of the Fishery Board for Scotland. 243 



place, leaving a track in the mud. When the box was cleaned a track of 

 byssi on the bottom was seen, showing the stages of its progression. 

 This track is reproduced in fig. 67. Nos. 1-11 indicate successive 

 positions of the mussel in its journey over the bottom of the box. The 

 track is reproduced about natural size ; the mussel is shown in natural 

 size in fig 69. The same two mussels were put into a glass aquarium, 

 and a track was also found on the bottom of it. This is shown in 

 fig. 68. The mussel had evidently proceeded by little, fairly regular 

 stages. The length of its stay at a place is indicated by the number 

 of byssus threads there. The shorter the stay the fewer the threads. In 

 some positions it has apparently only formed one thread, and then given 

 it up to shift another stage. 



I have noticed a mussel move itself by minute jerks within the limits of 

 move)nent permitted by the attached byssus threads. It is, of course, able 

 by means of the muscles of its byssus to throw a strain on or slacken 

 the byssus threads, independently of any pull which it may be making 

 with its adhering foot. 



The mussel may remain months without shifting out of the spot. Its 

 track is sometimes in a circle, coming back to cross itself. 



Lang mentions that the mussel is able to climb up a perpen- 

 dicular wall by lifting itself by the foot and then making a thread fast. 

 The thread may be made while the foot is in any condition of extension 

 or contraction; the length of the thread and its character various with that 

 condition. Fi-om the position thus secured, an^ suspended by the thread 

 now drawn tight by the byssus muscles, a further or a side position may 

 be got for other threads. From this situation it may then climb a 

 further stage. 



A mussel formed a thread, that is, withdrew its foot and exposed a new 

 thread, at 2 o'clock. At 2.23 it was noticed that the foot had been again 

 extruded. By 2.25 a new thread was exposed. It had taken less than 

 25 minutes to make the thread. 



The movement of the mussel was observed in different boxes in which 

 mud accumulated, and it was also noticed in some mussels that were on a 

 floating traj'. The latter were covered in part with filamentous algse. 



Figs. 70-80 represent periodic drawings of a mussel and its byssus 

 threads from the time it was put into a clean aquarium by itself. The faeces 

 was kept clear of it by being removed each day for the first month or so. 

 During the first night it had put its foot out in every direction, and had 

 cleared away the thin layer of mud, leaving an impress of the positions 

 that its foot had taken up. It had made only three threads. 



Its changes in position were watched from April till July ; during 

 this time it shifted its position slightly now and then. It began to move 

 in the beginning of May; at the end of the month the excreta had 

 accumulated near the inhalent aperture and the mussel had slewed round 

 to the right to a position at right angles to its original one. The 

 following day it had gone back to the left. 



On June 18 the mussel was now enveloped in a growth of algae, and it 

 had turned right round in the same spot, after breaking some of the 

 threads. On July 4 it occupied the position shown in fig. 80. It has 

 moved away a little, having thrown off two byssi that represent tem- 

 porary positions. On July 11 some algoid filaments were attached to the 

 shell near the edge, and the inhalent aperture was only opened a little. 

 The shells were separated, but the aperture was closed by the frills. It 

 had broken free and moved off a little way by July 13. On July 15 it 

 was over to the right, having made two small byssi tracks, and on the 

 following day it was found over to the left. Its shells were hardly open ; 



