Phylum Platyhelminthes

Platyhelminthes (Greek platy, broad or flat; helmin, worm) are the flatworms — bilaterally symmetrical, triploblastic, acoelomate animals that are nonetheless the first phylum in the classification scheme to achieve organ-system level of organisation, thanks to the appearance of the mesoderm layer. Their dorsoventrally flattened body gives the phylum its common name, and they show moderate cephalisation with generally unidirectional movement.

Most members of this phylum are endoparasites of animals, including humans, and parasitic forms carry hooks and suckers as organs of attachment to their host. The body is unsegmented, though some members show pseudosegmentation. Some parasitic flatworms absorb nutrients directly through their body surface rather than through a gut. Where a gut is present, it is often incomplete — as in the liver fluke, whose single opening serves as both mouth and anus. Specialised excretory cells called flame cells carry out osmoregulation and excretion, working through a network of fine tubules. …