Platyhelminthes show three distinct body axes, dorsal-ventral, anterior-posterior, and left-right, with a visible bilateral symmetry of the body. The space between the epidermis and digestive tract is filled with mesodermal parenchyma and muscle cells, into which all organs are embedded. These animals do not possess a secondary body cavity, a coelom. About one-seventh of the 37,000 described Platyhelminth species are free-living predators. All the rest are parasites. Most of them live as endoparasites inside their hosts, for example, in the liver’s bile ducts or the intestine in vertebrates. Other species colonize as ectoparasites, inhabiting the gills, the oral and pharyngeal cavities, or the rectum of their hosts. All parasites show complex life cycles.