) have described how the oyster thrives well on an iron threadwork net, 
stretched high above the bottom. 
E. Rauschenplat in his, in so many respects, excellent work »Ueber die 
Nahrung von Thieren aus der Kieler Bucht« (Wiss. Meeresunt. Kiel. Neue Folge. 
5. Bd. 1901) has described how the animals, which he calls plankton-feeders, never 
contain pure plankton, but at the same time large quantities of detritus. His 
explanation of this is, that ships' screws, fish nets, perhaps waves and currents 
stir up the bottom of the deep water in Kiel Bay, so that the detritus becomes 
mixed with plankton out on the muddy bottom; the animals attached to piles or 
other "objects above the bottom also contain detritus, and the reason for this, 
according to Rauschenplat, is that the waves break against these and tear 
away small parts of plants, of pieces of wood and suspended sand grains or masses 
of detritus collected among the heaps of Lamellibranchs, so that the water thus 
becomes unclean. He ascribes great value to the detritus as food; but that it 
should actually be present everywhere in the water-masses themselves, 
seems never to have occurred to him. I had also believed in this »pile- 
theory« for a long time, which indeed I had constructed for myself. It was Boy- 
sen Jensen's centrifuging experiments in 1909 on »pure« Limfjord water, which 
first showed me, that there is at least often and probably always more detritus 
than plankton present in the water itself. In 1908 appeared Lohmann's 
excellent work » Untersuchungen zur Feststellung des vollstindigen Gehaltes des Meeres 
an Plankton« (Wiss. Meeresunt. Kiel, N. E., Bd. 10. 1908), in which on the basis 
of centrifuging sea-water he shows, that the whole plankton content of the water 
cannot be determined by means of the older methods, certainly not by nets of 
fine millers' gauze; far too many small organisms pass through; various species 
are not caught at all. He endeavours further to show, that even if all the plankton 
is included, there is in winter not enough plant food to supply the requirements 
of the plankton animals: »so muss man nach einer weiteren Nahrungsquelle fur 
die Planktontiere suchen und die ist, wie mir dinkt, in dem iberall im Meere 
verbreiteten Detritus in grosser Menge gegeben«. Here for the first time I 
find the idea, that the detritus present in the water-masses themselves 
is an essential source of food for the plankton animals. He seems to 
have grave doubts as to whether the large diatoms of the net-plankton have any 
appreciable importance as source of food, and he believes that the view, that the 
Copepoda feed on them, has not been proved (see however Dakin's later investi- 
gations 1908). But to return to the food of the Limfjord oyster. Here we have a 
mass of water full of dust-fine detritus, »wet dust«, of the same nature as the 
contents of the stomach and gut of the oyster and the uppermost brown layer on 
the bottom of the fjord. Whether now the oyster obtains its detritus from the 
detritus lying on the bottom which has been stirred up in one way or another, or 
whether it obtains it from the detritus sinking down through the water itself, is 
of less importance. Oysters kept in a hox a couple of meters above the bottom 
contain essentially the same after 83 days as oysters lyimng under them on the 
bottom; a certain sorting out of the material can be detected however, hut no 
qualitative difference. It has thus taken long, before one could believe the evidence 
of one's eyes, that the oyster is a detritus-feeder, and that in addition it 
