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Flatworms and nudibranchs are colourful free-living animals on reefs, in mangroves and in saline lakes. Any resemblance between both groups is superficial and they are evolutionary very different. Flatworms are the most primitive 2-sided symmetrical, multi-cellular animals. They possess primitive eyes, an extensive intestinal system and a muscularly pharynx, which is protruded into a prey for the injection of enzymes. In this way digested food can be sucked up by the muscular pharynx. The food particles are taken up directly by the cells of the intestine. An anus is lacking. Flatworms form a thin layer and do not possess gills. Usually, two tentacles are visible at the head. Flatworms are hermaphrodites and possess male as well as female genitalia. Recently it became known that polyclad flatworms fight duels for a fathership. They do a lot of effort to fertilise another individual with their sperm.
Polyclads in particular possess a large colour variation for camouflage or warning, which indicates that the animals is not edible. They feed very often on tunicates from which they use the poisons for their own chemical defence against predators. Some flatworm species possess the same poison as puffer fish, one of the most deadly poisons. Interestingly, some fish species show mimicry with respect to colours, colour patterns and form of the flatworms although these fishes themselves are not poisonous (mimicry of Bates). Some flatworms and nudibranchs resemble eachother (mimicry of Mueller).
Polyclad flatworm, Pseudoceros lindae.
Nudibranchs are much better known. As an adult these animals bear no longer a shell. The snails have more body mass than the flatworms and possess external gills to obtain oxygen from the water. Location, number and form of the gills are important for their systematics. Nudibranchs are highly developed snails at which the shell reduced in the course of the evolution. Their defence is chemical by mucus, poisons and nematocysts obtained from their prey such as ascidians and hydroids. Just as at the polyclad flatworms many colour patterns are known for camouflage or warning for predators. Many species are insufficiently studied anatomically and highly variable species may consist of several species.
Acoel turbellarian flatworms under fungiid.