These are hollow, non-helical, filamentous appendages:

Explanation

These are hollow, nonhelical, filamentous appendages. Pili are smaller than flagella and are not involved in motility. True pili are only present in gram-negative bacteria. They are made up of a special protein called pilin. They are primarily involved in a mating process between cells called the conjugation process. Some pili function as a means of attachment of bacteria to various surfaces.