THE SEA BOTTOM 65 



attaining to a length of a foot and a diameter of three 

 inches, and has a thick brown or yellowish skin. It moves, 

 like its relatives the starfish, by means of five rows of 

 tube-feet, and has the curious habit, when irritated, of 

 shooting out masses of sticky white threads, which swell 

 up greatly in water and completely incapacitate attacking 

 animals. Because of this habit, it is also called the 

 cotton-spinner. On greater provocation it ejects its 

 stomach and entire viscera, later growing a fresh set. 

 Crustaceans are not especially common in mud but a 

 number of small species are found. 



Shallow Water Zone 



Many of the animals just mentioned, and also a host of 

 others, are also found in the Sub-littoral or Shallow Water 

 zone, for these two regions contain as dense and varied a 

 population as any on the earth's surface. The bottom is 

 usually soft, of sand, mud or muddy clay frequently mixed 

 with stones and rocks which, together with hosts of empty 

 shells, furnish a foundation for the attached animals. We 

 can here only refer to a few of the commonest or more 

 interesting inhabitants, some of which are also found in 

 shallower water, for the boundaries we make are largely 

 artificial. Carnivores and animals which feed on particles 

 in suspension — animate or inanimate — or swallow the bottom 

 mud and sand, are especially abundant for, though light 

 penetrates, if the water be clear, to depths of 100 fathoms 

 or even more, it is insufficient to support plant life in depths 

 considerably less than this. Down to sixty fathoms, 

 however, there is frequently an abundance of the limy 

 coralline weeds or NuUipores, of which the commonest, 

 Lithothamnion, forms an important constituent of the sea 

 bottom in many parts, 



