Which glial cells form an epithelium that lines a fluid-filled passageway within the spinal cord and brain?

The correct answer to this is:

“the ependymal cells”

Glial cells are cells that sustain and protect the neurons in the peripheral and central nervous system. They estimate to take up about half the volume of the nervous system. One of the glial cells is the ependymal cells. They create a simple cuboidal to columnar epithelium which is known as ependyma.

the dark variety, as they are not visible on the dark tree barks

explanation:

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.