250 Part III. — Twenty-fijth Annual Report 



Byssus stem. 



The byssus stem is a bundle of longitudinal plaited fibres. It is of 

 slightly greater diameter at the proximal end than distally. The byssus 

 stem grows independently of the formation of the threads. The growth 

 may be simply due to continued small pulls which the muscles 

 exert on the byssus thread. 



When a thread is being formed the foot of the mussel is extended, 

 and its tip having the depression is attached to the object. In this 

 position the groove is closed on every side. The byssus stem passes 

 through it. The secreting surface of small ducts surrounds the stem, and 

 the secretion that is poured out fills the groove, surrounds the stem, and 

 fills up the depression. Now from the short time which is required to 

 make a thread, it is evident that some special force is used to drive the 

 secretion out until it fills the whole groove, neck of pit, and depression. 

 This may be attained by the distension with fluid of the lacuute between 

 the muscles in the foot. And the internal pressure may be relieved by the 

 issue of the fluid from the median vessel into the depression. This will 

 relieve the sucker and permit of water getting into the groove in the foot. 

 The water would tend to harden the thread. This seems to me to be modus 

 of the formation of the thread. In some cases parts of threads are found 

 in the groove of the foot. Some sticky material that had a thread for- 

 mation was drawn out of the depression at the end of the groove, and it 

 was connected by a thread to what looked like a pore at one side. It 

 stuck to the glass. Under the microscope it was seen to be formed of 

 longitudinal fibres and moulded externally with a crenate edge. 



The complete thread (fig. 53) is flattened laterally in its proximal 

 half and is rounder at its distal half. It appears to be made in layers 

 laid one on top of the other. The part of the thread inside the mussel 

 remains pale, that outside grows yellow and finally black. Owing to 

 being flattened the thread shows a core very distinctly in its proximal 

 part. Each layer is corrugated as it is laid on. This layer formation is 

 probably due to the distention of the foot being exerted intermittently at 

 short intervals. The external corrugations on the thread depend on the 

 extent to which the foot is extended. The thread is crossed by fine lines in 

 its whole course. These probably represent rows of gland openings or the 

 division between. I think it is probable that the secretion is pressed 

 out of the ducts in little semi-plastic threads at right angles into the 

 grooves. There they are moulded by the sides of the groove, the complete 

 thread being built up of several layers. The buttons at the ends of the 

 thread were seen in one case to be tinged with violet. 



The proximal end of the thread is in the form of a ring round the 

 stem (fig. 57). 



When the threads have been laid on in rapid succession a longitudinal 

 section of the stem there shows the bases of the thread overlapping 

 like a series of steps (fig. 52). 



The threads may be made so quickly one after the other that the bases 

 overlap and increase the thickness of the stem very much (fig. ih.). 



In other cases the interval between the formation of the two threads 

 has been such that the stem of the byssus had grown in the interval 

 (fig. 57). The attachment made by the disc to a clear suriace, e.g., 

 glass, is very strong The foot of the mussel seems to a certain extent 

 to clean the surface to which the button is to adhere. 



Occasionally the thread breaks as soon as it is formed ; sometimes 

 the button alone is seen sticking to the object. Threads may be broken 

 by the muscles putting too great a pull on them. The adherent discs 



