The platyhelminthes are a group of wormlike soft-bodied animals comprising about 25,000 species. They have no body cavity or special respiratory structures and respiration occurs throughout the body surface. The gut usually has only one opening which serves as both mouth and anus. The majority are parasitic, including the flukes (class Trematoda), the monogeneans (class Monogenea) and the tapeworms (class Cestoda), but there is a fourth class, (Turbellaria) which consists of about 4,000 free-living species. Many platyhelminths are dorso-ventrally flattened which accounts for their common name, flatworm.
Very little information is available concerning survival threats to platyhelminthes, as it is very difficult to obtain population estimates. Water pollution seems to pose the largest threat.
Due to the great capability of adaptation and to the resistance of many platyhelminthes species to extreme environment conditions, man's alteration of the environment (such as the creation of dams) has often unintentionally favoured their propagation.