This is a baby nudibranch (Hermissenda crassicornis)on the back of the sea cucumber (Parastichopus californicus). The nudibranch (yes "naked lung") was about the size of an eraser head. The colorfull fingers on its back (called cerata) are actualy evaginations from its gut it uses for gas exchange. The cerata are also used for defense as they harbor stinging cells stolen from creatures related to jellyfish/anemones it consumes in its diet. For many years they where thought to be related to anemones/jellyfish until the advent of genetic sequencing showed these stinging cells (cnidae) where actually stolen (klepto-cnidae).
