Which metaphors does Holmes use to describe his correspondent to Mr. Watson? What do you think he meant?

From the text:

“Porlock, Watson, is a nom-de-plume, a mere identification mark; but behind it lies a shifty and evasive personality. In a former letter he frankly informed me that the name was not his own, and defied me ever to trace him among the teeming millions of this great city. Porlock is important, not for himself, but for the great man with whom he is in touch. Picture to yourself the pilot fish with the shark, the jackal with the lion—anything that is insignificant in companionship with what is formidable: not only formidable, Watson, but sinister—in the highest degree sinister. That is where he comes within my purview. You have heard me speak of Professor Moriarty?”