How do fish breathe?

Most fish do not breathe with lungs like land animals do.  Instead they have small slits called gills through which water passes.  The oxygen is then removed from the water by a network of blood vessels.  However, there are a few species of fish that can and do breathe air, including lungfish and bichirs.  There are also fish that breathe by absorbing oxygen through the skin and others that breathe by passing air through the gut.