of the Fishery Board for Scotland. 



251 



may loosen their holds. A byssus often contains many black threads 

 carrying their discs that had been pulled away from their points of 

 attachment. 



Finally, if the discs are firmly attached, and they generally stick well 

 if lately formed, the mussel can, when it desires to go, draw out the 

 byssus by its roots. Where the stem grows much the threads will 

 become so much the longer, and the mussel may no longer be able to put 

 a tight strain on them. It will then probably form new threads upon 

 which it can exert a strain. 



The stem and the thread are formed independently of one another. 

 The former is composed of the fibres from the byssus pit, possibly glued 

 together by secretion from the tips of the septa, while they are moulded 

 into shape by the neck of the pit. The mussel very often attaches some 

 of the threads to its own shell. One advantage which this may have 

 will be to tend to lessen the rocking of the mussel. 



A curious condition happened in the case of one mussel. It had 

 attached some of its byssus threads to the bottom of the tank and some 

 to its own shell. It drew out the byssus by its roots, and moved off, 

 but was checked at a little distance by the byssus thread attached to the 

 shell (fig. 69). It, however, when found, bad formed new byssus 

 threads on its new situation. Later on it snapped the threads that 

 detained it ; they were four in number, and together must have made 

 a fairly strong combination. 



The Formation of a new Byssus Stem. 



The rudiment of a new byssus stem is formed by the old root as it is 

 being drawn out. When the films forming the root slip out of the 

 loculi a secretion is poured out by the glands and this secretion accom- 

 panies the film outside, and is drawn out of the pit in a thickish thread. 

 The remains of this may be seen attached to cast'byssi. The loculi are 

 now full of secretion that will be moulded into films. And this little thread 

 of secretion that is left sticking out in the neck of the byssus pit, and 

 which is continous with that in the loculi, is sufficient for the mussel to 

 attach a thread to. It will harden at once. This is especially well seen 

 in the byssus tracks where the mussel has simply formed a few threads, 

 and then cast its byssus completely. Fig. 50 shows the formation of 

 threads on a new byssus. Very soon the byssus grows, increasing 

 rapidly to its normal thickness. 



The beginning of the stem is shown at (b), and the gradual widening 

 in diameter is shown by an optical section. The root is marked thus (v), 

 and the threads by the letter (t). 



The withdrawal of the films seems simply to open the ducts of the 

 glands, and give their secretion free exit. Then by filling the loculus 

 with secretion they seal themselves. 



Except for the bases of the threads no rind is found to the stem. There 

 appears to be a sort of outer skin, but it may be simply the flattening of 

 plastered films. 



In some cross sections of the byssus stem a cavity was shown in the 

 centre. 



The byssus stem probably grows through the strain put on it by the 

 muscles drawing it out bit by bit. 



Growth of the Spout-fish (Solen siliqua.) 



Several stages of the spout-fish have been kindly sent by Mr. Wm. 

 MacBride, jr., Pirnmill. They were collected on April 30th. 



