Ok I am new to Apex, this should be easy  but I am not finding a good resource manual. I have a Apex trigger to send an form email whenever a case changes owner.
What I'd like to do is include a link to the orginal case.

Here is what I think is the relevant lines as they exist

mail.setSubject('You are now the Owner of Case#:'+Trigger.new[i].CaseNumber);
                  mail.setPlainTextBody('Ownership of Case#:'+Trigger.new[i].CaseNumber+' has been set to you. Have a Nice Day.');

so I need to change this from plain text and include a field from the case containing the threadID so it'll link back to the case.
can that field be included?