30 THE DEPTHS OF THE SEA 



organs to which by analogy an auditory function has 

 been assigned. But it must not be forgotten that even 

 in the highest land-vertebrates the ear has two func- 

 tions. It is at once the organ of hearing and of 

 balancing. Part of the internal ear is occupied with 

 orientating the body. By means of it we can tell 

 whether we are keeping upright, going uphill or 

 descending, turning to the right or to the left ; and 

 it is probably this function which is the chief business 

 of the so-called ears of marine animals. Professor 

 Huxley once said that, unless one became a crayfish, 

 one could never be sure what the mental processes of 

 a crayfish were. This is doubtless true ; but experi- 

 ment has shown, both in crayfishes and cuttlefishes, 

 that, if the auditory organ be interfered with or in- 

 jured, the animal loses its sense of direction and 

 staggers hither and thither like a drunken man. It 

 is obvious that animals which move about at the 

 bottom require such balancing organs quite as much 

 as those which skim the surface, and it is in no wise 

 remarkable that such organs should be found in those 

 dwellers in the deep which move from place to place. 



If we could descend to the depths and look about 

 us, we should find the bottom of the sea near the 

 land carpeted with deposits washed down from the 

 shore and carried out to sea by rivers, and dotted 

 over with the remains of animals and plants which 

 inhabit shoal waters. This deposit, derived from the 

 land, extends to a greater or less distance around our 

 coast-line. In places this distance is very consider- 

 able. The Congo is said to carry its characteristic 

 mud 600 miles out to sea, and the Ganges and the 

 Indus to carry theirs 1,000 miles ; but sooner or later 

 we should pass beyond the region of coast mud and 

 river deposit, the seaward edge of which is the ' mud- 

 line ' of Sir John Murray. 



When we get beyond the mud-line, say a hundred 



