This snow white side-gilled slug is a pleasure to find.  At first glance it appears to be more like a cadlina, but on close examination it becomes clear that this animal lacks the gills typical of dorids. They have an oral veil and a white-margin running around the mantle and the foot, but their rhinophores are tubular and their breathing aparatus consists of a feathery side gill.  For an image of the anatomy of a live berthella, check out Morddyn’s Flickr site.  I wouldn’t recommend looking too far into his collection of live images of gastropods, unless you have a stomach for clothespinned flesh.  Note:  This berthella is often mis-identified as the Berthella californica and for the longest time I assumed that’s what this was.