Although expected to occur at Bodega Head based on range and habitat (low intertidal and subtidal rocky shores), this is the first time we've seen this intriguing mollusc. It's Berthella californica, a side-gilled sea slug. They're related to nudibranchs and sea hares, but are in a separate order called Notaspidea. Although not visible, they have a small, flattened internal shell that is covered by the mantle. This specimen was ~3 cm long.
A few things to look for in the photos: (1) a pair of rolled rhinophores (chemosensory organs, similar to antennae) — they were actively pulsating in the live animal, similar to a mouth opening and closing; (2) oral veil — leading edge, hammerhead-shaped; (3) foot — seen trailing behind; (4) gill — tip protruding from the back and right side of the animal; (5) distinct white spots throughout, and thin white border to mantle and foot.
[Note rolled rhinophore, above, and mouth-like rhinophore opening, below]
Top and bottom views of the gill tip, below.