How can I link a new contact to a new opportunity with web services?

We have successfully used web services to create a new contact and a new opportunity.  We need to link the contact to the opportunity (and vice versa) with web services.  Can someone please tell us how to do it or send us to the appropriate resource?   I assume there is another table that relates the two somewhere but we don't know what it is or how to access it.  Thank you.

Here is how the contact appears at the bottom section in an opportunity

Here is how the opportunity appears in the contact