<span>The immune system is a system of many biological structures and processes within an organism that protects against disease.</span>
The correct answer is cyanobacteria (ancient cyanobacteria).
The huge concentration of oxygen in the atmosphere is believed to have been initially produced by the actions of ancient cyanobacteria. They are usually observed as symbionts with a number of another group of species, like coral, lichens (fungi), Azolla (pteridophytes), angiosperms, and others.
The atmosphere of oxygen was generated by various cyanobacteria at the time of Proterozoic and Archaean periods.
The epidural space surrounding the spinal cord differs from
the surrounding of the brain is because the epidural space is set to be a
separation of the dura mater that surrounds the spinal cord and the spinal
dura, in which, allows separation because of its component of having fat and
blood vessels.
