Nudibranch Orienthella trilineata (northern form, about 10mm long) & look-alike amphipod Podocerus cristatus. Scanned from a 35 mm slide I took in the lab in 1984. I had found the specimens in close proximity to each other a few days earlier in the low intertidal at Middle Cove, Cape Arago, Oregon. The amphipod (and her recently hatched juveniles, visible at lower right when viewed large) are on a piece of the outer tube of the polychaete Pista elongata; the slug is on a piece of the hydroid Abietinaria.

Notice the white head tentacles of the slug and the white antennae on the amphipod. Here's what one form of the amphipod looks like in central California, where the southern form of O. trilineata ( with chromium orange on their rhinophores and cephalic tentacles) is prevalent: https://www.inaturalist.org/observations/1103734

Farther south, near Puerto Vallarta, the amphipod lacks white lines, just like the abundant local aeolids!:https://www.flickr.com/photos/8723238@N07/2686177157