THE IMPENETRABLE SEA 



to garden walls or the walls of houses. A gland within 

 the substance of the mussel's 'Toot" produces the material 

 for the threads, and it issues in a thick fluid which runs 

 along a groove extending down the hinder surface of the 

 "foot", which plants the threads wherever they are 

 required. 



The threads are arranged with consummate skill, first 

 in one direction then in another, the fluid running along 

 the groove and spreading out into a rounded disc at the 

 point where it touches the rock. There it becomes a 

 terminal sucker, at the end of an extremely strong thread, 

 which has been formed by the hardening of the fluid. 

 When the fastening operation is complete the mussel is 

 held in position by a mass of threads which diverge from 

 it like the guide-ropes of a tent — and no tent ever erected 

 by man was ever put up more efficiently. The threads are 

 directed forwards, so that (unless the mussel is tightly 

 packed in with others) the animal is able to swing round 

 slightly, with its narrower end facing the force of the sea, 

 thus enabhng it to resist wave action from any direction. 

 If the threads are broken the mollusc forms new ones 

 readily, and can move about for a while before attaching 

 itself again — it can even raise itself above mud and sedi- 

 ment. 



Mussels live on plankton, which they strain out of the 

 water that continually passes through their gill-plates, 

 which are enormously enlarged in proportion to their 

 size. The muscular power of mussels is amazing. When 

 one closes its shell tightly it uses force equivalent to lifting 

 132 times its own weight. 



Eider ducks are very fond of shellfish. As they are 

 feeding it may happen that one of them will pick up a 

 mussel in such a way that the animal will close its shells 

 firmly on the bird's beak or tongue. Holding on like 

 grim death the mussel may be fixed in such a manner 

 that it cannot be swallowed, or broken against the rocks. 

 If the bird tried to rid itself by dipping its beak in salt 



130 



