Invisible Font Directories, e.g., ".Aqua Kana"

I searched the forums using "dot font" and "invisible font" without success, then gave up. So here follows my (first) question (ever in discussions.apple.com). -- Background: Using Font Book to "export" all fonts from my "Computer" collection I obtained a folder which, when viewed via the Finder looks perfectly normal, but when looked at via Terminal.app with the unix command "ls -a" shows the additional directories: ".Aqua Kana", ".Helvetica LT MM", ".Keyboard" and ".Times LT MM". -- Question: Why do these font directories show up in this manner and why are they not seen in Font Book? -- Font Book shows no dots, i.e., there are no duplicate fonts. All fonts are validated. Everything works fine, I'm just curious.

867 MHz Powerbook G4 15" Mac OS X (10.4.9) Powerbook3,5