Flatworms are members of the phylum Platyhelminthes. There are more than 20,000 known species of flatworms. They can be found in freshwater, marine, or damp terrestrial environment. Most flatworms are free-living, however, some are parasites. Parasites live in the host body and can be harmful to the host. Flatworms have bilaterally-symmetrical flat bodies. They are ribbon-shaped and are flattened dorsoventrally. There are four major classes of flatworms such as Cestoda (tapeworms), Turbellaria (planarians), Trematoda (flukes), and Monogenea. The class Turbellaria (planarians) is free-living. Cestoda (tapeworms) and Trematoda (flukes) are parasitic.
Planarian is also known as Dugesia and lives in freshwater. It is hermaphroditic (having both male and female sex organs). It has a simple brain (ganglia) and nervous system, arrow-like head, and two eyespots. It has an ability of regeneration. It is a scavenger or a carnivore. Planaria are harmless flatworms.
Tapeworms are the intestinal worms. They live in the intestinal tract of many species, including dogs, cats, and even human beings. Tapeworms have segmented bodies and each segment is known as proglottid. Each proglottid is a reproductive organ. Tapeworms do not have a well-developed digestive system. They can grow very long. They remain attached to the intestine of the host using the hooks and suckers present on the head.
Flukes have complex life cycles and they live within one or more hosts. They are characterized by a well-developed digestive system with mouth at the anterior end and one or more suckers surrounding the mouth. Suckers are used to remain attached to the internal body surface of the host. Schistosoma (blood flukes) spends some part of its life in snails. When humans wade in the water containing snails, they get infected. A larva of a Schistosoma invades the blood vessels of humans. Its eggs are passed through human feces into water and the snail is infected.