What is the name of the structures that move substances across a tract surface?

cilia and flagella

Explanation:

In prokaryotic species , cilia are present , and in eukaryotic species , flagella is present .

Cilia and flagella both have same function , i.e. , to enable the movement of the cell , along with the movement of some substance and direct the flow of these substance along the tracts.

Cilia and flagella are composed of basal bodies.

Hence , from the given statement of the question,

The correct term is cilia and flagella .

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.