Plume de mer?

  

Close-up of the gill plume of the nudibranch sometimes known as a Sea Lemon (Peltodoris nobilis).  Note the extensive branching to increase surface area for respiration.

Evidence suggests that nudibranchs evolved from gastropod ancestors that lost their coiled shell and original internal gills.  Dorid nudibranchs like this one secondarily evolved an external gill plume for respiration.