There are a total of around 20,000 species of flatworms, some of which live in the sea, others in freshwater and some of which are parasitic. As the name suggests, flatworms are flat and relatively broad. They are primitive animals, but have a definite head and tail and a developed nervous system. The body and reproduction deviate so much from other worms that they have been categorized separately. The larger species glide through the water just like snails. Despite the fact that they are primitive, these animals are capable of learning. For example, they reacted to a lamp in experiments and can also learn to react to it always in the same way.