of the Fishery Board for Scotland. 249 



It is striated longitudinally, and splits up readily longitudinally. It 

 extends right to the limits of the loculus. It is difficult to get the film 

 separated whole, as it is so thin, and readily splits. Moreover, it is 

 thrown into a curve iuferiorly, as the loculus is. The top end of the 

 film is the thinnest pait. That means that the loculus is slightly wedge- 

 shaped. The edge of the film is all thready, as if in separating it fibres 

 had been broken that connected it to the glands round the edge of the 

 loculus. The film is lightly reticulated, but the main rows of striae run 

 longitudinally with cross wrinkles. The upper two-thirds is all punctu- 

 ated with dots, and it is whitish to the naked eye, while the lower 

 termination is clear and marked by parallel striation (fig. 62). Round 

 the outer rind of the pit are situated among the muscle fibres, the glands 

 which supply the secretion which forms the film. 



When the byssus is drawn out of a fresh mussel the films are whitish 

 and they stick together, i.e., they are more or less in a plastic condition. 

 In preserved mussels the fihn is hard, transparent ; the byssus cannot, 

 as a rule, be drawn out of preserved mussels. The septa are contracted 

 and thrown into ridges, which serve to hold the films last. In the fresh 

 mussel they often slide out easily. 



The sheath of the muscle includes a film-like tissue resembling the 

 above film, and both are probably derived from similar glands 



A mussel which had been opened, its adductor muscle being cut, had some 

 of the threads of its byssus attached to its own shell. It was seen to be 

 exerting a strain on the byssus. Part of the white portion of the stem 

 was drawn out of the pit, and the inner soft collar of the neck was 

 everted. The byssus root came gradually out, drawing a final thread of 

 adhesive matter with it. This thread was sticky and connected the 

 root still to the pit. Under the microscope it was seen to be rather 

 granular. The root was bathed in a similar fluid. The soft transparent 

 films were crumpled, but not regularly cross-barred, as in those mussels 

 killed in formaline. 



The force exerted on the muscles and on the pit when the byssus root 

 was withdrawn will cause a copious outflow of the secretion from the 

 glands into the loculi. It will lubricate the films in the passage out, and 

 fill the loculi with secretion, which will be moulded into new films. 



The films grow, and that process seems to take place as follows: — 

 Round the edge of the loculus the ducts pour out their secretion in a 

 semi-fluid condition as threads. These are moulded in the loculus into a 

 flat sheet, but the film shows its thread formation even in the bard con- 

 dition. The growth of the byssus may be due to successive pulls upon it, 

 which will result in drawing out the films a little, when the secretion 

 from the glands will pour out to a similar extent, so that the threads which 

 form the film retain their connection by plastic threads right into the gland 

 ducts. Inthe loculus the film hardens, but does not do so completely, 

 except at its lower extremity, when it comes out into the neck of the pit. 



The films thus retain a continuous connection down through the stem 

 of the byssus, and also above with the glands. 



The gelatinous termination of the septum dips down between the 

 thread-like terminations of the films as they leave the loculi, and in the neck 

 of the pit the films are moulded tightly into a rod-like stem of the byssus. 



It is possible that there is poured out from the ends of the septa a 

 secretion that serves to bind the films together into the byssus stem. 



The thin outer skin that clothes the septum on both sides is in its 

 nature similar to the film. In one case pieces of this outer clear layer 

 were bearing what looked like little short fibrillse. 



I made out no cilia on the septum, nor in the neck of the byssus-pit. 



