MUSCULAR ACTION OF THE FOOT. 177 



shell generally glides down the newly -constructed 

 filament, but this is not of necessity, nor does such 

 circumstance invariably occur. Indeed, while busily 

 engaged in attaching a disc to the glass, the muscles 

 of the foot will contract, and thus throw open the 

 folds of the groove, situated in the middle of that 

 organ ; when thus exposed, the byssus thread may be 

 seen in the furrow, stretched like the string of a harp 

 or dulcimer. 



While the end of the thread is being attached to 

 a certain spot, a conspicuous muscular action is per- 

 ceived going on in the foot, which alternately swells 

 and contracts, as if something were being pumped up 

 through the byssal channel, until it reached a certain 

 point. There being dilated and spread out in succes- 

 sive layers, it assumes a trumpet-like disc, which is 

 firmly fixed to the foreign object. Indeed, I am by 

 no means certain that the thread is not, when first 

 produced, exactly like a trumpet in shape. It also 

 conveys the idea of being blown out in a similar 

 manner to a piece of bottle glass. After being ex- 

 posed to the air for some little time, the hollowness 

 of the thread is not so apparent as when it is newly 

 fabricated. 



The mucous fluid, from which the fibres are 

 formed, is secreted in a gland situated at the base of 

 the foot, whence it is apparently expelled at the will 

 of the animal into the furrow already referred to, and 

 is there spun into threads. The toughness of these 



12 



