A phylum of acoelomate, triploblastic, dorsoventrally flattened (hence their name), bilaterally symmetrical worms in which the internal organs are well developed and metameric segmentation is absent. The gut may be present, but there is no anus and no blood-vascular system; protonephridia (see nephridium) are the excretory and osmoregulatory organs. Skeletal elements are rare. Morphologically they are quite diverse, and many are parasites. Most are hermaphrodites. The phylum lacks a definitive fossil record. From various evolutionary considerations it is probable that platyhelminths were in existence 1000–800 Ma ago.
Subjects: Zoology and Animal Sciences.