unzipping split zip files named filea, fileb, filec, file1, file2, I got my first mac today and Im really going crazy trying to do something quite simple

I have some split zip files called

somefilea.zip somefileb.zip    somefilec.zip somefiled.zip ... to     z.zip somefile1.zip somefile2.zip ...

I'm trying to merge these into the same folder and let the files they contain join into one folder.

I've tried app "the unarchiver" for this but I failed. It kept making new folders for every part.

Is there an unzip tool in which I can do this? 
On windows I'd select them all, right-click, select "extract" in the winrar menu and it would do what I want.
I managed to do it with StuffIt expander

I did have to click "merge" and "skip" 32 times but it eventually did the trick..