STILLNESS OF OCEAN DEPTHS 29 



variety of shape, and their only common feature is 

 that none of them are symmetrical. This radial sym- 

 metry is especially marked in the case of sessile 

 animals, those whose ' strength is to sit still,' attached 

 by their base to some rock or stone, or rooted by a 

 stalk into the mud. Such animals cannot move from 

 placei to place, and, like an oyster, are dependent for 

 their food on such minute organisms as are swept 

 towards them in the currents set by the action of their 

 cilia. A curious and entirely contrary effect is pro- 

 duced by this stillness on certain animals, which, 

 without being fixed, are, to say the least, singularly 

 inert. The sea-cucumbers or holothurians, which can 

 be seen lying still as sausages in any shallow sub- 

 tropical waters, are nevertheless rolled over from time 

 to time, and present now one, now another, surface to 

 the bottom. These have retained the five-rayed sym- 

 metry, which is so eminently characteristic of the 

 group Echinoderma, to which they belong. But the 

 holothurians in the deep sea, where nothing rolls 

 them about, continue throughout life to present the 

 same surface to the bottom ; and these have developed 

 a secondary bilateral symmetry, so that, like a worm 

 or a lobster, they have definite upper and lower sur- 

 faces. These bilateral holothurians first became known 

 by the dredgings of the Challenger, and formed one 

 of the most important additions to our knowledge 

 of marine zoology for which we are indebted to that 

 expedition. 

 At the bottom of the sea there is no sound — 



' There is no sound, no echo of sound, in the deserts of 

 the deep, 

 Or the great grey level plains of ooze where the shell- 

 burred cables creep.' 



The world down there is cold and still and noiseless. 

 Nevertheless, many of the animals of the depths have 



