Why is there no open or close operation in the interface to the flat file service or the directory service. What are the differences between our directory service Lookup operation and the UNIX open?

What will be an ideal response?

Because both services are stateless. The interface to the flat file service is designed to make open unnecessary. The Lookup operation performs a single-level lookup, returning the UFID corresponding to a given simple name in a specified directory. To look up a pathname, a sequence of Lookups must be used. Unix open takes a pathname and returns a file descriptor for the named file or directory.