102 REACTION: THE IXFLUE^XE OF COMMUNITY ON HABITAT 



rather than calcareous material (cf. Murray and Renarcl, 1891; ]\Iur- 

 ray and Hjort, 1912). 



In Danish waters, Petersen and Jensen (1911) have found that the 

 surface layer of bottonr deposits is 1-2 millimeters in thickness and 

 exhibits a distinct brown color. Petersen made a study of the brown 

 layer and stated that it was composed of fine particles loosely aggre- 

 gated so that the surface was fluffy in texture. In addition to some 

 inorganic particles, it contained the following: (1) shells of diatoms; 

 (2) fragments of tissue of higher plants; (3) chitinous needles and 

 bristles; (4) a few living organisms, comprising bacteria, diatoms, and 

 animals. 



The part played by excreta in bottom deposits is not definitely 

 known, but it is probably very considerable and may warrant the 

 frecjuent statement that such layers have repeatedly been passed 

 through the alimentary canal of animals living at the bottom. Moore 

 (1931, a, b) has studied the muds of the Clyde and found that as 

 much as 40 per cent of the fine material was consolidated into fecal 

 masses or pellets, though in extreme instances the mud was formed 

 entirely of these. He estimated that these deposits were accumulating 

 at about the rate of a half centimeter a year and that the pellets 

 themselves might persist for a hundred years. The upper 5 centi- 

 meters of mud represented the deposit of ten years and was very 

 loose, with a high water content and a complete deficit of oxygen at 

 the surface, while nitrogen and jihosphorus decreased with depth. As 

 to their origin, the pellets were derived largely from worms, lamelli- 

 branchs, and other bottom forms, though those of the plankton also 

 played an important part, especially of Calanus and several species of 

 euphausids. 



