242 



Part III. — Twenty-fifth Annual Report 



The Movement of the Mussel. 



In its larval stage stage the mussel floats about subject to the drift of 

 the water, and possessing an independent motion from its own cilia. 



When it sinks to the bottom, it travels about over seaweeds and stones, 

 by means of its long foot, which is ciliated It extends its foot, takes a 

 hold with the tip, and draws its body up to that position. It does this 

 actively and moves at a fair rate. 



It has also the power of attaching itself by means of its byssus. 

 M'lntosh iuforms us that these young mussels, when attached to zoophytes, 

 " can detach themselves at will, and if the water in which they are confined 

 be impure, they become restless, and soon, forsaking the zoophyte, swim to 

 the side of the vessel, where they swarm at the water line. In some 

 cases they have been noticed to migrate merely from the lower portion of 

 the zoophyte to the upper branches, near the surface of the water." 

 They measure about to ^ -inch. Towards the end of September many 

 minute mussels, \ inch long, are found on the ba.-es of the zoophytes. 



This power of moving by means of its foot and of detaching itself the 

 mussel retains during its whole life. As a rule, active continued move- 

 ment of the mussel in a line is only noticed in the earliest bottom stages. 



Some mussels about 1-^ inch in length, when put on August 5 into an 

 aquarium, became fairly active. They can move themselves along sharp end 

 first, leaving a trail like a snail track; it is narrow, as is the foot when ex- 

 tended. The mussel does not need to form byssus threads when it travels 

 on the level, at this size at least. It can draw itself round by putting out 

 its foot at right angles to its long axis and slewing its sharp end. iN'one 

 were seen to go round end first. One had its foot straight out in front of 

 the round end, but no actual movement was detected before the foot was 

 withdrawn. Big mussels do, I think, move round end first. 



All mussels are anchored by means of the threads which they form 

 with the foot, and they may be held fast by the threads of another, or of 

 several mussels. If they wish to shift their quarters in consequence of 

 unfavourable conditions, as, for example, the risk of becoming choked 

 with mud, the mussel may, without difficulty, do so if it is not bound by 

 other mussels. In the latter case, it may have to stay and be destroyed. 



When the mussel is not bound by other mussels, it may break its 

 attachment in several ways. It must first find a solid object to which the 

 tip of its foot may firmly adhere. Then by shortening its foot it throws 

 a strain on to the byssus. That strain may result in snapping the threads, 

 or some of them, upon w T hich the strain bears, and the mussel maybe able 

 to move a little away while the remaining threads still hold fast. The 

 mussel may then make new threads to stay it on either side in its new posi- 

 tion. The movement may have been made in any direction, end on or side- 

 ways. From the new position it may drag itself along in another direction, 

 snapping some more of the threads that detain it. The discs of some of 

 the threads may become detached from the stone to which they were ad- 

 hering when the strain was put on them. On an old byssus many 

 threads are found loose and bearing their discs. Finally the mussel 

 may cast off the whole byssus, when the byssus is too strongly attached 

 to break, by tearing it out by its root. So far as my observations have 

 shown, the mussel after this happens does not go very far before it 

 spins itself new byssus threads. In fact, it would appear simply to make 

 the one step, and then attach itself by one or several threads put out 

 anteriorly, posteriorly, and across {vide fig. 67). 



Two mussels were kept in a box in which the mud accumulated. One 

 of the mussels was observed after some months to have been shifting its 



