The name Platelminto means “worms with the body flattened dorsoventrally”, platýs = flat and helminths = worm. Platelminths are triblastic animals, acelomated, with bilateral symmetry, with an anterior and posterior, a dorsal and a ventral region. Among the flatworms there are free-living species that are predators or that feed on dead animals. They are present in the seas, in fresh water and in humid terrestrial environments .
There are also flatworms that are parasites of other animals, including humans. This is the case of Schistosoma mansoni , which causes schistosomiasis (water belly) and Taenia solium , which causes teniasis and cysticercosis. There are species of worms in which individuals are microscopic, but there are also species in which individuals are very long, reaching about 10 meters in length, such as some species of tapeworm or solitary (intestinal parasites).
Regardless of the length, the thickness of the body of the flatworms is very small. As a result, the innermost cells of the body are not far from the surface, which allows the exchange of respiratory gases and the release of nitrogenous excreta (ammonia) by diffusion through the body surface
Classes of flatworms
Whirlwinds are free-living flatworms, like freshwater planarians . Planarians are hermaphroditic animals, with rare self-fertilization. In the anterior region of the body there are two ocelli, sensory structures capable of perceiving light, but which do not form images. During the copulation of these animals, two individuals unite and exchange sperm and then separate. After fertilization, one or more zygotes are protected in capsules that are released through the genital pore. These are attached to the substrate and young planarians hatch from them. Planarians can also reproduce asexually by transverse fission.
Trematodes are parasitic flatworms that have one or two suction cups used to attach to the host’s body. When there are two suction cups, one is located around the mouth and the other is in the ventral position. The body lining is modified and plays an important role in protecting the animal against substances produced by the host. On the body surface, nutrients, mainly amino acids, are absorbed by pinocytosis and gas exchange and excreta release also occur.
An example of a trematode is a human parasite that stands out in Brazil called Schistosoma mansoni , which causes schistosomiasis or schistosomiasis. This species is one of the few among the flatworms that present separate sexes and sexual dimorphism, that is, it is possible to recognize males and females by the external aspect of the body. During the life cycle of this parasite there is a need for an intermediate host, a snail of the genus Biomphalaria .
Cestodes are parasitic flatworms , represented mainly by tapeworms (intestinal parasites). They have no mouth or other structures of the digestive system, nutrients are obtained only by pinocytosis or by absorption through the lining of the body. It is modified with characteristics that allow not only the absorption of nutrients, but also protection against substances produced by the host. The most important species for humans are: Taenia solium and Taenia saginata . These animals can reach the human intestine up to 10 m in length. Because there is usually only one individual in the host’s body, tapeworms are also called solitary.