GENERAL BIOLOGY 717 



pounds dissolved in sea-water, which must be formed, however, 

 when all is told, either by dissolution of the detritus or as 

 excreta from living organisms. 



It has long been recognised that the dust-like detritus plays Organic 

 an important part in the nourishment of certain bottom-animals ^^'"^^1^- 

 (see Chapter VII. and the reference to Murray's "mud-line"). 

 Investigations on the food of the oyster by Redeke and 

 American investigators have proved that detritus forms the 

 main contents of its stomach and intestines. Zoologists know 

 that great numbers of bottom forms (holothurians, worms, 

 and many others) are " mud-eaters," which live by passing 

 the soft mud of the sea-bottom through their digestive tract. 

 Lohmann and Rauschenplatt have lately shown that detritus 

 also plays an important part in the nourishment of pelagic 

 forms. Our ideas on this subject have recently been 

 advanced by the systematic investigations of C. G. J. Petersen.^ 



In the Limfjord Petersen studied how detritus was formed 

 by the disintegration of the dead plants along the coast, how it 

 was found suspended in the water, and finally settled on the 

 bottom as a soft layer 2 or 3 millimetres in thickness. In every 

 respect this fine mud was similar to that found in the digestive 

 tract of mussels and other animals. Petersen has proved this 

 phenomenon to be of general importance in all the waters 

 examined by him, and it will be necessary to examine the 

 conditions in various areas of the sea in a very extensive way 

 before we can arrive at a more perfect knowledge as to the 

 nutrition of animals. In the open ocean conditions are still 

 practically unexplored, and I will here only draw attention to 

 some points worthy of examination in the future. 



How far out to sea is the organic detritus carried ? 

 During our Atlantic cruise Gran was continually looking for 

 detritus, centrifuging water- samples for this purpose, but as 

 he tells us in Chapter VI. only insignificant quantities were 

 found in the open ocean. If we may draw conclusions from 

 bottom -deposits like Blue mud, there are vast differences in 

 various areas of the ocean. In Chapters IV. and VII. we 

 have seen that the terrigenous deposits on the eastern side 

 of the Atlantic are limited to the African and European coast 

 banks, while on the western side they extend far into the 

 ocean beyond the coast banks of America (see Map IV.). 

 These facts may obviously be explained as being due to 

 currents (see current-chart, Fig. 508), which on the western side 



^ Report of the Danish Biol. Statioti, No. XX. Copenhagen, 191 1. 



