42; 



DEPTHS OF THE OCEAN 



During his plankton work in the Liimfjord, Petersen 

 arrived at the conclusion that the plankton played a very 

 unimportant part in the food of bottom-animals (as, for instance, 

 the oyster). He commenced therefore to study the finely 

 granular mass found in the gut of the bottom animals. He 

 discovered that the uppermost layer of mud on the fjord bottom, 

 2 or 3 mm. in thickness, consisted of detritus containing minute 

 remains of organisms, mainly of decayed plants from the 

 littoral region, and that only this upper layer of the mud has 

 any nutritive value, the deeper blue-black layer not occurring 

 in the gut of the bottom animals. Starting from these re- 

 searches, Petersen studied the organic (nutritive) constituents 

 of the mud, especially of the upper layer, and investigated the 

 abundance of bottom-animals over different kinds of deposits. 

 For this purpose he constructed an apparatus (see Chapter X.) 

 for cutting away from the sea-bottom a square foot of its 

 surface. When this large "bottom sample" is sifted the 

 animals contained in the mud can be counted, and by com- 

 paring the quantities of mud-eating animals thus found per 

 square foot of bottom, the yielding power of different areas 

 may be estimated, much on the same principle as the productive 

 value of agricultural land is estimated. 



The "Michael Sars " had, during the Atlantic cruise, some 

 of Petersen's apparatus on board, but owing to difficulties in 

 using them in deep water, we did not succeed in obtaining 

 material of any value, a fact all the more regrettable, as there 

 is no doubt that Petersen's method gives far more exact results 

 as regards the quantities of certain animals living on the bottom 

 in shallow water than hauls with dredges and trawls. Neverthe- 

 less, the material at hand may be used to illustrate the question. 

 The most stringent quantitative science is in the first stages of 

 a new study satisfied to dispense with the demand for absolute 

 exactness, and contents itself with relative values — in other 

 words, with a comparison between different localities. 



Sir John Murray long ago attempted to compare the number 

 of animals taken in the dredge or trawl on different deposits, 

 based on the results of the " Challenger " Expedition, and I 

 reproduce some of his figures f^pm the second volume of the 

 " Challenger " Summary : — 



[Table 



