How do nudibranchs breathe?

Nudibranchs actually derive their name from their scientific name Nudibranchia, which means naked gills.  This is because of the horns and feathery gills that protrude from the nudibranchs' back.  Actually, to be more specific, some nudibranchs have these feathery gills that stick out of their back and that they use for respiration and others have tentacles that cover their bodies that are used for this purpose as well as for the purpose of self defense.