byssus

noun

1. 1.An exceptionally fine and valuable fibre or cloth of ancient times. Originally used for fine flax and linens, the word was later extended to fine cottons, silks, and sea silk.
2. 2.The long fine silky filaments excreted by several mollusks (particularly Pinna nobilis) by which they attach themselves to the sea bed, and from which sea silk is manufactured.
3. 3.The stipe or stem of some fungi which are particularly thin and thread-like.