UML sequence diagram - how to show a web service call

I recently started to model an application that makes extensive use of web service calls, and my customer asked me to include a UML sequence diagram to show the flow of calls in the system. This prompted me to wonder, “What is the correct way to show distributed systems (like a web service) in a UML sequence diagram?”

If this link is correct, in a UML diagram you show remote systems as actors.

This seems like a decent approach, leaving me with only a slight problem in that the remote system is treated as one amorphous blob, with no way I can see to declare classes/objects within that blob from my UML diagram. If a web service can be invoked by calls to different commands/controllers/classes, then I need a little more detail here.