[CentOS] CentOS 4.3 and apt-get

I see very clearly how yum accesses the CentOS repos when I look in

        /etc/yum.repos.d/CentOS-Base.repo

But I don't see any equivalent file in

        /etc/apt/sources.list.d

All I have in that directory are list files like 

        dag.list
        dries.list
        freshrpms.list
        ...etc..

How does apt-get know where to look for CentOS repo stuff?