Final answer:
a. Flagella
The hair-like projections used by bacilli and spirilla to move are called flagella. They are long structures used for cell movement, which are distinct from cilia, the short, abundant structures that move substances along cell surfaces.
Step-by-step explanation:
Hair-like projections used by bacilli and spirilla to move themselves are called flagella. These are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell. Examples of organisms with flagella include sperm and Euglena. While eukaryotic cells can have flagella, it is more rare compared to prokaryotes. On the contrary, cilia are short, hair-like structures used by certain cells, such as those in the respiratory tract, to move mucus and trapped particles. Unlike flagella, when cilia are present, they are abundant and cover the cell’s surface.