Marine flatworms are a group of organisms that have a soft, flattened, unsegmented form which resembles a leaf. They are extremely primitive, lack organs and body cavities, and have a very basic, incomplete digestive system. They breathe directly through their body's surface through a process called diffusion for which it is necessary for all cells of their body to be in contact with the water all the time, making their flattened form important. Flatworms feed on a variety of things ranging from algae to small fish and other invertebrates. They are brightly coloured, and thus, sometimes called "butterflies of the sea". Interestingly, some flatworms can mimic the appearance of sea slugs for camouflage, and some may even contain some minor concentrations of toxins for defence!