248 Part III. — Twenty-fifth Annual Report 



The neck of the pit grips the stem of the byssus tightly ; it is formed 

 by the continuation of the side of the groove of the foot. The neck has 

 a soft collar which is everted when a strong pull is made on the byssus, 

 and which is smooth on its inner surface (i-c and i'c, fig. 46). 



Muscles. — The byssus pit is supplied with two sets of muscles, one set 

 arising near the hinge from the surface of each half-shell. If the two 

 sets of muscles be torn apart, and the separation assisted by a knife, the 

 pit will be split into halves, and so will the byssus stem ; the pit is bi- 

 laterally symmetrical (fig. 63). One group of muscles occupies a position 

 where it gives off fibres to the outside of both halves of the pit. Between 

 the muscles are crowded large masses of yellow glands (g, fig. 23). 



The foot and the byssus pit are not strongly bound together, the only 

 connecting part being the neck of the pit. The neck does not belong to the 

 pit ; it belongs to the foot. The relation of the pit and the foot is expressed 

 thus — that the byssus stem passes through the groove of the foot at its 

 proximal closed end ((ig. 42). 



Septa. — The pit itself is roughly pear-shaped, the byssus-stem forming 

 the stalk (fig. 37). It is divided vertically by thin septa into a great 

 number of slit-like loculi, set parallel with the median vertical longitudi- 

 nal plane of tlie mussel. The septa are flat in their greater extent, but 

 inferiorly they are curved in a slight spiral as they go to join the byssus 

 stem. This condition is shown diagrammatically in fig. 96. When the 

 pit is split into two as above, some of the septa are cut across. Each 

 muscle is inserted across all the septa in its own half of the pit ; i.e., each 

 septum gets some strands of each muscle. 



A septum was dissected off (tig. 34). It is striated in texture, 

 and is transvei'sely ridged. The ridges are mostly developed on 

 the lower half ; towards the' upper limit they disappear. Some of the 

 fibres of the muscle on the outside of the pit come away with the septum ; 

 they are straight and have glands attached to them (g). The septum is 

 attached firmly to the binding tissue uniting the insertions of the muscles. 

 At one part of the outside of the pit the edges of the septa are free. 

 The septum shows a tendency to split along a line between the insertion of 

 two groups of muscles. In the upper narrow part of the septum the ridges 

 crossing obliquely are in connection with the muscles inserted on one side. 



In preserved specimens, killed in formalin, the ridges are very promi- 

 nent, the septum being thrown into wave-like section. When dissected 

 out of fresh specimens the septum is smooth, not ridged. It seems to be 

 muscular. The septum is coated on either side by a thin gelatinous layer 

 and it ends inferioily in a clear, gelatinous, tapered tip. This tip teazes 

 easily into strands. There is no sharp dividing line between the pit 

 muscle and the septum. The septa are simply external continuations 

 of the muscle fibres. The septum may thus be regarded as a sort of 

 combination muscle or tendon, formed out of fibres from three main 

 groups of muscles. This is shown diagrammatically in fig. 86. 



The septum itself shows numerous small glands (g) scattered through 

 it. The transparent part of the septum (tr) shows tubes in it. 



Film (figs. 61 and 62). — Each loculus is occupied by a thin gelatinous 

 film or lamella. It has the size and shape of the loculus and septum, 

 and it tapers away into a thread-like termination inferiorly where, it 

 unites with its fellows to form the byssus stem. Figs. 49 and 85 

 exhibit the structure diagrammatically. 



The film very often comes away with the septum when it is dissected 

 off, and if the septa are thrown into rugae the film is similarly rugose ; it 

 is moulded between the two adjacent septa. Fig. 36 shows the longi- 

 tudinal section of a ridged film. 



