The subarachanoid space lies between the arachnoid and pia mater. It is filled with cerebrospinal fluid. All blood vessels entering the brain, as well as cranial nerves pass through this space. The term arachnoid refers to the spider web like appearance of the blood vessels within the space.
Why do biologists organize living things into groups? What do the levels of classification indicate about the relationship between organisms? What characteristics are used to classify organisms into domains and kingdoms? What is the scientific study of how living things are classified called? Taxonomy
Here are some definitions: Classification is a very broad term which simply means putting things in classes. Any kind of organisational scheme is a classification: for example, sorting smarties by colour, coins by diameter or cities by population. Humans seem unable to resist the urge to classify.