158 THE BIOLOGY OF THE SEA-SHORE 



submergence may be of the greatest service to urchins in the 

 lower region of the shore by allowing them to escape into 

 deeper water when the outflowing tide threatens to leave 

 them in an unfavourable position. Similarly, in view of the 

 many occasions in the normal habitat when the tube feet 

 are unable to get a hold (sand, mud, rocks, etc., covered by 

 growths of fine algae, etc., or powdered with sand or mud), 

 the action of the lantern will lend valuable aid to that of the 

 spines. 



The employment of byssus threads by the mussel is 

 interesting because it illustrates a method of progress by 

 hauling on ropes, and then shortening them, which, prior 

 to Jennings' interpretation, was believed to be that of the 

 starfish. Williamson (1907), who has made a number of 

 interesting observations on the movements of the common 

 mussel {Mytilus edulis), quotes Lang as stating that the mussel 

 is able to climb up a perpendicular wall by lifting itself by 

 the foot and then making fast a thread, the length and 

 character of the latter varying with the degree of extension 

 of the foot. Suspended from the position thus secured the 

 animal may obtain further points of hold for new threads, 

 and from this situation may then climb a further stage. 

 The method described by Williamson by which mussels 

 endeavoured to keep in the water by slackening the byssus 

 will be noticed later in this chapter. We may notice here, 

 however, the way in which a mussel may break its attach- 

 ment when not bound by other mussels. A solid object is 

 found to which the tip of the foot may adhere ; the foot is 

 then shortened, and a strain is thrown on the byssus which 

 results in snapping some at least of the threads upon which 

 the strain bears. Even though some of the threads remain 

 the mussel is free to change its position to a certain extent, 

 and when the byssus is too strongly attached to break, the 

 mussel may get free by tearing it out by the roots. More- 

 over, without any aid from the foot the byssus threads may 

 be tautened and relaxed by the action of the byssus muscles. 

 (Williamson, loc. cit.) 



A feature of considerable biological interest in connection 



