This image shows a brightly-colored nudibranch (sea slug).  These forms lack any shell or true gills, and gas exchange occurs across the body surface.  Carnivorous nudibranchs that feed mainly on cnidarians have dorsal projections called cerata that cover their backs; these cerata have ducts that communicate with the stomach. After nutrients are absorbed by the stomach wall, undischarged nematocysts from prey such as the Portuguese man-of-war are carried from the stomach to the tips of the cerata where they are used for their own defense! The bright (aposematic) colors of these nudibranchs may advertise the fact that they are well defended with toxic secretions or nematocysts.