272 LIFE BY THE SEASHORE. 



been in large part described. It occurs in small numbers 

 on most parts of the coast, but in favourable situations 

 forms great mussel beds which are often carefully preserved, 

 and are of considerable commercial value. Before the 

 mussels can multiply with sufficient rapidity to form these 

 beds, they must have abundant food and a suitable sub- 

 stratum. For food they seem to depend largely on the 

 finer refuse brought down by rivers, and they rarely nourish 

 except where food of this kind is abundant. Where it is 

 abundant, however, they occur in numbers which are liter- 

 ally countless, as everyone who has seen a healthy mussel 

 bed must know. In one respect such beds are peculiarly 

 deceptive, as the unwary naturalist is likely to speedily 

 discover. My own first introduction took place in the 

 Firth of Forth, where a very low tide had laid bare a long 

 stretch of thickly covered rocks, dotted here and there with 

 huge starfish. Mindful of the tide, I hastened outwards 

 with more speed than discretion, and, planting a hasty foot 

 on a patch of mussels, found that it sank downwards over 

 the boot-top in a mass of fine mud before it reached the 

 firm rock. Later I learnt that this mud may reach a depth 

 of many feet; so that it is distinctly unwise to rashly under- 

 take the investigation of mussel beds. What happens is 

 this the mussels attach themselves to a smooth rock, and 

 by means of the fine cilia (whip -like threads) on the 

 surface of the gills and mantle produce rapid inhalent 

 currents. If the water contains many suitable solid par- 

 ticles they flourish apace, digesting these, and passing out 

 the indigestible residue in the form of fine mud. This mud 

 accumulates rapidly and would soon stifle the mussels, were 

 it not that as it is deposited they gradually lengthen their 

 attaching threads, so that they rise above the surface of the 

 rock. In still water the process may go on until the byssus 

 threads reach a length of several feet, the space between 

 the shell-fish and the rock being occupied by a mass of 

 solid mud. The result is that although a flourishing mussel 

 bed may be both useful and valuable, it is neither pretty 

 nor sweet-scented. 



The large horse-mussel does not, strictly speaking, live on 

 the shore rocks, but young forms are common there, and the 

 shells of the adult are not infrequently found on the sands. 



