Final answer:
The unique structure to parasitic worms such as tapeworms is the scolex, which allows them to attach to the host's intestinal wall, differing from excretory structures like flame cells and nephridia found in other organisms.
Step-by-step explanation:
The structure unique to parasitic worms such as tapeworms (cestodes) is the scolex. The scolex is an organ located at the anterior end of a tapeworm which allows it to attach to the intestinal wall of its host. Unlike structures such as flame cells and nephridia that are involved in excretion and osmoregulation, the scolex's primary function is to anchor the parasite within the host. Flame cells and nephridia can be found in other organisms and are not unique to parasitic worms. Flame cells, also known as protonephridia, are found in planaria and perform an excretory function similar to that of kidneys in humans, whereas nephridia are found in annelids like earthworms and serve as a more sophisticated excretory system. Malpighian tubules are part of the excretory system of insects and also serve a different role.