ii8 THE LIFE OF CRUSTACEA 



In the first place, the character of the sea-bottom 

 changes very greatly as we pass away from the 

 coast. Near the shore it is extremely diversified, 

 consisting in one place of rocks swept bare by the 

 tides or overgrown with jungles of sea-weed, in 

 another of banks of gravel or shingle, of sand or of 

 mud, but in all cases derived from the "waste" of 

 the land, as it is eaten away by the waves or washed 

 down by the rivers. As the distance from land 

 increases, the deposits become finer and finer, till 

 they shade off into a soft oozy mud, composed of 

 the finest particles brought down by the rivers. In 

 the neighbourhood of large rivers this mud may 

 sometimes extend for hundreds of miles from the 

 land, but there is a limit to the distance to which 

 even the finest particles can drift before they settle 

 to the bottom, and beyond this limit the floor of the 

 ocean is covered by sediments which owe their 

 origin, not to the land, but to the ocean itself. The 

 surface waters of the ocean everywhere teem with a 

 vast variety of floating animals and plants, and, as 

 these die, their remains sink to the bottom " like a 

 perpetual shower of rain." 



Among the most abundant floating organisms — 

 in the warmer seas, at any rate — are certain minute 

 animals known as Forammifera^ which belong to 

 the lowest class of the animal kingdom, and 

 have shells composed, in most cases, of carbonate 

 of lime. Over vast areas the bottom of the ocean 



