Why is the common mussel enveloped in a thread-like substance termed byssus?

Because these threads are united in the body to a secreting gland, furnished with powerful muscles, and, at the other extremity, glued to the rock or other body to which it connects itself. - Fleming.

Why are the pinnae, or sea-wings prized?

Because of their beard, by means of which they attach themselves, and from which gloves, etc. are manufactured at Smyrna, Tarentum, Palermo, etc.

A pair of gloves made of this material may be seen in the British Museum.