THE AMEBIC AN NATURALIST [Vol. XL VII 



The food of the animals of the Danish fjords is discussed by 

 Petersen. The oyster has commonly been considered as a plank- 

 ton feeder, although the materials found in its digestive tube are 

 indistinguishable from the surface brown layer of the bottom 

 and Petersen believes that the oyster feeds, in the main, upon 

 the organic parts of the dust-fine detritus. The growth of oysters 

 on objects raised from the bottom has been supposed to support 

 the view that they were plankton feeders, but now since it is 

 known, through the use of the centrifuge, that "pure" water 

 contains the dust-fine detritus which is everywhere available, it 

 also supports the view that the oyster feeds upon this as well. 

 Petersen remarks that Lohmann was the first to recognize this 

 detritus as an essential source of food for plankton animals, and 

 now Petersen gives it much greater extension and importance. 

 Thus in the past a clear distinction has not been made between 

 the ordinary plankton and detritus feeding animals. Such 

 feeders may be divided into two classes, those which filter the 

 material out of the water, and those which take it off the bottom. 

 In addition to the oyster the food of several other animals is 

 considered, such as Echinoderms, shrimp and Cardium. He 

 observed in an aquarium that the long-siphoned bivalve Abra 

 sucks in through the siphon the surface layer of detritus and he 

 suggests that the short-siphoned bivalves probably take their 

 detritus directly from the water. To the reviewer it seems that 

 here are several important suggestions for the students of our 

 American Unionidge. These molluscs are probably detritus feed- 

 ers also, rather than wholly plankton feeders, and this may be a 

 factor in their greater development in streams, compared with 

 ponds and lakes, on account of the superior powers of streams 

 in transporting detritus and other food. 



Instead of taking up the food relations of each species, Peter- 

 sen decided upon a larger unit, the animal community of Lim- 

 fjord, Thisted Bredning, and he studied the mass of animals 

 living on a square meter of the soft bottom. A review of the 

 food relations of such a sample area showed that it was il a 

 detritus-ruling Ln nullibra nch-trnrni rotnm xn > I >/ icitJi its preda- 

 tory animals. This animal community forms the basis for a 

 great part of the fish-life there." There are other communities. 

 Thus nearer the shore in about 5 meters and less of water the 

 Zostera zone begins, and this is a region which has not been 



